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Biology


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32-1 Introduction to the Mammals


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Mammals

What are the characteristics of mammals?


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Mammals

All mammals have two notable features: hair and mammary glands.

In females, mammary glands produce milk to nourish the young.


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Mammals

In addition to having hair and the ability to nourish their young with milk, all mammals

• breathe air.

• have four-chambered hearts.

• are endotherms that generate their body heat internally.


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Evolution of Mammals

When did mammals evolve?


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Mammalian fossils are characterized by a lower jaw with a large, teeth-bearing bone connected directly to the skull by a joint, and distinctive features of the limbs and the backbone.


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Mammals are descended from ancient reptiles.

Ancestors of modern mammals diverged from ancient reptiles during the Carboniferous Period.

For millions of years, various mammal-like reptiles lived alongside other reptile groups.


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The first true mammals appeared during the late Triassic Period, about 220 million years ago.

These mammals were very small and probably nocturnal.


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After the extinction of dinosaurs, mammals underwent an adaptive radiation.

They increased in size and occupied many new niches.

The Cenozoic Era is called the Age of Mammals.


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Form and Function in Mammals


The mammalian body has adapted in varied ways to a great many habitats.


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Body Temperature Control

Mammals are endotherms.

A high rate of metabolism helps mammals generate body heat.

Mammals have external body hair that helps them keep warm.

Subcutaneous fat, which is a layer of fat located beneath the skin, also helps conserve body heat.


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Many mammals have sweat glands that help cool the body.

• If its body temperature gets too high, the mammal sweats.

• Evaporation of sweat then cools the body.

• Some mammals pant to cool down.


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How do mammals maintain homeostasis?


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The ability of mammals to regulate their body heat from within is an example of homeostasis.


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Feeding

Because of its high metabolic rate, a mammal must eat a lot of food to maintain homeostasis.

Some mammals are herbivores. Some mammals are carnivores. Some mammals, including humans, are omnivores.

Certain whales are filter feeders.


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As mammals evolved, the form and function of their jaws and teeth became adapted to eat foods other than insects.

The joint between the skull and lower jaw allowed mammals to evolve larger, more powerful jaw muscles and different ways of chewing.


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Wolf

Molars and premolars

Horse

Jaw joint

Jaw joint

Canines

Incisors

HerbivoreCarnivore

Jaws and Teeth of Mammals


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Canines: Canines are pointed teeth. Carnivores use them for piercing, gripping, and tearing. In herbivores, they are reduced or absent.

Wolf Horse

Canines


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Incisors: Chisel-like incisors are used for cutting, gnawing, and grooming.

Wolf Horse Incisors


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Molars crush and grind food. The ridged shape of the wolf’s molars and premolars allows them to interlock during chewing, like the blades of scissors. The broad, flattened molars and premolars of horses are adapted for grinding tough plants.

Molars and premolarsWolf Horse


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A mammal’s digestive tract breaks down and absorbs the type of food that it eats.

Carnivores have a short intestine because enzymes quickly digest meat.

Herbivores have a longer intestine because tough, fibrous plant tissues take longer to digest.


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Cows have a stomach chamber called the rumen, in which swallowed plant food is stored and processed.

It contains bacteria that digest the cellulose of plant tissues.


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Respiration

All mammals use lungs to breathe.

A diaphragm is a large, flat muscle at the bottom of a mammal’s chest cavity that helps in breathing.


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When an animal inhales, chest muscles lift the rib cage up and out. The diaphragm pulls the chest cavity down.

The combined actions of the chest muscles and diaphragm increase the volume of the chest cavity.

The increase in volume pulls air into the lungs.


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When an animal exhales, chest muscles lower the rib cage. The diaphragm relaxes, and the volume of the chest cavity decreases.

Air is then pushed out of the lungs.


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Circulation

The mammalian circulatory system has two loops and a four-chambered heart.

The right side of the heart receives oxygen-poor blood from the body and pumps it to the lungs.

The left side receives oxygen-rich blood from the lungs and pumps it to the rest of the body.


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Mammalian heart

Right atrium

Right ventricle

Left atrium

Left ventricle

Complete division



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Excretion

Kidneys extract nitrogenous wastes from the blood in the form of urea.

Urea, other wastes, and water combine to form urine.

From the kidneys, urine flows to a urinary bladder, where it is stored until it is eliminated.


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The kidneys of mammals help maintain homeostasis by filtering urea from the blood, as well as by excreting excess water or retaining needed water.


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The kidneys control the amount of water in the body.

This enables mammals to live in many habitats in which they could not otherwise survive.


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Response

Mammals have well-developed brains with three main parts:

• cerebrum—controls thinking and learning

• cerebellum—controls muscular coordination

• medulla oblongata—regulates involuntary body functions


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Mammalian Brain


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The cerebrum has a well-developed outer layer called the cerebral cortex, which is the center of thinking and other complex behaviors.

Some behaviors, such as reading, are possible only with the human cerebral cortex.

Mammals other than humans also exhibit complex behaviors.


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Mammals rely on highly developed senses to detect and respond to stimuli from their external environment.

Many have well-developed senses of smell and hearing.


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All mammalian ears have the same basic parts, but they differ in their ability to detect sound.

• Dogs, bats, and dolphins detect sounds at higher frequencies than humans can.

• Elephants detect sounds at much lower frequencies.


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The ability to distinguish colors varies among species.

Color vision is most useful to animals that are active during the day.


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Chemical Controls

Mammals have endocrine glands that regulate body activities by releasing hormones.

Hormones are substances produced in one part of an organism that affect another part of the same organism.

Hormones are carried by the blood to the organs that they affect.


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Fighting Disease

The immune system helps protect animals from disease.

Barriers, such as the skin, prevent pathogens from entering the body.

Specialized cells and chemicals recognize and destroy pathogens.


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Movement

Mammals have backbones that flex vertically and side to side.

Shoulder and pelvic girdles are streamlined and flexible, permitting both front and hind limbs to move in many ways.

Variations in limb bones and muscles permit a variety of movements.


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Monkey Mole SealBat Horse

The limbs and digits (fingers and toes) of many mammals are adapted to their particular way of life. Note the variety of lengths and shapes of the limb bones. Homologous bones are the same color in all the drawings.


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Reproduction

Mammals reproduce by internal fertilization.

The male deposits sperm inside the reproductive tract of the female, where fertilization occurs.

All newborn mammals feed on their mother’s milk.


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Young mammals need care from one or both parents when they are born and for a long time afterward.

Parental care ensures that young will survive and reproduce.

The duration and intensity of parental care varies among different species.



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Some mammal species, such as lions, live in groups in which the young may be cared for by adults other than the parents.

Group living provides young mammals with complex social interaction among adults and juveniles.


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32-1


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32-1

One characteristic commonly used to determine whether fossils are mammals is

a. subcutaneous fat.

b. mammary glands.

c. a jaw joint that allows movement from side to side.

d. hair or fur.


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32-1

How many chambers do mammalian hearts have?

a. 2

b. 3

c. 4

d. 6


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32-1

Mammals are called endotherms because they

a. produce milk to nourish their young.

b. have well-developed sense organs.

c. have powerful jaws and specialized teeth.

d. generate their body heat internally.


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32-1

The first true mammals appeared in the fossil record

a. soon after the extinction of the dinosaurs.

b. about the same time as the first reptiles.

c. during the Triassic when dinosaurs were the dominant land animals.

d. long after the extinction of the dinosaurs.


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32-1

The duration of parental care in mammals

a. varies among different species.

b. is one to four weeks.

c. is six months to a year.

d. is intermediate compared to reptiles.

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