Theory of Evolution by Natural Selection

CHAPTER 7

Smith, T. M., & Smith, R. L. (2015). Elements of Ecology (9th ed.). Boston, MA: Pearson.

7.1 Size Imposes a Fundamental Constraint on the Evolution of Organisms

Living organisms occur in a wide range of sizes (Figure  7.1). The smallest animals are around 2–10 micrograms [μg], and the largest living animals are mammals (the blue whale weighing more than 100,000 kilograms (kg) in marine environments and the African elephant at 5000 kg on land). Each taxonomic group of animals has its own particular size range, largely as a result of morphological and physiological constraints. Some groups such as Bryozoa (aquatic colonial animals) contain species all within one or two orders of magnitude, whereas mammals are hugely variable in size. The smallest mammal is a species of shrew weighing only about 2 grams (g) fully grown—or about 100 million (108) times less than the blue whale.

Size has consequences for structural and functional relationships in animals, and as such, presents a fundamental constraint on adaptation. Most morphological and physiological features change as a function of body size in a predictable way—by a process known as scaling.

Geometrically similar objects, such as cubes or spheres, are referred to as being isometric (Greek for “having equal measurement”). The surface area (SA) and volume (V) of isometric objects are related to their linear dimensions (length = l) to the second and third power, respectively. For example, the surface area of a square is l2, where l is the length of each side. Therefore the surface area of a cube of length l is 6l2 (six sides). In contrast, the volume of the cube is l3 (Figure 7.2a).

Interpreting Ecological Data

1. Q1. The volume of a cube of length 4 is 64 (or 43), and the surface area is 96 (or 6 × 42). Now consider a three-dimensional rectangle having the following dimensions of length (l), height (h), and width (w): l = 16, h = 2, w = 2. The volume is l × h × w = 64. The surface area is 4(l × h) + 2(w × h). Calculate the surface area. How does the SA:V ratio differ for these two objects with the same volume?

2. Q2. Which of the two objects (cube or three-dimensional rectangle) would be a more efficient design for exchanging substances between the surface and the body interior?

An important consequence of the characteristics of isometric scaling is the relationship between the surface area and volume. If the ratio of surface area to volume (SA:V) is plotted against length (l) for a square, there is an inverse relationship between SA:V and l (Figure 7.2b); smaller bodies have a larger surface area relative to their volume than do larger objects of the same shape.

This relationship between surface area and volume imposes a critical constraint on the evolution of animals. The range of biochemical and physiological processes associated with basic metabolism (assimilation and respiration) requires the transfer of materials and energy between the organism’s interior and its exterior environment. For example, most organisms depend on oxygen (O2) to maintain the process of cellular respiration (see Section 6.1). Every living cell in the body, therefore, requires that oxygen diffuse into it to function and survive. Oxygen is a relatively small molecule that readily diffuses across the cell surface; in a matter of seconds, it can penetrate into a millimeter (mm) of living tissue. So the center of a spherical organism that is 1 mm in radius is close enough to the surface that as the organism uses oxygen in the process of respiration, its oxygen is replenished by a steady diffusion from the surface in contact with the external environment (air or water).

Now imagine a spherical organism with the radius of a golf ball: approximately 21 mm. It would now take more than an hour for oxygen to diffuse into the center. Although the layers of cells just below the surface would receive adequate oxygen, the continuous depletion of oxygen as it diffused toward the center and the greater distance over which oxygen would have to diffuse, would result in the death of the interior cells (and eventually the organism) because of oxygen depletion.

The problem is that, as the size (length or radius) of the organism increases, the surface area of the body across which oxygen diffuses into the organism decreases relative to the interior volume of the body that requires the oxygen (the SA:V ratio decreases as shown in Figure 7.2). So how can animals respond to this constraint so that an adequate flow of oxygen may reach the entire interior of the body in larger organisms?

A more complex, convoluted, or wrinkled surface, as shown in Figure 7.3, functions to increase the surface area of an object having the same volume as the golf-ball-shaped organism. The difference is that now (1) no point on the interior of the organism is more than a few millimeters from the surface, and (2) the total surface area over which oxygen can diffuse is much greater.

Another way of responding to the constraint is to actively transport oxygen into the interior of the body. Many of the smallest animals have a tube-like shape with a central chamber (Figure 7.4a). These animals draw water into their interior chamber (tube), allowing for the diffusion of oxygen and essential nutrients into the interior cells. Once again, the end result is the increase of the surface area for absorption (diffusion) relative to the volume (SA:V), which assures that every point (cell) in the interior is close enough to the surface to allow for the diffusion of oxygen. As body size increases, however, a more complex network of transport vessels (tubes) is needed for oxygen to reach every point in the body.

worms shown in (b).

Much of the shape of larger organisms is governed by the transport of oxygen and other essential substances to cells in the interior of the body. To allow for this, a complex set of anatomical structures has evolved in animals. Lungs function as interior chambers that bring oxygen close to blood vessels, where it can be transferred to molecules of hemoglobin for transport throughout the body. A circulatory system with a heart functioning as a pump assures that oxygen-containing blood is actively transported into the minute vessels or capillaries that permeate all parts of the body. These complex systems increase the surface area for exchange, assuring that all cells in the body are well within the maximum distance over which oxygen can diffuse at the rate necessary to support cellular respiration.

The same body size constraints apply to the wide range of metabolic processes that require the exchange of materials and energy between the external environment and the interior of the organism. Carbon and other essential nutrients must be taken in through a surface. The food canal (digestive system) in most animals is a tube in which the process of digestion occurs and through which dissolved substances must be absorbed into the circulatory system for transport throughout the body. In the smallest of animals, such as the Bryozoa (see Figure 7.1) or tube worms (Figure 7.4b), the central chamber into which water is drawn also functions as the food canal, where digestion occurs and substances are absorbed. Waste products then exit through the opening as water is expelled. In larger animals, the food canal is a tube extending from the mouth to the anus. As food travels through the tube it is broken down, and essential nutrients and amino acids are absorbed and transported into the circulatory system. The greater the surface area of the food canal, the greater its ability to absorb food. Because surface area increases as the square of length, the larger the animal (which increases as a cube), the greater the surface area of its food canal must be to maintain a constant ratio of surface area to volume.

From these simple examples, it should be clear that greater body size requires complex changes in the organism’s structure. These changes represent adaptations that maintain the relationship between the volume (or mass) of living cells that must be constantly supplied with essential resources from the outside environment and the surface area through which these exchanges occur.

We will examine various adaptations relating to the ability of animals to maintain the exchange of essential nutrients (food), oxygen, water, and thermal energy (heat) with the external environment. We will also consider how those adaptations are constrained by both body size and the physical environments in which the animals live (Section 7.11).

7.2 Animals Have Various Ways of Acquiring Energy and Nutrients

The diversity of potential energy sources in the form of plant and animal tissues requires an equally diverse array of physiological, morphological, and behavioral characteristics that enable animals to acquire (Figure 7.5) and assimilate these resources. There are many ways to classify animals based on the resources they use and how they exploit them. The most general of these classifications is the division based on how animals use plant and animal tissues as sources of food. Animals that feed exclusively on plant tissues are classified as herbivores. Those that feed exclusively on the tissues of other animals are classified as carnivores, and those that feed on both plant and animal tissues are called omnivores. In addition, animals that feed on dead plant and animal matter, called detritus, are detrital feeders, or detritivores (see Chapter 21). Each of these four feeding groups has characteristic adaptations that allow it to exploit its particular diet.

Herbivory

Because plants and animals have different chemical compositions, the problem facing herbivores is how to convert plant tissue to animal tissue. Animals are high in fat and proteins, which they use as structural building blocks. Plants are low in proteins and high in carbohydrates—many of them in the form of cellulose and lignin in cell walls, which have a complex structure and are difficult to break down (see Chapter 21). Nitrogen is a major constituent of protein. In plants, the ratio of carbon to nitrogen is about 50:1. In animals, the ratio is about 10:1.

Herbivores are categorized by the type of plant material they eat. Grazers feed on leafy material, especially grasses. Browsers feed mostly on woody material. Granivores feed on seeds, and frugivores eat fruit. Other types of herbivorous animals, such as avian sapsuckers (Sphyrapicus spp.) and sucking insects such as aphids, feed on plant sap; hummingbirds, butterflies, and a variety of moth and ant species feed on plant nectar (nectivores).

Grazing and browsing herbivores, with some exceptions, live on diets high in cellulose (complex carbohydrates made up of hundreds or thousands of simple sugar molecules). In doing so, they face several dietary problems. Their diets are rich in carbon but low in protein. Most of the carbohydrates are locked in indigestible cellulose, and the proteins exist in chemical compounds. Lacking the enzymes needed to digest cellulose, herbivores depend on specialized bacteria and protists living in their digestive tracts. These bacteria and protozoans digest cellulose and proteins, and they synthesize fatty acids, amino acids, proteins, and vitamins.

The highest-quality plant food for herbivores, vertebrate and invertebrate, is high in nitrogen in the form of protein. As the nitrogen content of their food increases, the animals’ assimilation of plant material improves, increasing growth, reproductive success, and survival. Nitrogen is concentrated in the growing tips, new leaves, and buds of plants. Its content declines as leaves and twigs mature and become senescent. Herbivores have adapted to this period of new growth. Herbivorous insect larvae are most abundant early in the growing season, and they complete their growth before the leaves mature. Many vertebrate herbivores, such as deer, give birth to their young at the start of the growing season, when the most protein-rich plant foods are available for their growing young.

Although availability and season strongly influence food selection, both vertebrate and invertebrate herbivores do show some preference for the most nitrogen-rich plants, which they probably detect by taste and odor. For example, beavers show a strong preference for willows (Salix spp.) and aspen (Populus spp.), two species that are high in nitrogen content. Chemical receptors in the nose and mouth of deer encourage or discourage consumption of certain foods. During drought, nitrogen-based compounds are concentrated in certain plants, making them more attractive and vulnerable to herbivorous insects. However, preference for certain plants means little if they are unavailable. Food selection by herbivores reflects trade-offs between quality, preference, and availability (see this chapter, Field Studies: Martin Wikelski).

Carnivory

Herbivores are the energy source for carnivores—the flesh eaters. Unlike herbivores, carnivores are not faced with problems relating to digesting cellulose or to the quality of food. Because the chemical composition of the flesh of prey and the flesh of predators is quite similar, carnivores encounter no problem in digesting and assimilating nutrients from their prey. Their major problem is obtaining enough food.

Among the carnivores, quantity is more important than quality. Carnivores rarely have a dietary problem because they consume animals that have resynthesized and stored protein and other nutrients from plants in their tissues.

Omnivory

Omnivores feed on both plants and animals. The food habits of many omnivores vary with the seasons, stages in the life cycle, and their size and growth rate. The red fox (Vulpes vulpes), for example, feeds on berries, apples, cherries, acorns, grasses, grasshoppers, crickets, beetles, and small rodents. The black bear (Ursus americanus) feeds heavily on vegetation—buds, leaves, nuts, berries, tree bark—supplemented with bees, beetles, crickets, ants, fish, and small- to medium-sized mammals.

The means of food resource acquisition functions as a major selective agent in the process of natural selection, directly influencing the physiology, morphology, and behavior of animal species. From the specific behaviors and morphologies necessary to locate, capture, and consume different food resources (see Figures 5.10, 5.15, 5.20, 5.21, and 7.5 for specific examples), to the different enzymes and digestive systems necessary to break down and extract essential nutrients from the plant and animal tissues upon which they feed, the means of acquiring food resources has been a major force in the evolution of animal diversity.

7.3 In Responding to Variations in the External Environment, Animals Can Be either Conformers or Regulators

Some environments change little on timescales relevant to living organisms, such as the deep waters of the oceans. However, the majority of environments on our planet vary on a wide range of timescales. Regular annual, lunar, and daily cycles (see Chapters 2 and 3) present organisms with predictable changes in environmental conditions, whereas changes on a much shorter timescale of hours, minutes, or seconds as a result of weather are much less predictable. When an animal is confronted with changes in its environment, it can respond in one of two ways: conformity or regulation.

In some species, changes in external environmental conditions induce internal changes in the body that parallel the external conditions (Figure 7.6a). Such animals, called conformers, are unable to maintain consistent internal conditions such as body fluid salinity or levels of tissue oxygen. Echinoderms such as the starfish, for example, are osmoconformers whose internal body fluids quickly come to equilibrium with the seawater that surrounds them. The degree to which conformers can survive in changing environments depends largely on the tolerance of their body tissues to internal changes brought about by the changes in the external environment.

Conforming largely involves changes at the physiological and biochemical levels. If the internal conditions are allowed to vary widely, be it in terms of temperature, salinity, or oxygen supply, then tissues and cells will need to have biochemical systems in place that can continue to function under the new conditions. In extreme conditions, changes in these systems must be sufficient enough to keep the animal functional, even if at a low level, to avoid potentially irreversible damages, such as freezing, hypoxia (lack of oxygen), or osmotic water loss. Typically the biochemical and physiological changes that occur are simple and energetically inexpensive but carry the cost of reduced activity and growth.

Regulators, as their name implies, use a variety of biochemical, physiological, morphological, and behavioral mechanisms to regulate their internal environments over a broad range of external environmental conditions (Figure 7.6b). For example, in contrast to an osmoconformer, an osmoregulator maintains the ion concentrations of its body fluids within a limited range of values when faced with changes in the ion concentration of the surrounding water.

In contrast to conformity, regulation may require substantial and energetically expensive changes in biochemistry, physiology, morphology, and behavior. Behavior is often the first line of defense; however, behavior is augmented by substantial physiological and biochemical adjustments.

The strategies of conformity and regulation, therefore, have different costs and benefits. The benefit of conformity is a low energetic expenditure associated with mechanisms that maintain internal environmental conditions, but it results in reduced activity, growth, and reproduction as environmental conditions deviate from those that optimize the function of cells, tissues, and organs. In contrast, regulation is generally expensive. For example, regulation of body temperature in terrestrial animals may account for as much as 90 percent of their total energy budget. The benefit, however, is in the level of performance and the greatly extended range of environmental conditions over which activity can be maintained (see Section 7.11).

Although conformity and regulation represent two distinct strategies for coping with variations in the external environment, a single species may exhibit a different strategy under different environmental conditions or during different activities (Figure 7.7). Extreme environmental conditions may exceed the ability of a species to regulate internal conditions, resulting in conformity with external environmental conditions (see Section 7.12). In addition, a species may be a regulator with respect to one feature of the environment, such as oxygen, but a conformer with respect to another, such as temperature.

7.4 Regulation of Internal Conditions Involves Homeostasis and Feedback

Organisms that maintain their internal environment within narrow limits need some means of regulating internal conditions relative to the external environment, including body temperature, water balance, pH, and the amounts of salts in fluids and tissues. For example, the human body must maintain internal temperatures within a narrow range around 37°C. An increase or decrease of only a few degrees from this range could prove fatal. The maintenance of a relatively constant internal environment in a varying external environment is called homeostasis.

Whatever the processes involved in regulating an organism’s internal environment, homeostasis depends on negative feedback—meaning that when a system deviates from the normal or desired state, referred to as the set point, mechanisms function to restore the system to that state. All feedback systems consist of a parameter or variable that is the focus of regulation (e.g., temperature or oxygen) and three components: receptor, integrator, and effector (Figure 7.8). The receptor measures the internal environment for the variable and transfers the information to the integrator. The integrator evaluates the information from the receptor (compares to set point) and determines whether action must be taken by the effector. The effector functions to modify the internal environment (the variable being regulated).

The thermostat that controls the temperature in your home is an example of a negative feedback system (see Figure 7.8). If we wish the temperature of the room to be 20°C (68°F), we set that point on the thermostat. When the temperature of the room air falls below that point, a temperature-sensitive device within the thermostat trips the switch that turns on the furnace. When the room temperature reaches the set point, the thermostat responds by shutting off the furnace. Should the thermostat fail to function properly and not shut off the furnace, then the furnace would continue to heat, the temperature would continue to rise, and the furnace would ultimately overheat, causing either a fire or a mechanical breakdown.

Among animals, the control of homeostasis is both physiological and behavioral. An example is temperature regulation in humans (see Figure 7.8). The normal temperature, or set point, for humans is 37°C. When the temperature of the environment rises, sensory mechanisms in the skin detect the change. They send a message to the brain, which automatically relays the message to receptors that increase blood flow to the skin, induces sweating, and stimulates behavioral responses. Water excreted through the skin evaporates, cooling the body. When the environmental temperature falls below a certain point, another reaction takes place. This time it reduces blood flow and causes shivering, an involuntary muscular exercise that produces more heat. If the environmental temperature becomes extreme, the homeostatic system breaks down. When it gets too warm, the body cannot lose heat fast enough to maintain normal temperature. Metabolism speeds up, further raising body temperature, until death results from heatstroke. If the environmental temperature drops too low, metabolic processes slow down, further decreasing body temperature until death by freezing ensues.

Field Studies Martin Wikelski

Department of Ecology and Evolutionary Biology, Princeton University, Princeton, New Jersey

The isolated archipelago of the Galápagos Islands off the western coast of South America is known for its amazing diversity of animal and plant life. It was the diversity of life on these islands that so impressed the young Charles Darwin and laid the foundations for his theory of natural selection (see Chapter  5). However, one member of the Galápagos fauna has consistently been met with revulsion by historic visitors: the marine iguana, Amblyrhynchus cristatus. Indeed, even Darwin himself commented on this “hideous-looking creature.”

Marine iguanas are widely distributed throughout the Galápagos Islands, and individuals of different populations vary dramatically in size (both length and weight). Because of these variations, many of the iguana populations were long considered separate species, yet modern genetic studies have confirmed that all of the populations are part of a single species. What could possibly account for the marked variation in body size among populations? This question has been central to the research of University of Konstanz ecologist Martin Wikelski. Studies by Wikelski and his colleagues during the past decade have revealed an intriguing story of the constraints imposed by variations in the environment of the Galápagos on the evolution of these amazing creatures.

In a series of studies, Wikelski and his colleagues have examined differences in body size between two populations of marine iguanas that inhabit the islands of Santa Fe and Genovesa. The populations of these two islands differ markedly in body size (as measured by the snout-vent length), with an average body length of 25 cm (maximum body weight of 900 g) for adult males on Genovesa as compared to 40 cm (maximum body weight of 3500 g) for adult males on the island of Santa Fe. Wikelski hypothesized that these differences reflected energetic constraints on the two populations in the form of food supply.

Marine iguanas are herbivorous reptiles that feed on submerged intertidal and subtidal algae (seaweed) along the rocky island shores, referred to as algae pastures. To determine the availability of food for iguana populations, Wikelski and colleagues measured the standing biomass and productivity of pastures in the tidal zones of these two islands. Their results show that the growth of algae pastures correlates with sea surface temperatures. Waters in the tidal zone off Santa Fe (the more southern island) are cooler than those off Genovesa, and as a result, both the length of algae plants and the productivity of pastures are five times greater off Santa Fe than Genovesa.

By examining patterns of food intake and growth of marked individuals on the two islands, Wikelski was able to demonstrate that food intake limits growth rate and subsequent body size in marine iguanas, which in turn depends on the availability of algae (Figure 1). Body size differences between members of the two island populations can be explained by differences in food availability.

Temporal variations in climate and sea surface temperatures also influence food availability for the marine iguanas across the Galápagos Islands. Marine iguanas can live for up to 30 years, and environmental conditions can change dramatically within an individual’s lifetime. El Niño events usually recur at intervals of three to seven years but were more prevalent in the decade of the 1990s (see Section 2.9). During El Niño years in the Galápagos, sea surface temperatures increase from an average of 18°C to a maximum of 32°C as cold ocean currents and cold-rich upwellings are disrupted. As a result, green and red algal species—the preferred food of marine iguanas—disappear and are replaced by the brown algae, which the iguanas find hard to digest. Up to 90 percent of marine iguana populations on islands can die of starvation as a result of these environmental changes.

In studying patterns of mortality during the El Niño events of the 1990s, Wikelski observed the highest mortality rate among larger individuals. This higher mortality rate was directly related to observed differences in foraging efficiency with body size. Wikelski and colleagues determined that although larger individuals have a higher daily intake of food, smaller individuals have a higher food intake per unit body mass, a result of higher foraging efficiency (food intake per bite per gram body mass). Large iguanas on both islands showed a marked decline in body mass during the El Niño events. The result is a strong selective pressure against large body size during these periods of food shortage (Figure 2).

Perhaps the most astonishing result of Wikelski’s research is that the marine iguanas exhibit an unusual adaptation to the environmental variations caused by El Niño. Change in body length is considered to be unidirectional in vertebrates, but Wikelski repeatedly observed shrinkage of up to 20 percent in the length of individual adult iguanas. This shrinking coincided with low food availability resulting from El Niño events.

Shrinking did not occur equally across all size classes. Wikelski found an inverse relationship between the initial body size of individuals and the observed change in body length during the period of food shortage—larger individuals shrank less than smaller individuals.

Shrinkage was found to influence survival. Large adult individuals that shrank more survived longer because their foraging efficiency increased and their energy expenditure decreased (Figure 3).

Given the disadvantage of larger body size during periods of low resource availability, what factors were selecting for larger body size in the marine iguana? What is the advantage of being big? Marine iguanas do not compete for food, either with other iguanas or other species of marine herbivores, and their potential predators are not size specific, so these factors were discounted as selective agents influencing body size. Instead Wikelski found that larger body size benefits males in attracting mates. Male iguanas establish display territories, and females select males for mating. Wikelski found that females favor larger males, and therefore larger males have greater reproductive success and relative fitness. It appears that the evolution of body size in the marine iguana is a continuous battle (trade-off) between the advantage of large body size in reproductive success and the disadvantage of large body size during regular periods of resource shortage.

Bibliography

1. Wikelski, M. 2005. “Evolution of body size in Galápagos marine iguanas.” Proceedings of the Royal Society B 272:1985–1993.

2. Wikelski, M., and C. Thom. 2000. “Marine iguanas shrink to survive El Niño.” Nature 403:37–38.

3. Wikelski, M., V. Carrillo, and F. Trillmich. 1997. “Energy limits on body size in a grazing reptile, the Galápagos marine iguana.” Ecology 78:2204–2217.

4. Wikelski, M. and F. Trillmich. 1997. “Body size and sexual size dimorphism in marine iguanas fluctuate as a result of opposing natural and sexual selection: An island comparison.” Evolution 51:922–936.

1. Does the mortality of iguanas during El Niño events represent a case of natural selection? Which of the three models of selection best describes the pattern of natural selection?

2. If the iguanas could not shrink during the period of resource shortage, how do you think the El Niño events would influence natural sel

7.5 Animals Require Oxygen to Release Energy Contained in Food

Animals obtain their energy from organic compounds in the food they eat; and they do so primarily through aerobic respiration, which requires oxygen (see Section 6.1). Most organisms are oxygen regulators, maintaining their own oxygen consumption even when external (ambient) oxygen levels drop below normal. Oxygen conformity in which oxygen consumption decreases in proportion to decreasing ambient oxygen concentrations is found, however, in some smaller aquatic organisms.

Oxygen is easily available in the atmosphere for terrestrial animals. However, for aquatic animals, oxygen may be limiting and its acquisition problematic (see Section 3.6). Differences between terrestrial and aquatic animals in the means of acquiring oxygen reflect the availability of oxygen in the two environments. Minute terrestrial organisms take in oxygen by diffusion across the body surface. With increasing body size, however, direct diffusion across the body surface is insufficient to supply oxygen throughout the body (see Section 7.1). Insects have tracheal tubes that open to the outside through openings (or spiracles) on the body wall (Figure  7.9a). The tracheal tubes carry oxygen directly to the interior of the body allowing diffusion to the cells.

Unable to meet oxygen demand through the direct diffusion of oxygen across the body surface, larger terrestrial animals (mammals, birds, and reptiles) have some form of lungs (Figure 7.9b). Unlike tracheal systems that branch throughout the insect body, lungs are restricted to one location. Structurally, lungs have innumerable small sacs that increase surface area across which oxygen readily diffuses into the bloodstream. Amphibians take in oxygen through a combination of lungs and vascularized skin (containing blood vessels). Lungless salamanders are an exception; they live in a moist environment and take in oxygen directly through the skin.

In aquatic environments, organisms must take in oxygen from the water or gain oxygen from the air in some way. Marine mammals such as whales and dolphins come to the surface to expel carbon dioxide and take in air containing oxygen to the lungs. Some aquatic insects rise to the surface to fill the tracheal system with air. Others, like diving beetles, carry a bubble of air with them when submerged. Held beneath the wings, the air bubble contacts the spiracles of the beetle’s abdomen.

A number of smaller aquatic animals are oxygen conformers, particularly sedentary marine invertebrates, most cnidarians (corals, jellyfish, and sea anemones), and echinoderms (starfish and sea urchins). Most, however, are oxygen regulators, and as with terrestrial animals, the mechanisms controlling oxygen uptake are related to size. Minute aquatic animals, zooplankton, take up oxygen from the water by diffusion across the body surface. Larger aquatic animals have gills, that is, outfoldings of the body surface that are suspended in the water and across which oxygen can diffuse. The gills of many aquatic invertebrates, such as starfish, are simple in shape and distributed over much of their body. In others, such as the crayfish or sea scallop, gills are restricted to specific regions of the body (Figure  7.9c). Fish, the major aquatic vertebrates, pump water through their mouth. The water flows over gills and exits through the back of the gill covers (Figure 7.9d). The close contact with and the rapid flow of water over the gills allows for exchanges of oxygen and carbon dioxide between water and the gills.

7.6 Animals Maintain a Balance between the Uptake and Loss of Water

Living cells, both plant and animal, contain about 75–95 percent water. Water is essential for virtually all biochemical reactions within the body, and it functions as a medium for excreting metabolic wastes and for dissipating excess heat through evaporative cooling. For an organism to stay properly hydrated, these water losses must be offset by the uptake of water from the external environment. This balance between the uptake and loss of water with the surrounding environment is referred to as an organism’s water balance (see Section 4.1).

Terrestrial animals have three major ways of gaining water and solutes: directly by drinking and eating and indirectly by producing metabolic water in the process of respiration (see Section 6.1). They lose water and solutes through urine, feces, evaporation from the skin, and from the moist air they exhale. Some birds and reptiles have a salt gland, and all birds and reptiles have a cloaca—a common receptacle for the digestive, urinary, and reproductive tracts. They reabsorb water from the cloaca back into the body proper. Mammals have kidneys capable of producing urine with high ion concentrations.

In arid environments, animals, like plants, face a severe problem of water balance. Survival depends on either evading the drought or by avoiding its effects. Animals of semiarid and desert regions may evade drought by leaving the area during the dry season and moving to areas where permanent water is available. Many of the large African ungulates (Figure 7.10) and many birds use this strategy.

(a) Many of the large ungulate species in the semiarid regions of Africa, such as the wildebeest shown here, migrate over the course of the year, following the seasonal shift in rainfall. (b) The changing distribution of wildebeest populations in the contiguous Serengeti, Masai Mara, and Ngorongoro Conservation areas in East Africa. This seasonal pattern of migration gives these species consistent access to food (grass production) and water.

Many animals that inhabit arid regions avoid the effects of drought by entering a period of physiological inactivity (dormancy) termed estivation. During hot, dry periods the spadefoot toad (Scaphiopus couchi) of the southern deserts of the United States remains below ground in a state of estivation and emerges when the rains return (Figure 7.11). Some invertebrates inhabiting ponds that dry up in summer, such as the flatworm Phagocytes vernalis, develop hardened casings and remain in them for the dry period. Other aquatic or semiaquatic animals retreat deep into the soil until they reach the level of groundwater. Many insects undergo diapause, a stage of arrested development in their life cycle from which they emerge when conditions improve.

Other animals remain active during the dry season but reduce respiratory water loss. Some small desert rodents lower the temperature of the air they breathe out. Moist air from the lungs passes over cooled nasal membranes, leaving condensed water on the walls. As the rodent inhales, this water humidifies and cools the warm, dry air.

There are other approaches to the problem. Some small desert mammals reduce water loss by remaining in burrows by day and emerging by night. Many desert mammals, from kangaroos to camels, extract water from the food they eat—either directly from the moisture content of the plants or from metabolic water produced during respiration—and produce highly concentrated urine and dry feces. Some desert mammals can tolerate a certain degree of dehydration. Desert rabbits may withstand water losses of up to 50 percent and camels of up to 27 percent of their body weight.

Unlike terrestrial animals, aquatic animals face the constant exchange of water with the external environment through the process of osmosis. As in the discussion of passive transport of water in plants, osmotic pressure moves water through cell membranes from the side of greater water concentration to the side of lesser water concentration (see Section 6.4). Aquatic organisms living in freshwater are hyperosmotic; they have a higher salt concentration in their bodies than does the surrounding water. Consequently, water moves inward into the body, whereas salts move outward. Their problem is the prevention of uptake, or the removal of excess water, and replacement of salts lost to the external environment. Because of the large disparity between the osmotic concentration of the freshwater and body fluids (e.g., blood), osmoconformity is not an option in freshwater environments. Freshwater fish maintain osmotic balance by absorbing and retaining salts in special cells in the gills and by producing copious amounts of watery urine (Figure 7.12a). Amphibians balance the loss of salts through the skin by absorbing ions directly from the water and transporting them across the skin and gill membranes. In the terrestrial phase, amphibians store water from the kidneys in the bladder. If circumstances demand it, they can reabsorb the water through the bladder wall.

The constraint imposed upon marine organisms is opposite of that faced by freshwater organisms. These organisms are hypoosmotic; they have a lower salt concentration in their bodies than does the surrounding water. When the concentration of salts is greater outside the body than within, organisms tend to dehydrate. Osmosis draws water out of the body into the surrounding environment. In marine and brackish environments, organisms have to inhibit water loss by osmosis through the body wall and prevent an accumulation of salts in the body (see Chapter 3).

Marine animals have evolved a variety of mechanisms that function to regulate water balance. Some animals are isosmotic; their body fluids have the same osmotic pressure as the surrounding seawater. For example, the bodies of invertebrates such as tunicates, jellyfish, many mollusks, and sea anemones are unable to actively adjust the amount of water in their tissues. These animals are osmoconformers, and their bodies gain water and lose ions until they are isosmotic to the surrounding water. In contrast, others function as osmoregulators, employing a variety of mechanisms to maintain constant salt concentration in their body. Marine bony (teleost) fish absorb saltwater into the gut. They secrete magnesium and calcium through the kidneys and pass these ions off as a partially crystalline paste. In general, fish excrete sodium and chloride, major ions in seawater, by pumping the ions across special membranes in the gills (Figure 7.12b). This pumping process is one type of active transport, moving salts against the concentration gradient, but it has a high energy cost. Sharks and rays retain enough urea to maintain a slightly higher concentration of solute in the body than exists in surrounding seawater. Birds of the open sea and sea turtles can consume seawater because they possess special salt-secreting nasal glands. Seabirds of the order Procellariiformes (e.g., albatrosses, shearwaters, and petrels) excrete fluids in excess of 5 percent salt from these glands. Petrels forcibly eject the fluids through the nostrils; other species drip the fluids out of the internal or external nares. In marine mammals, the kidney is the main route for elimination of salt; porpoises have highly developed kidneys to eliminate salt loads rapidly.

7.7 Animals Exchange Energy with Their Surrounding Environment

In principle, an animal’s energy balance is the same as that described for a plant (see Section 6.6). Animals, however, differ significantly from plants in their thermal relations with the environment. Animals can produce significant quantities of heat by metabolism, and their mobility allows them to seek out or escape heat and cold.

Body structure influences the exchange of heat between animals and the external environment. Consider a simple thermal model of an animal body (Figure 7.13). The interior or core of the body must be regulated within a defined range of temperature. In contrast, the temperature of the environment surrounding the animal’s body varies. The temperature at the body’s surface, however, is not the same as the air or water temperature in which the animal lives. Rather, it is the temperature at a thin layer of air (or water) called the boundary layer, which lies at the surface just above and within hair, feathers, and scales (see Section 6.6).

Therefore, body surface temperature differs from both the air (or water) and the core body temperature. Separating the body core from the body surface are layers of muscle tissue and fat, across which the temperature gradually changes from the core temperature to the body surface temperature. This layer of insulation influences the organism’s thermal conductivity; that is, the ability to conduct or transmit heat.

To maintain its core body temperature, the animal must balance gains and losses of heat to the external environment. It does so through changes in metabolic rate and by heat exchange. The core area exchanges heat (produced by metabolism and stored in the body) with the surface area by conduction, that is, the transfer of heat through a solid. Influencing this exchange are the thickness and conductivity of fat and the movement of blood to the surface. The surface layer exchanges heat with the environment by conduction, convection, radiation, and evaporation, which are all influenced by the characteristics of skin and body covering.

External environmental conditions heavily influence how animals confront thermal stress. Because air has a lower specific heat and absorbs less solar radiation than water does (Section 3.2), terrestrial animals face more radical and dangerous changes in their thermal environment than do aquatic animals. Incoming solar radiation can produce lethal heat. The loss of radiant heat to the air, especially at night, can result in deadly cold. Aquatic animals live in a more stable energy environment, but they have a lower tolerance for temperature changes (Section 7.9).

7.8 Animal Body Temperature Reflects Different Modes of Thermoregulation

Different animal species exhibit different ranges of body temperature in their natural environments. In some, body temperature varies; these species are referred to as poikilotherms (from the Greek poikilos meaning “changeable”). In others species, termed homeotherms (from the Greek homoeo meaning “same”), body temperature is constant or nearly constant. These terms, poikilotherm and homeotherm, are not, however, synonymous with conformers and regulators discussed in Section 7.3. In fact, probably the only true thermoconformers are those animals that live in environments, such as the deep regions of the oceans, that have little to no variations in ambient temperature, and body temperature is virtually identical to the unchanging water temperature. Whether poiklotherms or homeotherms, all animals exhibit some degree of regulation.

Although environmental temperatures vary widely, both temporally and spatially, in most organisms body temperature is regulated through behavior, physiology, and morphology. The term thermoregulation does not merely refer to an organism’s internal temperature differing from that of the surrounding environment; rather regulation implies maintaining the average body temperature or variations in body temperature within certain bounds. This requires mechanisms for the organism to sense and respond to its thermal environment. There are two categories of thermal regulation that emphasize the source of thermal energy used to influence body temperature: ectothermy and endothermy.

Ectothermy is the process of maintaining body temperature through the exchange of thermal energy with the surrounding environment. Species that use this mechanism of thermoregulation are called ectotherms. In contrast, endothermy is the process of maintaining body temperature through internally generated metabolic heat. Species that use this mechanism of thermoregulation are called endotherms. Although in practice all animals generate some internal heat as a function of metabolic processes, and all animals use external sources of thermal energy to modify body temperatures (such as basking in the sun or seeking shade), these two categories are largely distinct. Endothermic species have the special ability to raise their metabolic activity markedly in excess of their immediate needs, using the resulting metabolic heat to maintain body temperature. In contrast, ectothermic species lack this ability and depend on external sources.

So how is the classification of animals based on variations in body temperature (poikilotherm and homeotherm) related to the classification of species based on primary means of temperature regulation (ectotherm and endotherm)? Although some species that inhabit environments where the thermal environment is fairly constant, such as the cold deep waters of the ocean or the litter layer of a tropical rain forest, may exhibit little if any variations in body temperature, the term homeotherm is generally applied to endothermic animals that maintain a constant body temperature through metabolic processes (endothermy). The only animals that fall within the category of endothermic homeotherms are birds and mammals. All other animals are typically classified as poikilotherms.

To simplify our proceeding discussion, we will discuss mechanisms in thermoregulation in terms of the two categories of animals based on variations in body temperature: poikilotherms, who use primarily ectothermy, and homeotherms, who primarily regulate body temperature using endothermy.

7.9 Poikilotherms Regulate Body Temperature Primarily through Behavioral Mechanisms

The performance (common measures include locomotion, growth, development, fecundity, and survivorship) of poikilotherms varies as a function of body temperature. As with plants (see Section 6.6, Figure 6.6), each species has minimum and maximum temperatures at which performance approaches zero (Tmin and Tmax) and a temperature or range of temperatures over which performance is optimal (Topt; Figure  7.14). Likewise, the relationship between body temperature and performance varies among species and is correlated to the temperature characteristics of the environments they inhabit. Jonathon Stillman and George Somero of Stanford University examined the upper thermal tolerance limits (Tmax) of 20 species of porcelain crabs, genus Petrolisthes, from intertidal and subtidal habitats (see Chapter 25, Figure 25.1) throughout the eastern Pacific. The researchers found that the upper thermal tolerance limit (Tmax) was positively correlated with surface water temperature and maximum temperature in the microhabitats in which the species were found (Figure 7.15).

Poikilotherms have a low metabolic rate and a high ability to exchange heat between body and environment (high thermal conductivity; see Figure 7.13). During normal activities, poikilotherms carry out aerobic respiration. Under stress and while pursuing prey, the poikilotherms’ inability to supply sufficient oxygen to the body requires that much of their energy production come from anaerobic respiration, in which oxygen is not used. This process depletes stored energy and accumulates lactic acid in the muscles. (Anaerobic respiration can occur in the muscles of marathon runners and other athletes, causing leg cramps.) Anaerobic respiration metabolism limits poikilotherms to short bursts of activity and results in rapid physical exhaustion.

To maintain body temperature in the “preferred” or optimal range, terrestrial and amphibious poikilotherms rely largely on behavioral thermoregulation. They seek out appropriate microclimates where environmental temperatures allow for body temperatures to approach optimal values. Insects such as butterflies, moths, bees, dragonflies, and damselflies bask in the sun to raise their body temperature to the level necessary to become highly active. When they become too warm, these animals seek the shade. Semiterrestrial frogs, such as bullfrogs (Rana catesbeiana) and green frogs (Rana clamitans), exert considerable control over their body temperature. By basking in the sun, frogs can raise their body temperature as much as 10°C above ambient temperature. Because of associated evaporative water losses, such amphibians must be either near or partially submerged in water. By changing position or location or by seeking a warmer or cooler substrate, amphibians can maintain body temperatures within a narrow range.

Lizards raise and lower their bodies and change body shape to increase or decrease heat conduction between them and the rocks or soil they rest on. They also seek sunlight or shade or burrow into the soil to adjust their temperatures. Desert beetles, locusts, and scorpions exhibit similar behavior. They raise their legs to reduce contact between their body and the ground, minimizing conduction and increasing convection by exposing body surfaces to the wind.

The work of Gabriel Blouin-Demers and Patrick Weatherhead of Carleton University (Ottawa, Canada) illustrates the role that behavior plays in the thermoregulation of snakes. The researchers conducted a series of studies to examine how the body temperatures of individual black rat snakes (Elaphe obsoleta) varied on a daily basis under field conditions. Individual snakes were implanted with sensors that allowed their movement and body temperature to be monitored. Although it is relatively straightforward to monitor the temperatures of the various environments used by the snakes over the course of the day, the more relevant measure is the body temperature that occurs when the snake occupies each of these environments, referred to as the operative environmental temperature. For example, the body temperature of a snake lying on bare soil would not be the same as either the air temperature at the soil surface or the surface temperature of the soil. As presented in Section 7.7, the temperature of the snake is influenced by the physical characteristics of the snake (body shape, color, and thermal conductivity) and the exchange of heat between the snake and the surrounding environment. To better estimate the range of body temperatures that each environment represented, the researchers used physical models of a black rat snake constructed from painted copper tubing that matched the reflectance and conductance properties of the snake’s body. The preferred (selected) body temperature(s) of the black rat snakes was established using thermal gradients in the laboratory.

Daily variations in average body temperature for the month of July are presented in Figure 7.16. By selecting a variety of microhabitats (rocks, bare ground, forest, fields, in the open or in the shade; Figure 7.17), individuals were able to maintain their preferred temperature during most of the active period of the day regardless of variations in the operative environmental temperatures. Both the thermal environment and the behavior of the snakes determined the daily pattern of body temperature.

When faced with longer-term changes in environmental temperatures, such as seasonal changes, poikilotherms are able to undergo the process of temperature acclimation (see Chapter  5, Section 5.4). Acclimation allows an animal’s relationship between body temperature and performance to shift. For example, under acclimation, an animal’s metabolic reactions in cold temperatures are increased to a level that is closer to that of warm-acclimated individuals, even though their body temperatures are that of the environment (Figure  7.18). This type of thermal acclimation involves specific biochemical changes (such as shifts in enzyme systems).

Interpreting Ecological Data

1. Q1. How does the temperature at which the maximum sustained swimming speed occurs differ for each of the two water acclimation treatments? How do these differences relate to the water temperatures to which the crocodiles were acclimated (acclimation treatment temperatures)?

2. Q2. How would you describe the trade-off that occurs between temperature acclimation and swimming speed for the crocodiles (costs and benefits of acclimation to the two different temperatures)?

The thermal conductivity of water is approximately 25 times greater than air, meaning that heat is transferred 25 times faster than in air. For this reason, animals in water reach an equilibrium with their surrounding environment much faster than terrestrial animals. As a result, it is much more difficult for the body temperature of aquatic animals to be independent of the surrounding water temperature. Aquatic poikilotherms, when completely immersed, maintain no appreciable difference between their body temperature and the surrounding water. Aquatic poikilotherms are poorly insulated. Any heat produced in the muscles moves to the blood and on to the gills and skin, where heat transfers to the surrounding water by convection. Exceptions are sharks and tunas, which use a form of countercurrent exchange—a blood circulation system that allows them to keep internal temperatures higher than external ones. (See Section 7.13 and Figure 7.23 for discussion of countercurrent heat exchange.) Because seasonal water temperatures are relatively stable, fish and aquatic invertebrates maintain a constant temperature within any given season. They adjust seasonally to changing temperatures by acclimation or physiological adjustment to a change in environmental conditions (for an example of seasonal acclimation see Chapter 5, Section 5.4 and Figure  5.8). They undergo these physiological changes over a period of time. Because water temperature changes slowly through the year, aquatic poikilotherms may adjust slowly. The process of thermal acclimation involves changes in both the upper (Tmax) and lower (Tmin) limits of tolerance to temperature (see Figure 5.4). If they live at the upper end of their tolerable thermal range, poikilotherms’ physiologies adjust at the expense of the ability to tolerate the lower range. Similarly, during periods of cold, the animals’ physiological functions shift to a lower temperature range, which would have been debilitating before. Fish are highly sensitive to rapid change in environmental temperatures. If they are subjected to a sudden temperature change (faster than biochemical and physiological adjustments can occur), they may die of thermal shock.

7.10 Homeotherms Regulate Body Temperature through Metabolic Processes

Homeothermic birds and mammals meet the thermal constraints of the environment by being endothermic. Their body temperature is maintained by the oxidization of glucose and other energy-rich molecules in the process of respiration. The process of oxidation is not 100 percent efficient, and in addition to the production of chemical energy in the form of adenosine triphosphate (ATP), some energy is converted to heat energy (see Section 6.1). Because oxygen is used in the process of respiration, an organism’s basal metabolic rate is typically measured by the rate of oxygen consumption. Recall from Section 6.1 that all living cells respire. Therefore, the rate of respiration for homeothermic animals is proportional to their body mass (grams body mass0.75; however, the exponent varies across different taxonomic groups, ranging from 0.6 to 0.9; Figure 7.19).

For homeotherms, the thermoneutral zone is a range of environmental temperatures within which the metabolic rates are minimal (Figure 7.20). Outside this zone, marked by upper and lower critical temperatures, metabolic rate increases.

Maintenance of a high body temperature is associated with specific enzyme systems that operate optimally within a high temperature range, with a set point of about 40°C. Because efficient cardiovascular and respiratory systems bring oxygen to their tissues, homeotherms can maintain a high level of energy production through aerobic respiration (high metabolic rates). Thus, they can sustain high levels of physical activity for long periods. Independent of external temperatures, homeotherms can exploit a wider range of thermal environments. They can generate energy rapidly when the situation demands, escaping from predators or pursuing prey.

To regulate the exchange of heat between the body and the environment, homeotherms use some form of insulation—a covering of fur, feathers, or body fat (see Figure 7.13). For mammals, fur is a major barrier to heat flow, but its insulation value varies with thickness, which is greater in large mammals than in small ones. Small mammals are limited in the amount of fur they carry because a thick coat would reduce their ability to move. The thickness of mammals’ fur changes with the season, a form of acclimation (see Section 5.4). Aquatic mammals—especially of Arctic regions—and Arctic and Antarctic birds such as auklets (Alcidae) and penguins (Spheniscidae) have a heavy layer of fat beneath the skin. Birds reduce heat loss by fluffing the feathers and drawing the feet into them, making the body a round, feathered ball. Some Arctic birds, such as ptarmigan (Lagopus spp.), have feathered feet—unlike most birds, which have scaled feet that function to lose heat.

Although the major function of insulation is to keep body heat in, it also keeps heat out. In a hot environment, an animal must either rid itself of excess body heat or prevent heat from being absorbed in the first place. One way is to reflect solar radiation from light-colored fur or feathers. Another way is to grow a heavy coat of fur that heat does not penetrate. Large mammals of the desert, notably the camel, use this method. The outer layers of hair absorb heat and return it to the environment.

Some insects—notably moths, bees, and bumblebees—have a dense, furlike coat over the thoracic region that serves to retain the high temperature of flight muscles during flight. The long, soft hairs of caterpillars, together with changes in body posture, act as insulation to reduce convective heat exchange.

When insulation fails, many animals resort to shivering, which is a form of involuntary muscular activity that increases heat production. Many species of small mammals increase heat production without shivering by burning (oxidizing) highly vascular brown fat. Found about the head, neck, thorax, and major blood vessels, brown adipose tissue (fat) is particularly prominent in hibernators, such as bats and groundhogs (Marmota monax).

Many species employ evaporative cooling to reduce the body heat load. Birds and mammals lose some heat through the evaporation of moisture from the skin. When their body heat is above the upper critical temperature, evaporative cooling is accelerated by sweating and panting. Only certain mammals have sweat glands—in particular, horses and humans. Panting in mammals and gular fluttering in birds function to increase the movement of air over moist surfaces in the mouth and pharynx. Many mammals, such as pigs, wallow in water and wet mud to cool down.

7.11 Endothermy and Ectothermy Involve Trade-offs

Prime examples of the trade-offs involved in the adaptations of organisms to their environment are endothermy and ectothermy, which are the two alternative approaches to regulation of body temperature in animals. Each strategy has advantages and disadvantages that enable the organisms to excel under different environmental conditions. For example, endothermy allows homeotherms to remain active regardless of variations in environmental temperatures, whereas environmental temperatures largely dictate the activity of poikilotherms (ectothermy). However, the freedom of activity enjoyed by homeotherms comes at a great energy cost. The maintenance of internal body temperature in homeotherms requires a high metabolic rate, and heat lost to the surrounding environment must be continuously replaced by additional heat generated through respiration. As a result, metabolic costs weigh heavily against homeotherms. In contrast, ectotherms, not needing to burn calories to provide metabolic heat, allocate more of their energy intake to biomass production than to metabolic needs. Ectotherms, therefore, require fewer calories (food) per gram of body weight. A homeotherm must take in some 20 times more food energy than a poikilotherm of equal body mass. Because they do not depend on internally generated body heat, ectotherms can curtail metabolic activity in times of food and water shortage and temperature extremes. Low energy demands enable some terrestrial poikilotherms to colonize areas with limited food and water.

One of the most important features influencing its ability to regulate body temperature is an animal’s size. Poikilotherms (ectothermy) absorb heat across their body’s surface but must absorb enough energy to heat the entire body mass (volume). Therefore, the ratio of surface area to volume (SA:V) is a key factor controlling the uptake of heat and the maintenance of body temperature. As an organism’s size increases, the SA:V ratio decreases (see Figure 7.2b). Because the organism must absorb sufficient energy across its surface to warm the entire body mass, the amount of energy or the period of time required to raise body temperature increases. For this reason, ectothermy imposes a constraint on maximum body size for poikilotherms and restricts the distribution of the larger poikilotherms to the warmer, aseasonal regions of the subtropics and tropics. For example, large reptiles such as alligators, crocodiles, iguanas, komodo dragons, anacondas, and pythons are all restricted to warm tropical environments.

The constraint that size imposes on homeotherms (endothermy) is opposite that presented earlier for poikilotherms. For homeotherms, it is the body mass (or volume) that produces heat through respiration, while heat is lost to the surrounding environment across the body surface. The smaller the organism, the larger the SA:V ratio, therefore, the greater the relative heat loss to the surrounding environment. To maintain a constant body temperature, the heat loss must be offset by increased metabolic activity (respiration). Thus, small homeotherms have a higher mass-specific metabolic rate (metabolic rate per unit body mass; Figure 7.21) and consume more food energy per unit of body weight than do large ones. Small shrews (Sorex spp.), for example, ranging in weight from 2 to 29 g (see Figure 7.1), require a daily amount of food (wet weight) equivalent to their own body weight. Therefore, small animals must spend most of their time seeking and eating food. The mass-specific metabolic rate (respiration rate per gram of body weight) of small endotherms rises so rapidly that below a certain size, they do not meet their energy demands. On average, 2 g is about as small as an endotherm may be and still maintain a metabolic heat balance; however, this minimum constraint depends on the thermal environment. Some shrews and hummingbirds undergo daily torpor (see Section 7.13) to reduce their metabolic needs. As a result of the conflicting metabolic demands of body temperature and growth, most young birds and mammals are born in an altricial state, meaning they are blind, naked, helpless, and begin life as ectotherms. They depend on the body heat of their parents to maintain their body temperature, which allows most of these young animals’ energy to be allocated to growth.

7.12 Heterotherms Take on Characteristics of Ectotherms and Endotherms

Species that sometimes function as homeotherms while at other times as poikilotherms are called temporal heterotherms. At different stages of their daily and seasonal cycle or in certain situations, these animals take on characteristics of endotherms or ectotherms. They can undergo rapid, drastic, repeated changes in body temperature.

Interpreting Ecological Data

1. Q1. How does the variable plotted on the y-axis of this graph (mass-specific metabolic weight) differ from the variable plotted on the y-axis of Figure  7.19?

2. Q2. What does the graph imply about the rates of cellular respiration for a mouse compared to a horse?

3. Q3. How would the graph differ if the y-axis was plotted on a logarithmic scale (log10)?

Insects are ectothermic and poikilothermic; yet in the adult stage, most species of flying insects are heterothermic. When flying, they have high rates of metabolism, with heat production as great as or greater than that of homeotherms. They reach this high metabolic state in a simpler way than do homeotherms because they are not constrained by the uptake and transport of oxygen through the lungs and vascular system. Insects take in oxygen by demand through openings in the body wall and transport it throughout the body in a tracheal system (see Section 7.5).

Temperature is crucial to the flight of insects. Most cannot fly if the temperature of the body muscles is less than 30°C, nor can they fly if muscle temperature is higher than 44°C. This constraint means that an insect must warm up to take off, and it must get rid of excess heat in flight. With wings beating up to 200 times per second, flying insects can produce a prodigious amount of heat.

Some insects, such as butterflies and dragonflies, warm up by orienting their bodies and spreading their wings to the sun. Most warm up by shivering their flight muscles in the thorax. Moths and butterflies vibrate their wings to raise thoracic temperatures above ambient temperatures. Bumblebees pump their abdomens without any external wing movements. They do not maintain any physiological set point, and they cool down to ambient temperatures when not in flight.

To reduce metabolic costs during periods of inactivity, some small homeothermic animals become heterothermic and enter into torpor daily. Daily torpor is the dropping of body temperature to approximately ambient temperature for a part of each day, regardless of season.

Some birds, such as the common poorwill (Phalaenoptilus nuttallii) and hummingbirds (Trochilidae), and small mammals, such as bats, pocket mice, kangaroo mice, and white-footed mice, undergo daily torpor. Such daily torpor seems to have evolved as a way to reduce energy demands over that part of the day when the animals are inactive, allowing them to save the energy that would otherwise be used to maintain a high (normal) body temperature. Nocturnal mammals, such as bats, go into torpor by day; and diurnal animals, such as hummingbirds, go into torpor by night. As the animal goes into torpor, its body temperature falls steeply and oxygen consumption drops (Figure 7.22). With the relaxation of homeothermic responses, the body temperature declines to within a few degrees of ambient temperature. Arousal returns the body temperature to normal rapidly as the animal renews its metabolic heat production.

Some birds, such as the common poorwill (Phalaenoptilus nuttallii) and hummingbirds (Trochilidae), and small mammals, such as bats, pocket mice, kangaroo mice, and white-footed mice, undergo daily torpor. Such daily torpor seems to have evolved as a way to reduce energy demands over that part of the day when the animals are inactive, allowing them to save the energy that would otherwise be used to maintain a high (normal) body temperature. Nocturnal mammals, such as bats, go into torpor by day; and diurnal animals, such as hummingbirds, go into torpor by night. As the animal goes into torpor, its body temperature falls steeply and oxygen consumption drops (Figure 7.22). With the relaxation of homeothermic responses, the body temperature declines to within a few degrees of ambient temperature. Arousal returns the body temperature to normal rapidly as the animal renews its metabolic heat production.

To escape the rigors of long, cold winters, some heterothermic mammals go into a long, seasonal torpor called hibernation. Hibernation is characterized by the cessation of activity and controlled hypothermia (reduction of body temperature). Homeothermic regulation is relaxed, and the body temperature is allowed to approach ambient temperature. Heart rate, respiration, and total metabolism fall, and body temperature sinks below 10°C. Associated with hibernation are high blood levels of carbon dioxide and an associated decrease in blood pH (increased acidity). This state, called acidosis, lowers the threshold for shivering and reduces the metabolic rate. Hibernating homeotherms, however, are able to rewarm spontaneously using only metabolically generated heat.

Among homeotherms, entrance into hibernation is a controlled process difficult to generalize from one species to another. Some hibernators, such as the groundhog (Marmota monax), feed heavily in late summer to store large fat reserves from which they will draw energy during hibernation. Others, like the chipmunk (Tamias striatus), lay up a store of food instead. All hibernators, however, convert to a means of metabolic regulation different from that of the active state. Most hibernators rouse periodically and then drop back into torpor. The chipmunk, with its large store of seeds, spends much less time in torpor than do species that store large amounts of fat.

Although popularly said to hibernate, black bears (Ursus americanus), grizzly bears (Ursus arctos), and female polar bears (Ursus maritimus) do not. Instead, they enter a unique winter sleep from which they easily rouse. They do not enter extreme hypothermia but allow body temperatures to decline only a few degrees below normal. The bears do not eat, drink, urinate, or defecate, and females give birth to and nurse young during their sleep; yet they maintain a metabolism that is near normal. To do so, the bears recycle urea, normally excreted in urine, through the bloodstream. The urea is degraded into amino acids that are reincorporated in plasma proteins.

Hibernation provides selective advantages to small homeotherms. For them, the maintenance of high body temperature during periods of cold and limited food supply is too costly. It is far less expensive to reduce metabolism and allow the body temperature to drop. Doing so eliminates the need to seek scarce food resources to maintain higher body temperatures.

7.13 Some Animals Use Unique Physiological Means for Thermal Balance

Because of an animal’s limited tolerance for heat, storing body heat does not seem like a sound option to maintain thermal balance in the body. But certain mammals, especially the camel, oryx, and some gazelles, do just that. The camel, for example, stores body heat by day and dissipates it by night, especially when water is limited. Its temperature can fluctuate from 34°C in the morning to 41°C by late afternoon. By storing body heat, these animals of dry habitats reduce the need for evaporative cooling and thus reduce water loss and food requirements.

Many ectothermic animals of temperate and Arctic regions withstand long periods of below-freezing temperatures in winter through supercooling and developing a resistance to freezing. Supercooling of body fluids takes place when the body temperature falls below the freezing point without actually freezing. The presence of certain solutes in the body that function to lower the freezing point of water influences the amount of supercooling that can take place (see Chapter  3). Some Arctic marine fish, certain insects of temperate and cold climates, and reptiles exposed to occasional cold nights employ supercooling by increasing solutes, notably glycerol, in body fluids. Glycerol protects against freezing damage, increasing the degree of supercooling. Wood frogs (Rana sylvatica), spring peepers (Hyla crucifer), and gray tree frogs (Hyla versicolor) can successfully overwinter just beneath the leaf litter because they accumulate glycerol in their body fluids.

Some intertidal invertebrates of high latitudes and certain aquatic insects survive the cold by freezing and then thawing out when the temperature moderates. In some species, more than 90 percent of the body fluids may freeze, and the remaining fluids contain highly concentrated solutes. Ice forms outside the shrunken cells, and muscles and organs are distorted. After thawing, they quickly regain normal shape.

To conserve heat in a cold environment and to cool vital parts of the body under heat stress, countercurrent heat exchange has evolved in some animals (Figure 7.23). For example, the porpoise (Phocaena spp.), swimming in cold Arctic waters, is well insulated with blubber. It could experience an excessive loss of body heat, however, through its uninsulated flukes and flippers. The porpoise maintains its body core temperature by exchanging heat between arterial (coming from the lungs) and venous (returning to the lungs) blood in these structures (see Figure 7.23). Veins completely surround arteries, which carry warm blood from the heart to the extremities. Warm arterial blood loses its heat to the cool venous blood returning to the body core. As a result, little body heat passes to the environment. Blood entering the flippers cools, whereas blood returning to the deep body warms. In warm waters, where the animals need to get rid of excessive body heat, blood bypasses the heat exchangers. Venous blood returns unwarmed through veins close to the skin’s surface to cool the body core. Such vascular arrangements are common in the legs of mammals and birds as well as in the tails of rodents, especially the beaver (Castor canadensis).

Many animals have arteries and veins divided into small, parallel, intermingling vessels that form a discrete vascular bundle or net known as a rete. In a rete, countercurrent heat exchange occurs as blood flows in opposite directions. Countercurrent heat exchange can also keep heat out. The oryx (Oryx beisa), an African desert antelope exposed to high daytime temperatures, experiences elevated body temperatures yet keeps the highly heat-sensitive brain cool by a rete in its head. The external carotid artery passes through a cavernous sinus filled with venous blood that is cooled by evaporation from the moist mucous membranes of the nasal passages (Figure 7.24). Arterial blood passing through the cavernous sinus cools on the way to the brain, lowering the temperature of the brain by 2°C to 3°C compared to the body core.

Countercurrent heat exchangers are not restricted to homeotherms. Certain poikilotherms that assume some degree of endothermism employ the same mechanism. The swift, highly predaceous tuna (Thunnus spp.) and the mackerel shark (Isurus tigris) possess a rete in a band of dark muscle tissue used for sustained swimming effort. Metabolic heat produced in the muscle warms the venous blood, which gives up heat to the adjoining newly oxygenated blood returning from the gills. Such a countercurrent heat exchange increases the power of the muscles because warm muscles contract and relax more rapidly. Sharks and tuna maintain fairly constant body temperatures, regardless of water temperatures.

7.14 An Animal’s Habitat Reflects a Wide Variety of Adaptations to the Environment

One of the most fundamental factors defining the relationship between an organism and the environment is the place where it is found, its habitat. The millions of species that inhabit our planet are not found everywhere, nor are they distributed at random across Earth’s environments. There is a correspondence between the different species of organisms and the environments in which they are found. Each species on our planet occupies a unique geographic area where its members live and reproduce. So why are some species found in one location—one habitat—but not in another?

The most fundamental constraint on the distribution of species is the ability of the environment to provide essential resources and environmental conditions capable of sustaining basic life processes. For a species to persist in a given location, it must have the physiological potential to survive and reproduce in that habitat. As we have seen, physical and chemical (abiotic) features of the environment, such as pH, temperature, or salinity have a direct influence on basic physiological processes necessary to sustain life. Because physiological processes proceed at different rates under different conditions, any one organism has only a limited range of conditions under which it can survive. For a species to succeed in a given location, however, it must do more than survive. A species’ habitat must provide the wide array of resources necessary to sustain growth and reproduction. The environment must provide essential food resources, cover from potential predators, areas for successful reproduction (courtship, mating, nesting, etc.), and a substrate for a wide array of life activities. As such, a species’ habitat is a reflection of the wide variety of adaptations relating to these processes (e.g., feeding and mating behaviors). For plants and sessile animal species, there is only the hope that gametes or individuals dispersed by a variety of means (wind, water, animals, etc.) arrive at a site that is suitable for successful establishment. In mobile animal species, however, individuals actively choose a specific location to inhabit. The process of selecting a specific location to inhabit is called habitat selection.

Given the importance of habitat selection on an organism’s fitness, how are organisms able to assess the suitability of an area in which to settle? What do they seek in a living place? Such questions have been intriguing ecologists for many years. Habitat selection has been most widely studied in birds, particularly in species that defend breeding territories—areas of habitat that the individual defends against other individuals and in which it carries out its life activities (feeding, mating, and rearing of offspring). (See Section 11.10 for discussion of territoriality.) The advantage of studying habitat selection in territorial species is that territories can be delineated, and the features of the habitat within the territory can be described and contrasted with the surrounding environment.

Of particular importance is the contrast between those areas that have been chosen as habitats and adjacent areas that have not. Using this approach, a wide variety of studies examining the process of habitat selection in birds has demonstrated a strong correlation between the selection of an area as habitat and structural features of the vegetation. These studies suggest that habitat selection most likely involves a hierarchical approach. Birds appear initially to assess the general features of the landscape: the type of terrain; presence of lakes, ponds, streams, and wetland; gross features of vegetation such as open grassland, shrubby areas, and type and extent of forest. Once in a broad general area, the birds respond to more specific features, such as the structural configuration of the vegetation, particularly the presence or absence of various vertical layers such as shrubs, small trees, tall canopy, and degree of patchiness (Figure 7.25). Frances James, an avian ecologist at Florida State University, coined the term niche gestalt to describe the vegetation profile associated with the breeding territory of a particular species.

In addition to the physical structure of the vegetation, the actual plant species present can be important. Certain species of plants might produce preferred food items, such as seeds or fruits, or influence the type and quantity of insects available as food for insectivorous birds.

The structural features of the vegetation that define its suitability for a given species may be related to a variety of specific needs, such as food, cover, and nesting sites. The lack of an adequate nesting site may prevent an individual from occupying an otherwise suitable habitat. Animals require sufficient shelter to protect themselves and young from enemies and adverse weather conditions. Cavity-nesting animals require suitable dead trees or other structures in which they can access cavities (Figure 7.26).

Habitat selection is a common behavioral characteristic of a wide array of vertebrates other than birds; fish, amphibians, reptiles, and mammals furnish numerous examples. Garter snakes (Thamnophis elegans) living along the shores of Eagle Lake in the sagebrush-ponderosa pine ecosystems of northeastern California select rocks of intermediate thickness (20–30 cm) over thinner and thicker rocks as their retreat sites. Shelter under thin rocks becomes lethally hot; shelter under thicker rocks does not allow the snakes to warm their bodies to preferred temperatures (see Figure 7.17). Insects, too, cue in on habitat features. Thomas Whitham of the University of Utah studied habitat selection by the gall-forming aphid Pemphigus betae, which parasitizes the narrow-leaf cottonwood (Populus angustifolia). He found that aphids select the largest leaves to colonize and discriminate against small leaves. Beyond that, they select the best positions on the leaf. Occupying this particular habitat, which provides the best food source, produces individuals with the highest fitness.

Ecological Issues & Applications Increasing Global Temperature Is Affecting the Body Size of Animals

Body size is one of the most important phenotypic traits of animals, influencing virtually all physiological and ecological processes (Section 7.1). Variation in body size, both geographically and through time, is a common phenomenon and assumed to be a product of adaptation through natural selection (see Section 5.6). For example, temperatures have a direct effect on an animal’s energy balance, and the relationship between body size and heat exchange (see Section 7.11) is the basis for Bergmann’s rule. Bergmann’s rule states that for endotherms, body size for a species tends to increase with decreasing mean annual temperature. The result is a cline in body size with latitude (for discussion of clines, see Section 5.8): increasing body size with increasing latitude. Similar changes in body size in response to temperature have been observed over time. For example, Ross Secord of the University of Nebraska and colleagues examined shifts in body size in the earliest known horses (family Equidae) during the Paleocene-Eocene Thermal Maximum (PETM) approximately 56 million years ago. A high-resolution record of continental climate and body size of fossil horses shows a directional body size decrease of approximately 30 percent over the first 130,000 years of the PETM (a period of warming), followed by a 76 percent increase in the recovery phase of the PETM as temperatures cooled.

Given the strong selective (evolutionary) influence that temperature has on body size, could patterns of recent climate warming over the past century (0.6°C increase in mean global temperature; see discussion in Chapter 2, Ecological Issues & Applications) as a result of human activities have influenced the body size of animals? Numerous studies have documented recent changes in animal body size for local populations over the timescale of decades to a century that are correlated to changes in local temperature. For example, Celine Teplitsky of the University of Helsinki (Finland) and colleagues examined data on mean body mass of red-billed gulls (Larus novaehollandiae scopulinus) from New Zealand over a 47-year period (1958–2004). Results of their analyses show that mean body mass had decreased over time as ambient temperatures increased (Figure  7.27). Similar patterns have been observed from a wide array of bird and mammal species. Some of the most pronounced changes have occurred in animal species that inhabit the northern latitudes, where the largest changes in temperature have occurred over recent decades (see Chapter  2, Ecological Issues & Applications). Yoram Yom-Tov of Tel Aviv University (Israel) and colleagues examined variations in body size of the stone marten (Martes foina) collected in Denmark between 1858 and 1999. Analyses show that skull size (and by implication body size) had two periods of decrease and that these two periods coincided with the periods of increase in ambient temperature (of 0.7°C and 0.55°C, respectively). The changes in temperature for the region of Denmark during these periods are equivalent to the observed global rise in mean global temperature during the 20th century (approximately 0.6°C).

Although numerous studies have found evidence of decreasing body size with increasing temperatures across various species of endotherms—a pattern consistent with the hypothesis that smaller body size is more energetically efficient under a warmer climate—other studies have observed increases in body size with rising temperatures. For example, in contrast to the pattern of decreasing body size in the stone marten that he observed in Denmark, Yoram Yom-Tov observed an increase in average body size with increasing ambient temperature for Eurasian otter (Lutra lutra) collected in Sweden between 1962 and 2008. To understand the apparent discrepancy in these studies, it is important to understand that temperature has direct and indirect effects on animals through a variety of processes other than thermoregulation that may complicate the story. For example, temperature can have a significant influence on both food availability and nutrition. In cold regions, the direct effect of an increase in temperature may be a reduction in the cost of body maintenance, thus enabling animals to divert energy toward growth, which results in an increase in body size. Increases in temperature can also have an effect on the availability of food resources. In the case of the observed increase in body size in the Eurasian otter in Sweden, Yom-Tov found that increasing temperature reduced the length of time of ice cover on freshwater lakes in Sweden, thus increasing the access of otter to food resources (fish and invertebrates). Similarly, elevated plant growth and thus food availability under warmer climate conditions is suggested as an explanation for increase in the body size of the masked shrew (Sorex cinereus) and the American marten (Martes americana) in Alaska, and the weasel (Mustela nivalis) and stoat (Mustela ereminea) in Sweden.

The story that emerges from these and many other studies is that rising global temperatures are having an impact on the performance and fitness of animal species. As we shall see later in Chapter 27, just as with body size, the variety of responses to a warming climate are as diverse and complex as the array of biological processes that respond both directly and indirectly to temperature and other features of the climate system.

Summary

Body Size 7.1

Size has consequences for structural and functional relationships in animals; as such, it is a fundamental constraint on adaptation. For objects of similar shape, the ratio of surface area to volume decreases with size; that is, smaller bodies have a larger surface area relative to their volume than do larger objects of the same shape. The decreasing surface area relative to volume with increasing body size limits the transfer of materials and energy between the organism’s interior and its exterior environment. An array of adaptations function to increase the surface area and enable adequate exchange of energy and materials between the interior cells and the external environment.

Acquisition of Energy and Nutrients 7.2

To acquire energy and nutrients, herbivores consume plants, carnivores consume other animals, and omnivores feed on both plant and animal tissues. Detritivores feed on dead organic matter.

Directly or indirectly, animals get their nutrients from plants. Low concentrations of nutrients in plants can adversely affect the growth, development, and reproduction of plant-eating animals. Herbivores convert plant tissue to animal tissue. Among plant eaters, the quality of food, especially its protein content and digestibility, is crucial. Carnivores must secure a sufficient quantity of nutrients already synthesized from plants and converted into animal flesh.

Conformers and Regulators 7.3

When an animal is confronted with changes in the environment, it can respond in one of two ways: conformity or regulation. In conformers, changes in the external environmental conditions induce internal changes in the body that parallel external conditions. Conformity largely involves changes at the physiological and biochemical level that are simple and energetically inexpensive but are accompanied by reduced activity and growth. Regulators use a variety of biochemical, physiological, morphological, and behavioral mechanisms that regulate their internal environments. Regulation is energetically expensive, but the benefit is a high level of performance and a greatly extended range of environmental conditions over which an activity can be maintained.

Regulation of Internal Conditions 7.4

To confront daily and seasonal environmental changes, organisms must maintain some equilibrium between their internal and external environment. Homeostasis is the maintenance of a relatively constant internal environment in a variable external environment through negative feedback responses. Through various sensory mechanisms, an organism responds physiologically or behaviorally to maintain an optimal internal environment relative to its external environment. Doing so requires an exchange between the internal and external environments.

Oxygen Uptake 7.5

Animals generate energy by breaking down organic compounds primarily through aerobic respiration, which requires oxygen. Differences between terrestrial and aquatic animals in their means of acquiring oxygen reflect the availability of oxygen in the two environments. Most terrestrial animals have some form of lungs, whereas most aquatic animals use gills to transfer gases between the body and the surrounding water.

Water Balance 7.6

Terrestrial animals must offset water loss from evaporation, respiration, and waste excretion by consuming or conserving water. Terrestrial animals gain water by drinking, eating, and producing metabolic water. Animals of arid regions may reduce water loss by becoming nocturnal, producing highly concentrated urine and feces, becoming hyperthermic during the day, using only metabolic water, and tolerating dehydration.

Aquatic animals must prevent the uptake of, or rid themselves of, excess water. Freshwater fish maintain osmotic balance by absorbing and retaining salts in special cells in the body and by producing copious amounts of watery urine. Many marine invertebrates’ body cells maintain the same osmotic pressure as that in seawater. Marine fish secrete excess salt and other ions through kidneys or across gill membranes.

Energy Exchange 7.7

Animals maintain a fairly constant internal body temperature, known as the body core temperature. They use behavioral and physiological means to maintain a heat balance in a variable environment. Layers of muscle fat and surface insulation of scales, feathers, and fur insulate the animal body core against environmental temperature changes. Terrestrial animals face a more dynamic and often more threatening thermal environment than do aquatic animals.

Thermal Regulation 7.8

Animals fall into three major groups relating to temperature regulation: poikilotherms, homeotherms, and heterotherms. Poikilotherms, so named because they have variable body temperatures influenced by ambient temperatures, are ectothermic. Animals that depend on internally produced heat to maintain body temperatures are endothermic. They are called homeotherms because they maintain a rather constant body temperature independent of the environment. Many animals are heterotherms that function either as endotherms or ectotherms, depending on external circumstances.

Poikilotherms 7.9

Poikilotherms gain heat from and lose heat to the environment. Poikilotherms have low metabolic rates and high thermal conductance. Environmental temperatures control their rates of metabolism. Poikilotherms are active only when environmental temperatures are moderate; they are sluggish when temperatures are cool. They have, however, upper and lower limits of tolerable temperatures. Most aquatic poikilotherms maintain no appreciable difference between body temperature and water temperature.

Poikilotherms use behavioral means of regulating body temperature. They exploit variable microclimates by moving into warm, sunny places to heat up and by seeking shaded places to cool off. Many amphibians move in and out of water. Insects and desert reptiles raise and lower their bodies to reduce or increase conductance from the ground or for convective cooling. Desert animals enter shade or spend the heat of day in underground burrows.

Homeotherms 7.10

Homeotherms maintain high internal body temperature through the oxidiziation of glucose and other energy-rich molecules. They have high metabolic rates and low thermal conductance. Body insulation of fat, fur, feathers, scales, and furlike covering on many insects reduces heat loss from the body. A few desert mammals employ heavy fur to keep out desert heat and cold. When insulation fails during the cold, many homeotherms resort to shivering and burning fat reserves. For homeotherms, evaporative cooling by sweating, panting, and wallowing in mud and water is an important way of dissipating body heat.

Trade-offs in Thermal Regulation 7.11

The two approaches to maintaining body temperature, ectothermy and endothermy, involve trade-offs. Unlike poikilotherms, homeotherms are able to remain active regardless of environmental temperatures. For homeotherms, a high rate of aerobic metabolism comes at a high energy cost. This cost places a lower limit on body size. Because of the low metabolic cost of ectothermy, poikilotherms can curtail metabolic activity in times of food and water shortage and temperature extremes. Their low energy demands enable some terrestrial poikilotherms to colonize areas of limited food and water.

Heterotherms 7.12

Based on environmental and physiological conditions, heterotherms take on the characteristics of endotherms or ectotherms. Some normally homeothermic animals become ectothermic and drop their body temperature under certain environmental conditions. Many poikilotherms, notably insects, must increase their metabolic rate to generate heat before they can take flight. Most accomplish this feat by vibrating their wings or wing muscles or by basking in the sun. After flight, their body temperatures drop to ambient temperatures.

During environmental extremes, some animals enter a state of torpor to reduce the high energy costs of staying warm or cool. Their metabolism, heartbeat, and respiration slows, and their body temperature decreases. Hibernation (seasonal torpor during winter) involves a complete rearrangement of metabolic activity so that it runs at a very low level. Heartbeat, breathing, and body temperature are all greatly reduced.

Unique Physiological Means to Maintain Thermal Balance 7.13

Many homeotherms and heterotherms employ countercurrent circulation, the exchange of body heat between arterial and venous blood reaching the extremities. This exchange reduces heat loss through body parts or cools blood flowing to such vital organs as the brain.

Some desert mammals use hyperthermia to reduce the difference between body and environmental temperatures. They store up body heat by day, then release it to the cool desert air by night. Hyperthermia reduces the need for evaporative cooling and thus conserves water. Some cold-tolerant poikilotherms use supercooling, the synthesis of glycerol in body fluids, to resist freezing in winter. Supercooling takes place when the body temperature falls below freezing without freezing body fluids. Some intertidal invertebrates survive the cold by freezing, then thawing with warmer temperatures.

Habitat 7.14

The place where an animal is found is called its habitat. A species’ habitat must provide the wide array of resources necessary to sustain growth and reproduction. The environment must provide essential food resources, cover from potential predators, areas for successful reproduction (courtship, mating, nesting, etc.), and a substrate for a wide array of life activities. In mobile animals, the behavioral process of selecting a location to occupy is called habitat selection.

Climate Change and Body Size Ecological Issues & Applications

Bergmann’s rule states that for endotherms, body size for a species tends to increase with decreasing mean annual temperature. Studies have documented recent changes in animal body size for local populations over the timescale of decades to a century that are correlated to changes in local temperature. Recent global warming has resulted in both increases and decreases in the average size of different animal species. Decreases in body size have been related to the benefit of smaller body size in thermal balance, whereas increases in body size have been associated with increases in food availability under warmer climates.

(Smith 148-149)

Smith, Thomas M. Smith and Robert L. Elements of Ecology, 9th Edition. Pearson Learning Solutions, 06/2016. VitalBook file.

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