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20.1: Mammalian Traits

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Lesson 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. By raising its fur, this cat is “saying” that it’s big and dangerous. This might discourage a predator from attacking.

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 it’s 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. They cause “goosebumps,” as shown here.

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. That’s 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.

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. 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. Most mammals also 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. How would you classify your own diet?

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.

Mammalian Teeth (Human). With their different types of teeth, mammals can eat a wide range of foods.

Lungs and Heart of Mammals

Keeping the rate of metabolism high takes a constant and plentiful supply of oxygen. That’s 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). That’s 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.

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.

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. It’s 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 pride’s 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. That’s 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.

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.

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.

Lesson 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.

Lesson Review Questions

Recall

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?

Apply Concepts

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

Think Critically

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

8. 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?

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