Biology Lesson 05

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Biology Lesson 13: Mammals and Animal Behavior
You might think that these young tigers are fighting but theyre 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
Vocabulary
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.
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Most mammals share several other traits. The traits in the following list are typical of but not necessarily unique to mammals.
Structure and Function in Mammals
Many structures and functions in mammals are related to endothermy. Mammals can generate and conserve heat when its 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. Thats 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.
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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. Thats 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). Thats 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. Its 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 prides 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. Thats 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:
Section Summary
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.
Section 2: Reproduction in Mammals
Section Objectives
Vocabulary
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.
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 mothers 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 mothers immune system as a foreign parasite.
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 cant 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 mothers death.
Marsupials
Marsupials have a different way of reproducing that reduces the mothers 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 mothers 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 mothers 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 mothers uterus reduces the risk of her immune system attacking the embryo. In addition the marsupial mother doesnt 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 mothers 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 sweat milk from a patch on their belly.
Pros and Cons of Monotreme Reproduction
The mothers risks are less in monotremes than in therian mammals. The mother doesnt 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
Points to Consider
Monotremes are less similar to therian mammals than the two groups of therian mammals are to each other.
Section 3: Evolution and Classification of Mammals
Section Objectives
Vocabulary
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:
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
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 didnt 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.
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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 Earths 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 Earths 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. Thats 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:
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
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!