Chapter 33: Mammals

Section 1: Mammals

Mammals

Includes many diverse species that vary greatly in appearanceRange in size from a tiny mouse to a huge elephantMammals can be found flying in the air, running along the ground, and swimming in the seaAlthough 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 internallyMost species are experts at maintaining a constant body temperatureMammals use various combinations of fur, hair, and subcutaneous fat to conserve body heatMany mammals also have sweat glands that help cool the bodySweat 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 viviparousThis means that young mammals develop within the mother for a time and then are born aliveFemale mammals have mammary glands, which produce milk to nourish the young for some time after they are bornMammary 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 teethCombined with their jaws, the teeth of mammals bite, chew, and grind food efficientlyMammals have well-developed breathing muscles, including a diaphragmMammals 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.

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.

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.

Evolution of Mammals

The first mammals were very smallBy 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 preyCarnivores use an up and down chopping movement of their jaws to chew their foodThe 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 digestedHerbivorous animals have evolved strong lips and flat edged incisors that grasp and tear this tough vegetationThey 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 ownThe vertebrate digestive system has never evolved the ability to produce enzymes that digest celluloseTo 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 celluloseAfter a certain amount of time, the mammal regurgitates the plant food from the rumen into its mouthThere 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 intestinesMany of the same kinds of microorganisms that digest cellulose are found in the cecumThe 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 musclesChest muscles pull air in and push air out by moving the ribs up and down to increase and decrease the size of the chest cavityWhen 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 lungsMany 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 vesselsThe main pump, the heart, sends deoxygenated blood to the lungsAfter it leaves the lungs, the now oxygenated blood returns to the heart and is pumped throughout the rest of the body via blood vesselsThe 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 vertebratesMammalian kidneys extract nitrogenous wastes from the blood in the form of ureaUrea, water, and other wastes form urineFrom the kidneys, urine flows to a urinary bladder, where it is stored until it is eliminatedThe 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 animalsThe 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 environmentEyes vary a great deal from one mammal species to anotherWith 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 abilitiesThe senses of smell and taste are often more highly developed in other mammals than in humansMore 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 movementRunning mammals can achieve great speeds on level groundClimbing mammals have hands and feet with flexible digits that can grasp vines and branchesFlying mammals have arms modified to support flaps of skin that form wingsAquatic 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 reproductionEgg-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 aliveThe fertilized egg grows into an embryo inside the mother’s reproductive tractThe embryo is supplied with nourishment by a yolk sac on the eggBecause 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 aloneInstinctively, it crawls across its mother’s fur into a pouch called the marsupiumIt 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 marsupialsBut in placental mammals, the embryo’s chorion, amnion, and allantois develop differentlyTissues from these membranes join with tissues from the mother’s uterus to form an organ called the placentaNutrients, 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 periodThe gestation period of mammals ranges from a few weeks in mice and rats to as long as two years in elephantsThe gestation period in humans is nine months

Reproduction

After birth, most placental mammals provide their young with a period of careThe duration of parental care varies among different speciesDuring the time infant and mother live together, the infant learns a great deal about its surroundings from its motherMany 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 mammalsThe structure of teeth and the number and kinds of bones in the head are two important features by which mammals are classifiedBut perhaps the most important characteristic used to classify mammals is the method of reproduction

Monotremes

Egg-laying mammalsVery rareOnly three species exist todayLive in Australia and New Guinea Duckbill platypus Echidna (2)

Marsupials

Pouched mammalsFound in AustraliaHerbivores that feed primarily on range grassesOpossums are the only marsupials found in North America todayIn the past, many other marsupials lived in South America

Placentals

Living placental mammals are placed in 16 ordersPlacental mammals have slightly higher metabolic rates than those of marsupialsThey are also much more abundant than the marsupials

Order Insectivora

Insect eatersIncludes tree shrews, hedgehogs, shrews, and molesHave extremely high metabolic rates and must eat almost constantly

Order Chiroptera

Contains many different species of batsAccount for ¼ of all mammal speciesMany bats are active only at nightUse echolocation to help them navigateEmit high pitched sounds that bounce off objectsColonies 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, gophersTwo long front teeth that continue to grow during a rodent’s lifeShort gestation period

Order Lagomorpha

Rabbits and haresResemble rodentsSharp front teeth and eat plant materialTheir gestation period is short, and the number of young they produce is high

Order Carnivora

Meat eatersCats, dogs, wolves, bears, weasels, hyenasMost are terrestrialKill with sharp teeth and claws

Order Cetacea

Truly aquatic mammals: whales, dolphins, and porpoisesHave lungs and a circulatory system designed to permit long, deep divesLayer of blubber keeps them warmHave lost both their external ears and their hind legsMate and bear their young in the waterAll are carnivores

Order Sirenia

Aquatic animals related to elephantsPeaceful, slow-moving herbivores that live in rivers and streams in Africa, South America, and FloridaThe manatee, or sea cow, lives in quiet waters in southern Florida, where it is often injured by careless boaters

Order Artiodactyla

Contains the large grazing animals: cattle, sheep, goats, hippopotami, giraffes, and pigsThe original five toes on each foot have been reduced to twoEven-toed ungulates “hoofed mammal”

Order Perissodactyla

Horses, zebras, tapirs, and rhinoceroses – the odd-toed ungulatesHave hooves formed from the center toe of each foot

Order Proboscidea

Mammals with trunks, the great elephantsOnly two species, the Indian elephant and the African elephant still surviveBoth species are in danger of becoming extinct

Order Primates

Includes our own speciesClosely related to the ancient insectivoresMost complicated behaviorsMonkeys and apes represent two main branches within this order

Order Primates

Primates were split apart by the moving continents One branch, the New World monkeys, live

almost entirely in trees They have long arms for swinging from

branch to branch and long prehensile tails, which they use for grasping while climbing

The other branch, the Old World monkeys lack prehensile tails

How Mammals Fit into the World

Herbivorous mammals are major consumers of plant materialThese herbivores are food for carnivorous mammalsBats eat mosquitoes and insectsDomesticated mammals provide food in the form of meat and dairy productsSome mammals carry diseaseOthers destroy the land around them