32-1 Introduction to the
Mammals
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32-1 Introduction to the Mammals
Mammals
Class Mammalia
All mammals have two notable features: hair and
mammary glands.
In females, mammary glands produce milk to
nourish the young.
In addition to having hair and the ability to nourish
their young with milk, all mammals
•
•
•
breathe air
have four-chambered hearts
are endotherms
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32-1 Introduction to the Mammals
Evolution of Mammals
Evolution of Mammals
Mammalian fossils are characterized by
• a lower jaw with a large, teeth-bearing bone
connected directly to the skull by a joint
• complex teeth that are replaced once in a lifetime
• distinctive features of the limbs and the backbone.
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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32-1 Introduction to the Mammals
Evolution of Mammals
The first true mammals appeared during the late
Triassic Period, about 220 million years ago.
These mammals were very small, resembling the
modern tree shrew.
While dinosaurs were dominate during the
Cretaceous Period, mammals were generally small
and remained out of sight, and were most likely
nocturnal, or active at night.
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32-1 Introduction to the Mammals
Evolution of Mammals
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.
Three major groups of mammals had evolved by the
beginning of the Cenozoic Era, and surviving
members include monotremes, marsupials, and
placental mammals.
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32-1 Introduction to the Mammals
Form and Function in
Mammals
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.
Hair is part of the integumentary system: the outer
coving of the body, the skin and all associated
structures.
Subcutaneous fat, which is a layer of fat located
beneath the skin, also helps conserve body heat.
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32-1 Introduction to the Mammals
Form and Function in
Mammals
Many mammals have sweat glands that help cool the
body and are regulated by an internal negative
feedback mechanism.
•If body temperature is too high, the mammal sweats.
•Evaporation of sweat then cools the body.
•Mammals that lack sweat glands pant to cool down.
The ability of mammals to regulate their body heat
from within is an example of homeostasis.
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32-1 Introduction to the Mammals
Form and Function in
Mammals
Feeding
Because of its high metabolic rate, a mammal must eat
a lot of food to maintain homeostasis (as much as 10X
the amount of a reptile of similar size).
Mammals can be herbivores, carnivores, or omnivores.
Certain whales are filter feeders.
Early mammals ate insects.
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32-1 Introduction to the Mammals
Form and Function in
Mammals
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.
Carnivores’ jaws usually move up and down as they
chew and herbivores’ jaws move side-to-side.
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32-1 Introduction to the Mammals
Form and Function in
Mammals
Canines: Canines are pointed teeth. Carnivores use
them for piercing, gripping, and tearing. In herbivores,
they are reduced or absent.
Canines
Wolf
Horse
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32-1 Introduction to the Mammals
Form and Function in
Mammals
Incisors: Chisel-like incisors are used for cutting,
gnawing, and grooming.
Wolf
Incisors
Horse
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32-1 Introduction to the Mammals
Form and Function in
Mammals
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.
Wolf
Molars and premolars
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Horse
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32-1 Introduction to the Mammals
Form and Function in
Mammals
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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32-1 Introduction to the Mammals
Form and Function in
Mammals
Many herbivores have specialized digestive organs to
break down plant matter.
Cows and their relatives have a stomach chamber
called the rumen, in which swallowed plant food is
stored and processed.
It contains symbiotic bacteria that digest the cellulose
of plant tissues.
After some time, the grazer regurgitates the food into
the mouth and the food is chewed and swallowed
again.
After several such cycles, it moves through the rest of
the stomach to the intestines.
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32-1 Introduction to the Mammals
Form and Function in
Mammals
Respiration
All mammals use lungs to breathe. The lungs are
controlled by two sets of muscles.
A diaphragm is a large, flat muscle at the bottom of
a mammal’s chest cavity that helps in breathing.
When an animal inhales, chest muscles lift the rib
cage up and out, increasing the volume of the chest
cavity. At the same time, the diaphragm pulls the
chest cavity down to further increase the volume.
The increase in volume pulls air into the lungs.
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32-1 Introduction to the Mammals
Form and Function in
Mammals
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.
Lungs contain alveoli: small sacs, site of gas
exchange.
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32-1 Introduction to the Mammals
Form and Function in
Mammals
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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32-1 Introduction to the Mammals
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32-1 Introduction to the Mammals
Form and Function in
Mammals
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.
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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32-1 Introduction to the Mammals
Form and Function in
Mammals
Kidneys also retain salts, sugars, and other
compounds the body cannot afford to lose.
They 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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32-1 Introduction to the Mammals
Form and Function in
Mammals
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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32-1 Introduction to the Mammals
Form and Function in
Mammals
Mammalian
Brain
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32-1 Introduction to the Mammals
Form and Function in
Mammals
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.
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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32-1 Introduction to the Mammals
Form and Function in
Mammals
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 ultrasound.
•
Elephants detect sounds at much lower
frequencies - infrasound.
The ability to distinguish colors varies among species.
Color vision is most useful to animals that are active
during the day, or diurnal.
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32-1 Introduction to the Mammals
Form and Function in
Mammals
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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32-1 Introduction to the Mammals
Form and Function in
Mammals
Fighting Disease
The immune system helps protect animals from
disease, and when they do get sick, it helps them
recover.
Barriers, such as the skin, prevent pathogens from
entering the body.
Specialized cells and chemicals recognize and
destroy pathogens.
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32-1 Introduction to the Mammals
Form and Function in
Mammals
Movement
Mammals have backbones that flex vertically and
side to side. This flexibility allows mammals to move
with a bouncing, leaping stride.
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 including to run, walk, climb, burrow,
hop, pounce, swing, fly, leap, and swim.
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32-1 Introduction to the Mammals
Form and Function in
Mammals
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.
Monkey
Horse
Mole
Bat
Seal
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32-1 Introduction to the Mammals
Form and Function in
Mammals
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.
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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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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