18'2 Mammals

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18.2 Mammals
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Objectives

• Describe the characteristics of mammals.
• Explain how mammals differ from other animal
  groups.
• Compare and contrast mammals and birds.

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• Look closely at the surface of your forearm.
• What do you see?
• The little hairs poking out of your skin are a
  characteristic shared by none of the animals you
  have studied so far.
• Only one group of animals has true hair.
• They-and you-are the mammals.

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• Like birds, mammals are vertebrates and
  endotherms.
• They also have four-chambered hearts.
• Their hair and mammary glands set mammals apart
  from other animals.

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• Mammary glands, active only in female mammals,
  produce milk to feed offspring.
• The milk is a balanced diet of fats, sugars,
  protein, minerals, and vitamins.

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Origin of Mammals

• Mammals evolved from ancient reptiles, just as
  the birds did.
• The early mammals and birds were both endotherms,
  adapted for a more active life in cold climates
  than their reptile ancestors.
• Otherwise, the first mammals were very different
  from the first birds.

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• While birds evolved feathers as a body covering,
  mammals developed hair.
• Birds evolved wings from their front limbs.
• The limbs of mammals, in contrast, became more
  efficient at moving on land, in water, or in
  trees.

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• The earliest mammals lived during the age of
  dinosaurs.
• Fossils show that they were probably no bigger
  than rats and similar to rats in appearance.
• They were very likely active at night, hiding
  from meat-eating dinosaurs by day.

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• When the dinosaurs died out, however, mammals
  began to evolve rapidly.
• In taking the place of the dinosaurs, they
  developed a variety of shapes, sizes, and
  lifestyles.
• After a while mammals came to live in nearly
  every habitat on the earth.

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Characteristics of Mammals

• The ability to maintain a constant internal
  temperature is a major reason why mammals have
  become so widespread.
• Mammals also have several characteristics
  different from those of birds, the other group of
  endotherrns.

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Hair and Skin

• All mammals have hair at some time in their
  lives.
• For many mammals, hair insulates the body,
  preventing heat loss.

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• For others, however, hair is not important.
• Adult dolphins, for example, have no hair at all.
  
• They rely instead on a layer of fat under the
  skin to keep in body heat.

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• Skin is also an important insulator for land
  mammals that live in cold environments, such as
  polar bears.
• In addition to their heavy coats, polar bears
  have a thick layer of skin that keeps their
  bodies from losing heat.
• For larger mammals living in warm environments,
  the ability to sweat is an important adaptation.
• Sweat evaporates from their bodies, keeping them
  cool.

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Teeth

• Mammals have three different kinds of teeth
  incisors, canines, and molars.
• Incisors are chisel-shaped teeth that are used
  for cutting and gnawing.

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• Canines are long, pointed teeth that can stab
  prey and tear flesh.
• Molars are broad, flat teeth that are good for
  grinding food.
• The kinds of teeth a mammal has depends on the
  food it eats.
• Some mammals with a varied diet have all three
  kinds of teeth and can handle any kind of food.

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Movement

• Mammals are active animals.
• They move to find food and escape enemies.
• All mammals, except dolphins and whales, have two
  pairs of limbs they use to move.

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• The structure of these limbs, however, varies
  considerably.
• Depending on the species, they can be adapted for
  running, swimming, climbing, digging, or flying.
• Some mammals, such as rabbits, move by hopping.

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Body Systems

• As in birds, the organ systems of mammals help
  adapt them to an active life.
• The four-chambered heart efficiently transports
  oxygen to the body cells, supporting the high
  rate of metabolism necessary for endotherms.

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• A mammal's lungs are made up of millions of tiny
  sacs.
• These sacs greatly increase the surface area of
  lung tissue in contact with air.
• As a result, the lungs can transfer more oxygen
  into the blood.

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• Another adaptation for breathing is a muscle in
  the chest called the diaphragm (DY uh FRAM).
• The diaphragm expands and contracts the chest
  cavity, causing air to enter the lungs.

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• Mammals also have complex nervous systems.
• Their brains are large, especially the part
  called the cerebrum (SUR uh bruhm).
• The cerebrum is where the higher brain functions
  occur.

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• With their large cerebrums, mammals are able to
  perform complex behaviors such as learning.
• Most mammals also have well-developed senses.

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End