Childrens Illustrated Encyclopedia

330

Mammals 330-

most mammals, including

monkeys, cats, and dogs,

are called placental

mammals because the

young develop inside the

mother’s womb, or uterus,

and are fed by means of the

placenta. the placenta is a

specialized organ embedded in

the wall of the womb. It carries

nutrients and other essential

materials from the mother’s

blood to the baby’s blood. these

nutrients help the young grow

and develop. after the young are

born, the placenta comes out of

the uterus as afterbirth.

monotreme mammals

Five kinds of mammals lay eggs. they are called

monotreme mammals, and include the platypus

and the four types of echidna (spiny ant-eater).

all are found in australasia. after about 10 days,

the young hatch out of the eggs, and then feed

on their mother’s milk.

short beaked anteater

the short-beaked echidna, lays a

single egg in a temporary pouch

on its abdomen. after the young

echidna hatches, it stays in the

pouch for 6 to 8 weeks. once

its spines start developing, it

is kept in a burrow.

PrImates

monkeys, apes, and humans belong to a

group called primates. Primates are able

to grasp with their hands. most primates

have thumbs and big toes, with flat fingernails

rather than claws. members of the primate

group range in size from the mouse lemur,

which weighs only 2 oz (60 g), to the

gorilla, which weighs up to 610 lb (275 kg).

Pouched mammals

kangaroos, opossums,

wallabies, koalas, wombats,

and bandicoots are all known

as pouched mammals or

marsupials. these animals

carry their young in their

pouches until the young are

developed and strong enough to

leave. once it has left the pouch, the

joey (young wallaby) returns to the pouch if it

needs to suckle or if it is tired or threatened. marsupials

are found in australia and new Guinea, south america,

and north america. a few marsupials, such as the shrew

opossum of south america, do not have pouches.

the anImal GrouP called mammals includes the

heaviest, tallest, and fastest animals on land—the elephant,

the giraffe, and the cheetah. mice, whales, rhinoceroses,

bats, and humans are also mammals. like birds,

mammals are warm-blooded (endothermic), but three

features set them apart from all other creatures.

all mammals are covered in fur or hair, all feed

their young on milk, and all have a unique type of

jaw. the jawbone helps us to identify the fossilized

bones of prehistoric mammals that lived on earth

millions of years ago. mammals are also members of the

group known as vertebrates because they all have

vertebrae (backbones). today, there are more than

5,000 kinds of mammals, including carnivores (meat

eaters) such as tigers; herbivores (plant eaters)

such as rabbits; and omnivores (meat and plant

eaters) such as bears. cattle, sheep, goats, and

most other farm animals are mammals, and

many pets are mammals, too, including cats,

dogs, and guinea pigs. mammals live nearly everywhere. they are

found on land, in the sea, and in the sky, from the coldest arctic

to the most searing heat of the desert.

MAMMALS

A wallaby’s large tail is so strong that it

can act as a prop for the wallaby

to lean on.

Young male

joey

A mammal’s

body is

covered in fur.

marsuPIal younG

marsupials are not completely

developed when they are born.

after birth, the baby crawls

through its mother’s fur into

a pocketlike pouch on the

abdomen, where it attaches

itself to her teat, and stays

there until it is fully developed.

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