Chapter 33: Mammals Section 1: Mammals
Mammals • Includes many diverse species that vary greatly in appearance • Range in size from a tiny mouse to a huge elephant • Mammals can be found flying in the air, running along the ground, and swimming in the sea • Although they differ in size and habitats, all members of the class Mammalia share certain characteristics
What Is a Mammal? • Mammals are endothermic animals, which means they are able to generate substantial heat internally • Most species are experts at maintaining a constant body temperature • Mammals use various combinations of fur, hair, and subcutaneous fat to conserve body heat • Many mammals also have sweat glands that help cool the body • Sweat produced by sweat glands evaporates from the skin, lowering body temperature whenever necessary
What Is a Mammal? • With the exception of a few species, all mammals are viviparous • This means that young mammals develop within the mother for a time and then are born alive • Female mammals have mammary glands, which produce milk to nourish the young for some time after they are born • Mammary glands, which give mammals their name, are probably the most important characteristic that scientists use to include an animal in class Mammalia
What Is a Mammal? • Mammals have several kinds of teeth • Combined with their jaws, the teeth of mammals bite, chew, and grind food efficiently • Mammals have well-developed breathing muscles, including a diaphragm • Mammals have a four-chambered heart
One characteristic that unites all mammals is hair. This brown bear and her cubs can sleep through winter’s cold insulated by their thick coats and a layer of fat beneath their skin.
All female mammals nurse their young, feeding them milk they produce in mammary glands. These glands are the source of the class Mammalia’s name. Be they lions or whales, all mammals breathe air. Breathing is easy for land mammals, but sea mammals must return to the surface to breathe.
Evolution of Mammals • The first mammals were very small • By the end of the Cretaceous Period, the mammals had split into three groups • Monotremes • Only three species survive today • Duckbill platypus • Marsupials • Has a pouch in which its young lives for a time • Kangaroo • Placental mammals • Mammals you are most familiar with • Mice, cats, whales, elephants, etc.
Evolution of Mammals • Because the fossil record is incomplete, it is hard to say precisely where and when each of these three groups appeared
The fossil record shows that the first mammals resembled this tree shrew. Tree shrews are omnivores; they eat both plants and animals.
Form and Function in Mammals • Mammals have limbs and organ systems that have evolved many shapes to serve many functions in different environments
Feeding • Carnivorous mammals, such as cats and dogs, have strong, sharp teeth called incisors and canines that are used for biting and ripping flesh from their prey • Carnivores use an up and down chopping movement of their jaws to chew their food • The behavioral and physical characteristics of many mammals allow them to capture prey
Feeding • Herbivorous mammals, from cows to giraffes, eat plants that are tough and require thorough chewing in order to be digested • Herbivorous animals have evolved strong lips and flat edged incisors that grasp and tear this tough vegetation • They chew by moving their jaws from side to side, using flattened molars to grind the plant food into a pulp
Feeding • Despite this efficient chewing, the cellulose that most plant tissues contain is impossible for mammals to digest on their own • The vertebrate digestive system has never evolved the ability to produce enzymes that digest cellulose • To help in the digestion of plant material, many grazing mammals have a chamber in their digestive tract called the rumen, in which newly swallowed plant food is stored and processed for a time
Feeding • The rumen contains thriving colonies of bacteria that produce enzymes needed to break down cellulose • After a certain amount of time, the mammal regurgitates the plant food from the rumen into its mouth • There the partially digested food is again chewed and mixed with saliva • “chewing their cud” • The 2nd time the food is swallowed, it moves through the rest of the digestive tract, where digestion is completed and nutrients are absorbed
Feeding • Some herbivores, such as rabbits, lack a rumen but have a large dead-end sac, or cecum, forming part of their intestines • Many of the same kinds of microorganisms that digest cellulose are found in the cecum • The ancestors of modern humans had a cecum, but over time it has shrunk to the small, sometimes troublesome pouch we call the appendix
A mammal’s teeth provide important clues to its diet. The sharp, pointed teeth of a wolf show that this animal is a carnivore. The large flat grinding teeth of a deer indicate that this animal eats vegetation.
Respiration • All mammals use lungs powered by two sets of muscles • Chest muscles pull air in and push air out by moving the ribs up and down to increase and decrease the size of the chest cavity • When the large muscle known as the diaphragm contracts, it pulls the bottom of the chest cavity downward, further increasing the cavity size and causing air to rush into the lungs • Many mammals are able to use exhaled air to vibrate their vocal cords and produce a variety of sounds, such as a roar, a bark, or even a song
Internal Transport • The mammalian circulatory system is an arrangement of pumps and blood vessels • The main pump, the heart, sends deoxygenated blood to the lungs • After it leaves the lungs, the now oxygenated blood returns to the heart and is pumped throughout the rest of the body via blood vessels • The two separate circuits – one to and from the lungs, the other to and from the rest of the body – efficiently transports gases and nutrients to every cell of a mammal’s body
Excretion • Mammals have the most highly developed kidneys of all vertebrates • Mammalian kidneys extract nitrogenous wastes from the blood in the form of urea • Urea, water, and other wastes form urine • From the kidneys, urine flows to a urinary bladder, where it is stored until it is eliminated • The kidneys can also retain salts, sugars, and other compounds the body cannot afford to lose
This desert fox survives in areas that have scant supplies of water. Mammals that live in desert regions have very efficient kidneys. Thus their urine contains little water.
Response • Mammals have the most highly developed brains of any animals • The brain consists of three parts: cerebrum, cerebellum, and medulla • Cerebrum: thinking, learning, understanding • Cerebellum: movement • Medulla: breathing, heart rate
Response • Mammals depend on highly developed senses to provide themselves with information about their environment • Eyes vary a great deal from one mammal species to another • With the exception of apes, monkeys, and humans, mammals do not see color well
Response • All mammalian ears are built on the same basic plan, they also vary a great deal in their abilities • The senses of smell and taste are often more highly developed in other mammals than in humans • More than any other animal group, mammals depend on complex behaviors for protection
Compared with other animals, a mammal has a large brain. Each antelope, alert to danger, is processing a great deal of information about the environment in its brain. If there is real danger, a delay of a fraction of a second could mean that the antelope will not survive.
Movement • From the four limbs they inherited from their ancestors, mammals have evolved different structures for movement • Running mammals can achieve great speeds on level ground • Climbing mammals have hands and feet with flexible digits that can grasp vines and branches • Flying mammals have arms modified to support flaps of skin that form wings • Aquatic mammals have arms modified into flippers, which they use to control their speed and direction in the water
This baby monkey swinging from a tree branch shows only some of the movements mammals are capable of. Its long tail is useful in maintaining balance as it moves through the trees.
Reproduction • The three groups of mammals differ greatly in their methods of reproduction • Egg-laying mammals, the monotremes, are the most primitive mammals and reproduce much like reptiles • Oviparous • Lays eggs that are incubated outside the mothers body • Once the young hatch, however, they nurse on milk provided by the mother
Reproduction • Marsupials are viviparous and bear their young alive • The fertilized egg grows into an embryo inside the mother’s reproductive tract • The embryo is supplied with nourishment by a yolk sac on the egg • Because this yolk is not large enough to nourish the embryo through its entire developmental period, the embryo must leave its mothers womb very early
Reproduction • At such an early age of development, the embryo is unable to survive alone • Instinctively, it crawls across its mother’s fur into a pouch called the marsupium • It spends the next several months there, growing sufficiently large and independent so that it can leave the pouch
Reproduction • The early stages of placental embryos are much like those of marsupials • But in placental mammals, the embryo’s chorion, amnion, and allantois develop differently • Tissues from these membranes join with tissues from the mother’s uterus to form an organ called the placenta • Nutrients, oxygen, carbon dioxide, and wastes are exchanged between embryo and mother through the placenta
Reproduction • The time the embryo spends inside the uterus is called the gestation period • The gestation period of mammals ranges from a few weeks in mice and rats to as long as two years in elephants • The gestation period in humans is nine months
Reproduction • After birth, most placental mammals provide their young with a period of care • The duration of parental care varies among different species • During the time infant and mother live together, the infant learns a great deal about its surroundings from its mother • Many biologists believe that this long learning period is one of the most important benefits of the prolonged childhood of many mammals
A platypus is a strange animal. A female platypus feeds milk to her young – but only after the young have hatched from eggs laid by the mother.
The baby kangaroo, safely hitching a ride in its mother’s pouch, views the world around it with amusement. If danger threatens, mother and baby can quickly hop away.
This orangutan mother will care for her baby for some time. Rarely setting foot on the ground, the baby clings tightly to its mother’s fur.
Chapter 33: Mammals Section 2: Important Orders of Living Mammals
Important Orders of Living Mammals • Scientists use several important characteristics to classify mammals • The structure of teeth and the number and kinds of bones in the head are two important features by which mammals are classified • But perhaps the most important characteristic used to classify mammals is the method of reproduction
Monotremes • Egg-laying mammals • Very rare • Only three species exist today • Live in Australia and New Guinea • Duckbill platypus • Echidna (2)
Marsupials • Pouched mammals • Found in Australia • Herbivores that feed primarily on range grasses • Opossums are the only marsupials found in North America today • In the past, many other marsupials lived in South America
Placentals • Living placental mammals are placed in 16 orders • Placental mammals have slightly higher metabolic rates than those of marsupials • They are also much more abundant than the marsupials
Order Insectivora • Insect eaters • Includes tree shrews, hedgehogs, shrews, and moles • Have extremely high metabolic rates and must eat almost constantly
Order Chiroptera • Contains many different species of bats • Account for ¼ of all mammal species • Many bats are active only at night • Use echolocation to help them navigate • Emit high pitched sounds that bounce off objects • Colonies of bats sleep together in trees, hanging upside down and wrapping their wings around their body
Order Edentata • “Without teeth” • Sloths, anteaters, armadillos
Order Rodentia • Mice, rats, squirrels, beavers, porcupines, gophers • Two long front teeth that continue to grow during a rodent’s life • Short gestation period