Biology Lesson 05

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Biology Lesson 13: Mammals and Animal Behavior

You might think that these young tigers are fighting, but theyâre really playing. Like most other young mammals, tigers like to play. Why do mammals play? Is playing just for fun, or does it serve some other purpose as well?

Playing is actually an important way of learning. By playing, these tigers are learning moves that will help them become successful predators as adults. Playing is just one of many ways that mammals and other animals learn how to behave. In this chapter, you will learn more about mammals such as tigers. You will also learn more about animal behavior and other ways that animals learn.

Section 1: Mammalian Traits

Section Objectives

• List characteristics of mammals.
• Describe structure and function in mammals.

Vocabulary

• alveoli (singular, alveolus)
• arboreal
• cerebrum
• lactation
• mammal
• mammary gland
• neocortex

Introduction

Mammals are a class of endothermic vertebrates. They have four limbs and produce amniotic eggs. Examples of mammals include bats, whales, mice, and humans. Clearly, mammals are a very diverse group. Nonetheless, they share many traits that set them apart from other vertebrates.

Characteristics of Mammals

Two characteristics are used to define the mammal class. They are mammary glands and body hair (or fur).

1. Female mammals have mammary glands. The glands produce milk after the birth of offspring. Milk is a nutritious fluid. It contains disease-fighting molecules as well as all the nutrients a baby mammal needs. Producing milk for an offspring is called lactation.

2. Mammals have hair or fur. It insulates the body to help conserve body heat. It can also be used for sensing and communicating. For example, cats use their whiskers to sense their surroundings. They also raise their fur to look larger and more threatening (see Figure below).

Cat Communicating a Warning.

(image in .pdf file)

Most mammals share several other traits. The traits in the following list are typical of, but not necessarily unique to, mammals.

• The skin of many mammals is covered with sweat glands. The glands produce sweat, the salty fluid that helps cool the body.
• Mammalian lungs have millions of tiny air sacs called alveoli. They provide a very large surface area for gas exchange.
• The heart of a mammal consists of four chambers. This makes it more efficient and powerful for delivering oxygenated blood to tissues.
• The brain of a mammal is relatively large and has a covering called the neocortex. This structure plays an important role in many complex brain functions.
• The mammalian middle ear has three tiny bones that carry sound vibrations from the outer to inner ear. The bones give mammals exceptionally good hearing. In other vertebrates, the three bones are part of the jaw and not involved in hearing.
• Mammals have four different types of teeth. The teeth of other vertebrates, in contrast, are all alike.

Structure and Function in Mammals

Many structures and functions in mammals are related to endothermy. Mammals can generate and conserve heat when itâs cold outside. They can also lose heat when they become over-heated. How do mammals control their body temperature in these ways?

How Mammals Stay Warm

Mammals generate heat mainly by keeping their metabolic rate high. The cells of mammals have many more mitochondria than the cells of other animals. The extra mitochondria generate enough energy to keep the rate of metabolism high. Mammals can also generate little bursts of heat by shivering. Shivering occurs when many muscles contract a little bit all at once. Each muscle that contracts produces a small amount of heat.

Conserving heat is also important, especially in small mammals. A small body has a relatively large surface area compared to its overall size. Because heat is lost from the surface of the body, small mammals lose a greater proportion of their body heat than large mammals. Mammals conserve body heat with their hair or fur. It traps a layer of warm air next to the skin. Most mammals can make their hair stand up from the skin, so it becomes an even better insulator (see Figure below). Mammals also have a layer of fat under the skin to help insulate the body. This fatty layer is not found in other vertebrates.

Goosebumps. Mammals raise their hair with tiny muscles in the skin. Even humans automatically contract these muscles when they are cold.

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How Mammals Stay Cool

One way mammals lose excess heat is by increasing blood flow to the skin. This warms the skin so heat can be given off to the environment. Thatâs why you may get flushed, or red in the face, when you exercise on a hot day. You are likely to sweat as well. Sweating also reduces body heat. Sweat wets the skin, and when it evaporates, it cools the body. Evaporation uses energy, and the energy comes from body heat. Animals with fur, such as dogs, use panting instead of sweating to lose body heat (see Figure below). Evaporation of water from the tongue and other moist surfaces of the mouth and throat uses heat and helps cool the body.

Panting Dog. This dog is overheated. It is losing excess body heat by panting.

(image in .pdf file)

Eating and Digesting Food

Maintaining a high metabolic rate takes a lot of energy. The energy must come from food. Therefore, mammals need a nutritious and plentiful diet. The diets of mammals are diverse. Except for leaf litter and wood, almost any kind of organic matter may be eaten by mammals. Most mammals feed on a variety of other species. The few exceptions include koalas, which feed only on eucalyptus plants, and giant pandas, which feed only on bamboo. Types of mammalian diets and examples of mammals that eat them are given in Table below .

Table 20.1 Some mammals are strictly herbivores or strictly carnivores. However, most mammals will eat other foods if necessary. Some mammals are omnivores. They routinely eat a variety of both plant and animal foods. How would you classify your own diet?

Mammalian Diets

Type of Diet

Foods Eaten

Examples of Mammals with this Type of Diet

herbivorous diet: plants

leaves, grasses, shoots, stems, roots, tubers, seeds, nuts, fruits, bark, conifer needles, flowers

rabbit, mouse, sea cow, horse, goat, elephant, zebra, giraffe, deer, elk, hippopotamus, kangaroo, monkey

carnivorous diet: animals

other mammals, birds, reptiles, amphibians, fish, mollusks, worms, insects

aardvark, anteater, whale, hyena, dog, jackal, dolphin, wolf, weasel, seal, walrus, cat, otter, mole

omnivorous diet: plants and animals

any of the foods eaten in herbivorous and carnivorous diets

bear, badger, mongoose, fox, raccoon, human, rat, chimpanzee, pig

Different diets require different types of digestive systems. Mammals that eat a carnivorous diet generally have a relatively simple digestive system. Their food consists mainly of proteins and fats that are easily and quickly digested. Herbivorous mammals, on the other hand, tend to have a more complicated digestive system. Complex plant carbohydrates such as cellulose are more difficult to digest. Some herbivores have more than one stomach. The stomachs store and slowly digest plant foods.

Mammalian teeth are also important for digestion. The four types of teeth are specialized for different feeding functions, as shown in Figure below. Together, the four types of teeth can cut, tear, and grind food. This makes food easier and quicker to digest.

Lungs and Heart of Mammals

Keeping the rate of metabolism high takes a constant and plentiful supply of oxygen. Thatâs because cellular respiration, which produces energy, requires oxygen. The lungs and heart of mammals are adapted to meet their oxygen needs.

The lungs of mammals are unique in having alveoli. These are tiny, sac-like structures. Each alveolus is surrounded by a network of very small blood vessels (see Figure below). Because there are millions of alveoli in each lung, they greatly increase the surface area for gas exchange between the lungs and bloodstream. Human lungs, for example, contain about 300 million alveoli. They give the lungs a total surface area for gas exchange of up to 90 square meters (968 square feet). Thatâs about as much surface area as one side of a volleyball court!

Alveoli of Mammalian Lungs. Clusters of alveoli resemble tiny bunches of grapes. They are surrounded by many blood vessels for gas exchange.

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Mammals breathe with the help of a diaphragm. This is the large muscle that extends across the bottom of the chest below the lungs. When the diaphragm contracts, it increases the volume of the chest. This decreases pressure on the lungs and allows air to flow in. When the diaphragm relaxes, it decreases the volume of the chest. This increases pressure on the lungs and forces air out.

The four-chambered mammalian heart can pump blood in two different directions. The right side of the heart pumps blood to the lungs to pick up oxygen. The left side of the heart pumps blood containing oxygen to the rest of the body. Because of the dual pumping action of the heart, all of the blood going to body cells is rich in oxygen.

The Mammalian Brain

Of all vertebrates, mammals have the biggest and most complex brain for their body size (see Figure below). The front part of the brain, called the cerebrum, is especially large in mammals. This part of the brain controls functions such as memory and learning.

Vertebrate Brains. Vertebrate brains come in a range of sizes. Even the brains of mammals show a lot of variation in size. The area of the neocortex is greatest in humans.

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The brains of all mammals have a unique layer of nerve cells covering the cerebrum. This layer is called the neocortex (the pink region of the brains in Figure above). The neocortex plays an important role in many complex brain functions. In some mammals, such as rats, the neocortex is relatively smooth. In other mammals, especially humans, the neocortex has many folds. The folds increase the surface area of the neocortex. The larger this area is, the greater the mental abilities of an animal.

Intelligence of Mammals

Mammals are very intelligent. Of all vertebrates, they are the animals that are most capable of learning. Mammalian offspring are fed and taken care of by their parents for a relatively long time. This gives them plenty of time to learn from their parents. By learning, they can benefit from the experiences of their elders. The ability to learn is the main reason that the large mammalian brain evolved. Itâs also the primary reason for the success of mammals.

Social Living in Mammals

Many mammals live in social groups. Social living evolved because it is adaptive. Consider these two examples:

1. Herbivores such as zebras and elephants live in herds. Adults in the herd surround and protect the young, who are most vulnerable to predators.

2. Lions live in social groups called prides. Adult females in the pride hunt cooperatively, which is more efficient than hunting alone. Then they share the food with the rest of the pride. For their part, adult males defend the prideâs territory from other predators.

Locomotion in Mammals

Mammals are noted for the many ways they can move about. Generally, their limbs are very mobile. Often, they can be rotated. Many mammals are also known for their speed. The fastest land animal is a predatory mammal. Can you guess what it is? Racing at speeds of up to 112 kilometers (70 miles) per hour, the cheetah wins hands down. In addition, the limbs of mammals let them hold their body up above the ground. Thatâs because the limbs are attached beneath the body, rather than at the sides as in reptiles (see Figure below).

Limb Positions in Reptiles and Mammals. The sprawling limbs of a reptile keep it low to the ground. A mammal has a more upright stance.

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Mammals may have limbs that are specialized for a particular way of moving. They may be specialized for running, jumping, climbing, flying, or swimming. Mammals with these different modes of locomotion are pictured in Figure below.

Mammalian Locomotion. Mammals have many different modes of locomotion.

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The deer in the Figure above is specialized for running. Why? It has long legs and hard hooves. Can you see why the other animals in the figure are specialized for their particular habitats? Notice how arboreal, or tree-living animals, have a variety of different specializations for moving in trees. For example, they may have:

• A prehensile, or grasping, tail. This is used for climbing and hanging from branches.
• Very long arms for swinging from branch to branch. This way of moving is called brachiation.
• Sticky pads on their fingers. The pads help them cling to tree trunks and branches.

Section Summary

• Mammals are a class of endothermic vertebrates. They have four limbs and produce amniotic eggs. The mammal class is defined by the presence of mammary glands and hair (or fur). Other traits of mammals include sweat glands in their skin, alveoli in their lungs, a four-chambered heart, and a brain covering called the neocortex.
• Mammals have several ways of generating and conserving heat, such as a high metabolic rate and hair to trap heat. They also have several ways to stay cool, including sweating or panting. Mammals may be herbivores, carnivores, or omnivores. They have four types of teeth, so they can eat a wide range of foods. Traits of the heart and lungs keep the cells of mammals well supplied with oxygen and nutrients.
• Mammals have a relatively large brain and a high level of intelligence. They also have many ways of moving about and may move very quickly.

Extra Practice

1. A certain mammal has very long forelimbs. What does that suggest about where the animal lives and how it moves?

2. Explain how mammalian teeth differ from the teeth of other vertebrates. How are mammalian teeth related to endothermy?

3. Compare and contrast the mammalian brain with the brains of other vertebrates. How is the brain of mammals related to their ability to learn?

Points to Consider

Most mammals are born as live young, as opposed to hatching from eggs. Giving birth to live young has certain advantages over egg laying.

• What do you think the advantages of live births might be? How might this form of reproduction help ensure that the offspring survive?
• Do you think that giving birth to live young, as opposed to laying eggs, might have disadvantages? What might the disadvantages be?

Section 2: Reproduction in Mammals

Section Objectives

• Describe female reproductive structures of therian mammals.
• Outline reproduction in placental mammals.
• Explain how marsupials reproduce.
• Describe monotreme reproduction.

Vocabulary

• marsupial
• monotreme
• placenta
• placental mammal
• therian mammal
• uterus (plural, uteri)
• vagina

Introduction

Most mammals are viviparous. Their young are born live. They are born either as relatively large, well-developed fetuses or as tiny, immature embryos. Mammals that are viviparous are called therian mammals. Only a few mammals lay eggs instead of giving birth to an infant or embryo.

Therian Mammals

Like other female vertebrates, all female mammals have ovaries. These are the organs that produce eggs (see Figure below). Therian mammals also have two additional female reproductive structures that are not found in other vertebrates. They are the uterus and vagina.

• The uterus (plural, uteri) is a pouch-like, muscular organ. The embryo or fetus develops inside the uterus. Muscular contractions of the uterus push the offspring out during birth.
• The vagina is a tubular passageway through which the embryo or fetus leaves the motherâs body during birth. The vagina is also where the male deposits sperm during mating.

Female Reproductive System of a Therian Mammal (Human). The female reproductive system of all therian mammals is similar to that of humans.

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Therian mammals are divided into two groups: placental mammals and marsupial mammals. Each group has a somewhat different reproductive strategy.

Placental Mammals

Placental mammals are therian mammals in which a placenta develops during pregnancy. The placenta sustains the fetus while it grows inside the motherâs uterus. Placental mammals give birth to relatively large and mature infants. Most mammals are placental mammals.

The Placenta

The placenta is a spongy structure. It consists of membranes and blood vessels from both mother and embryo (see Figure below). The placenta passes oxygen, nutrients, and other useful substances from the mother to the fetus. It also passes carbon dioxide and other wastes from the fetus to the mother. The placenta lets blood from the fetus and mother exchange substances without actually mixing. Thus, it protects the fetus from being attacked by the motherâs immune system as a âforeign parasite.â

Placenta of a Placental Mammal (Human). The placenta allows the exchange of gases, nutrients, and other substances between the fetus and mother.

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Pros and Cons of Placental Reproduction

The placenta permits a long period of fetal growth in the uterus. As a result, the fetus can become large and mature before birth. This increases its chances of surviving.

On the other hand, supporting a growing fetus is very draining and risky for the mother. The mother has to eat more food to nourish the fetus. She also becomes heavier and less mobile as the fetus gets larger. As a result, she may be less able to escape from predators. Because the fetus is inside her, she canât abandon it to save her own life if she is pursued or if food is scarce. Giving birth to a large infant is also risky. It may even result in the motherâs death.

Marsupials

Marsupials have a different way of reproducing that reduces the motherâs risks. A marsupial is a therian mammal in which the embryo is born at an early, immature stage. The embryo completes its development outside the motherâs body in a pouch on her belly. Only a minority of therian mammals are marsupials. They live mainly in Australia. Examples of marsupials are pictured in Figurebelow.

Marsupials. Marsupials include the kangaroo, koala, and opossum.

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The Marsupial Embryo

The marsupial embryo is nourished inside the uterus with food from a yolk sac instead of through a placenta. The yolk sac stores enough food for the short period of time the embryo remains in the uterus. After the embryo is born, it moves into the motherâs pouch, where it clings to a nipple (see Figure below). It remains inside the pouch for several months while it continues to grow and develop. Even after the offspring is big enough to leave the pouch, it may often return to the pouch for warmth and nourishment. Eventually, the offspring is mature enough to remain outside the pouch on its own.

Marsupial Embryo in the Pouch. A kangaroo embryo suckles a nipple inside its mother.

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Pros and Cons of Marsupial Reproduction

In marsupials, the short period of development within the motherâs uterus reduces the risk of her immune system attacking the embryo. In addition, the marsupial mother doesnât have to eat extra food or carry a large fetus inside her. The risks of giving birth to a large fetus are also avoided. Another pro is that the mother can expel the embryo from her pouch if she is pursued by a predator or if food is scarce. On the other hand, a newborn marsupial is tiny and fragile. Therefore, it may be less likely to survive than a newborn placental mammal.

Monotremes

Only five living species of mammals are not therian mammals. They are called monotremes. Monotremes are mammals that reproduce by laying eggs. The only living monotreme species are the platypus and echidnas (see Figure below and Figure below). They are found solely in Australia and New Guinea (an island not far from Australia).

Platypus. The platypus is a monotreme, a mammal that reproduces by laying eggs.

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Echidna. Like the platypus, the echnida is a monotreme. The only living monotreme species inhabit Australia and New Guinea.

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Eggs and Lactation in Monotremes

Female monotremes lack a uterus and vagina. Instead, they have a cloaca with one opening, like the cloacas of reptiles and birds. The opening is used to excrete wastes as well as lay eggs.

Monotreme eggs have a leathery shell, like the eggs of reptiles. The eggs are retained inside the motherâs body for at least a couple of weeks. During that time, the mother provides the eggs with nutrients. Platypus females lay their eggs in a burrow. Echidna females have a pouch in which they store their eggs. Female monotremes have mammary glands but lack nipples. Instead, they âsweatâ milk from a patch on their belly.

Pros and Cons of Monotreme Reproduction

The motherâs risks are less in monotremes than in therian mammals. The mother doesnât need to eat more or put herself at risk by carrying and delivering a fetus or an embryo. On the other hand, externally laid eggs are more difficult to protect than an embryo in a pouch or a fetus in a uterus. Therefore, monotreme offspring may be less likely to survive than the offspring of therian mammals.

Section Summary

• Therian mammals are viviparous. They give birth to an embryo or infant rather than laying eggs. The female reproductive system of a therian mammal includes a uterus and a vagina. There are two groups of therian mammals: placental mammals and marsupials.
• Placental mammals give birth to a relatively large and mature fetus. This is possible because they have a placenta to nourish the fetus and protect it from the motherâs immune system. This allows for a long period of growth and development before birth. Because the offspring is relatively large and mature at birth, it has a good chance of surviving. However, carrying and giving birth to a large fetus is risky for the mother. It also requires her to eat more food.
• Marsupials give birth to a tiny, immature embryo. The embryo then continues to grow and develop in a pouch on the motherâs belly. This is less risky for the mother. However, the embryo is fragile, so it may be less likely to survive than the fetus of a placental mammal.
• Monotremes reproduce by laying eggs. They have a cloaca instead of a uterus and vagina. The eggs pass through the opening of the cloaca. This form of reproduction is the least risky for the mother. However, eggs are harder to protect than is an embryo or a fetus in a pouch or uterus. Therefore, monotreme offspring may have a lower chance of surviving than the offspring of therian mammals.

Points to Consider

Monotremes are less similar to therian mammals than the two groups of therian mammals are to each other.

• How might the different groups of mammals have evolved?
• Which group of mammals do you think evolved first?

Section 3: Evolution and Classification of Mammals

Section Objectives

• Describe the therapsid ancestors of mammals.
• Outline the evolution of monotreme, marsupial, and placental mammals.
• Summarize the evolution of modern mammals.
• Contrast traditional and phylogenetic classifications of mammals.

Vocabulary

• therapsid

Introduction

Which mammalian trait evolved first? What was the first mammal like? When did the earliest mammal live? Detailed answers to these questions are still in dispute. However, scientists generally agree on the major events in the evolution of mammals. These are summarized in Table below. Refer back to the table as you read about the events in this Section.

Table 20.2. This table shows part of the geologic time scale. It includes only events related to the evolution of mammals. *mya = millions of years ago

Major Events in Mammalian Evolution

Era

Period

Epoch

Major Events

Start (mya)*

Cenozoic

Neogene

Holocene

Rise of human civilization; spread and dominance of modern humans

0.01

-

-

Pleistocene

Spread and then extinction of many large mammals; appearance of modern humans

1.8

-

-

Pliocene

Appearance of many existing genera of mammals, including the genus Homo

5.3

-

-

Miocene

Appearance of remaining modern mammal families; diversification of horses and mastodons; first apes

23.0

-

Paleogene

Oligocene

Rapid evolution and diversification of placental mammals

33.9

-

-

Eocene

Appearance of several modern mammal families; diversification of primitive whales

55.8

-

-

Paleocene

Appearance of the first large mammals

65.5

Mesozoic

Cretaceous

-

Emergence of monotreme, marsupial, and placental mammals; possible first appearance of four clades (superorders) of placental mammals (Afrotheria, Xenarthra, Laurasiatheria, Supraprimates)

145.5

-

Jurassic

-

Spread of mammals, which remain small in size

199.6

-

Triassic

-

Evolution of cynodonts to become smaller and more mammal-like; appearance of the first mammals

251.0

Paleozoic

Permian

-

Evolution and spread of synapsids (pelycosaurs and therapsids)

299.0

-

Carboniferous

-

Appearance of amniotes, the first fully terrestrial vertebrates

359.0

Mammalian Ancestors

Ancestors of mammals evolved close to 300 million years ago. They were amniotes called synapsids. Figure below shows how modern mammals evolved from synapsids. The stages of evolution from synapsids to mammals are described below.

Phylogeny of Mammalian Evolution. This diagram represents the evolution of mammals.

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Pelycosaurs

Synapsids called pelycosaurs became the most common land vertebrates during the first half of the Permian Period. A pelycosaur genus called Dimetrodon is shown in Figure below. Dimetrodon had sprawling legs and walked like a lizard. It also had a fairly small brain. However, it had started to develop some of the traits of mammals. For example, it had teeth of different types.

Pelycosaur Synapsid: Dimetrodon. Dimetrodon was a pelycosaur. It lived about 275 million years ago.

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Therapsids

Some pelycosaurs gave rise to a group of animals called therapsids. The earliest therapsids lived about 260 million years ago. At first, the therapsids looked a lot like Dimetrodon. But after a while, they could easily be mistaken for mammals. They evolved a number of mammalian traits, such as legs positioned under the body instead of along the sides. Therapsids became the most common and diverse land vertebrates during the second half of the Permian Period.

The Permian Period ended about 250 million years ago with a mass extinction. Most therapsids went extinct. Their niches were taken over by sauropsids. These were the amniotes that evolved into dinosaurs, reptiles, and birds. Not all therapsids went extinct, however. The few that remained no longer had to compete with many other therapsids. Some of them eventually evolved into mammals.

Cynodonts

The surviving therapsids were small animals. Some of the most successful were the cynodonts (see Figure below). They flourished worldwide during the first half of the Triassic Period. Some of them ate insects and were nocturnal, or active at night. Being nocturnal may have helped save them from extinction. Why? A nocturnal niche was one of the few niches that dinosaurs did not take over in the Triassic Period.

Cynodonts became more mammal-like as they continued to evolve. Some of their mammalian traits may have been adaptations to their nocturnal niche. For example:

• The ability to regulate body temperature might have been selected for because it would allow nocturnal animals to remain active in the cool of the night.
• A good sense of hearing might have been selected for because it would be more useful than good vision when hunting in the dark.

Probable Mammalian Ancestor: Cynodont. Cynodonts were mammal-like therapsids. They may have been ancestral to mammals. They were about the size of a rat.

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By the end of the Triassic Period, cynodonts had become even smaller in size. They also had evolved many mammalian traits. For example, they had

• Four different types of teeth.
• A relatively large brain.
• Three tiny bones in the middle ear.
• A diaphragm for breathing.
• Endothermy.
• Lactation.
• Hair.

Cynodonts probably gave rise to mammals about 200 million years ago. However, they are not considered to be mammals themselves. In fact, competition with early mammals may have led to their extinction. They went extinct sometime during the Jurassic or Cretaceous Period.

Evolution of Early Mammals

The earliest mammals evolved from cynodonts. But the evolution of mammals didnât end there. Mammals continued to evolve. Monotreme mammals probably split off from other mammals first. They were followed by marsupials. Placental mammals probably evolved last.

Evolution of Monotremes

The first monotremes may have evolved about 150 million years ago. Early monotreme fossils have been found in Australia. An example is a genus called Steropodon, shown in Figure below. It may have been the ancestor of the platypus. Early monotremes retained some of the traits of their therapsid ancestors. For example, they laid eggs and had a cloaca. These traits are still found in modern monotremes.

Probable Monotreme Ancestor: Steropodon. Like the platypus, Steropodon probably had a bill.

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Evolution of Marsupials

The first marsupials may have evolved about 130 million years ago. One of the earliest was the extinct genus Sinodelphys. A fossil of this mammal is shown in Figure below. It is a remarkable fossil find. It represents a nearly complete animal. Even tufts of hair and imprints of soft tissues were preserved.

Early Marsupial: Sinodelphys. The dark shapes on these two rock slabs are two halves of the fossil named Sinodelphys. The head is at the top of the image. The legs point toward the center.

(image in .pdf file)

Sinodelphys was about 15 centimeters (6 inches) long. Its limb structure suggests that it was a climbing animal. It could escape from predators by climbing into trees. It probably lived on a diet of insects and worms.

Evolution of Placental Mammals

The earliest placental mammals may have evolved about 110 million years ago. The ancestor of placental mammals may be the extinct genus Eomaia. Fossils of Eomaia have been found in what is now China. It was only about 10 centimeters (4 inches) long. It was a tree climber and probably ate insects and worms. Eomaia had several traits of placental mammals. Figure below shows how Eomaia may have looked.

Probable Ancestor of Placental Mammals: Eomaia. Eomaia lived a little over 100 million years ago.

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The placental mammal descendants of Eomaia were generally more successful than marsupials and monotremes. On most continents, placental mammals became the dominant mammals, while marsupials and monotremes died out. Marsupials remained the most common and diverse mammals in Australia. The reason for their success there is not yet resolved.

Evolution of Modern Mammals

The Cretaceous Period ended with another mass extinction. This occurred about 65 million years ago. All of the dinosaurs went extinct at that time. Did the extinction of the dinosaurs allow mammals to take over?

Traditional View

Scientists have long assumed that the extinction of the dinosaurs opened up many niches for mammals to exploit. Presumably, this led to an explosion of new species of mammals early in Cenozoic Era. Few mammalian fossils from the early Cenozoic have been found to support this theory. Even so, it was still widely accepted until recently.

View from the Mammalian Supertree

In 2007, an international team of scientists compared the DNA of almost all known species of living mammals. They used the data to create a supertree of mammalian evolution. The supertree shows that placental mammals started to diversify as early as 95 million years ago.

What explains the diversification of mammals long before the dinosaurs went extinct? What else was happening at that time? One change was a drop in Earthâs temperature. This may have favored endothermic mammals over ectothermic dinosaurs. Flowering plants were also spreading at that time. They may have provided new and plentiful foods for small mammals or their insect prey.

The supertree also shows that another major diversification of mammals occurred about 50 million years ago. Again, worldwide climate change may have been one reason. This time Earthâs temperature rose. The warmer temperature led to a greater diversity of plants. This would have meant more food for mammals or their prey.

Classification of Placental Mammals

Traditional classifications of mammals are based on similarities in structure and function. Increasingly, mammals are being classified on the basis of molecular similarities.

Traditional Classification

The most widely accepted traditional classification of mammals divides living placental mammals into 17 orders. These orders are shown in Table below. Placental mammals are still commonly placed in these orders. However, this classification is not very useful for studies of mammalian evolution. Thatâs because it groups together some mammals that do not seem to be closely related by descent from a recent common ancestor.

Table 20.3: This classification of mammals was widely accepted for more than 50 years.

Orders of Placental Mammals (Traditional Classification)

Order

Example

Example

Sample Trait

Insectivora

mole

small sharp teeth

Edentata

anteater

few or no teeth

Pholidota

pangolin

large plate-like scales

Chiroptera

bat

digits support membranous wings

Carnivora

coyote

long pointed canine teeth

Rrodentia

mouse

incisor teeth grow continuously

Lagomorpha

rabbit

chisel-like incisor teeth

Perissodactyla

horse

odd-toed hooves

Artiodactyla

deer

even-toed hooves

Cetacea

whale

paddlelike forelimbs

Primates

monkey

five digits on hands and feet

Proboscidea

elephant

tusks

Hyracoidea

hyrax

rubbery pads on feet

Dermoptera

colugo

membrane of skin between legs for gliding

Pinnipedia

seal

feet with fins

Sirenia

manatee

paddle-like tail

Tubulidentata

aarvark

teeth without enamelPhylogenetic Classification

Phylogenetic Classification

The mammalian supertree classifies placental mammals phylogenetically. It groups together mammals that are closely related because they share a recent common ancestor. These groups are not necessarily the same as the traditional groups based on structure and function. The supertree classification places placental mammals in four superorders. The four superorders and some of the mammals in them are:

• Afrotheriaâaardvarks, elephants, manatees.
• Xenarthraâanteaters, sloths, armadillos.
• Laurasiatheriaâbats, whales, hoofed mammals, carnivores.
• Supraprimatesâprimates, rabbits, rodents.

All four superorders appear to have become distinct from one another between 85 and 105 million years ago. The exact relationships among the superorders are still not clear. Revisions in this classification of mammals may occur as new data become available.

Section Summary

• Amniotes called synapsids were the ancestors of mammals. Synapsids named pelycosaurs had some of the traits of mammals by 275 million years ago. Some of them evolved into therapsids, which became widespread during the Permian Period. The few therapsids that survived the Triassic takeover were small, arboreal insect eaters. They were also nocturnal. Being active at night may explain why they survived and evolved still more mammalian traits.
• Monotremes evolved about 150 million years ago. Like modern monotremes, they had a cloaca and laid eggs. Marsupials evolved about 130 million years ago. They were very small and ate insects and worms. Placental mammals evolved about 110 million years ago. They were also small and climbed trees. Placental mammals became the dominant land mammals. Most marsupials and monotremes died out except in Australia.
• Mammals used to be classified on the basis of similarities in structure and function into 17 different orders. Recently, DNA analyses have shown that the traditional orders include mammals that are not closely related. Phylogenetic classification, based on DNA data, groups placental mammals in four superorders. The superorders appear to have become distinct from each other 85â105 million years ago.

Lesson 13 Review Questions

Directions: Answer each of the following questions.

1. List five traits that are shared by all mammals, including the two traits that are used to define the mammal class.

2. Describe how mammals stay warm.

3. What is the function of sweating?

4. Identify mammals that are herbivores, carnivores, and omnivores.

5. What are alveoli? What is their function?

6. What are the functions of the uterus and vagina in therian mammals?

7. What is the placenta? What is its role?

8. Where does a marsupial embryo develop? How is it nourished?

9. Describe eggs and egg laying in monotremes.

10. How does lactation differ in monotremes and therian mammals?

11. What were the synapsids? When were they most widespread?

12. Identify the therapsids. How were they related to mammals?

13. Describe cynodonts. What is their place in the evolution of mammals?

14. Outline the evolution of monotreme, marsupial, and placental mammals.

15. What is the mammalian supertree?

Essay submission: Select 1 Biology topic from this lesson, and submit a 3-5 paragraph essay about the topic. Remember to cite your sources!