homework encyclopedia

120 Nature

1 DOLPHIN

Dolphins and whales form a group

of mammals called cetaceans.

Cetaceans spend their whole lives

in water and even give birth there.

They resemble fish but have lungs

not gills, and so must come to

the surface to breathe air.

LIONS HUNTING 3

Predators such as lions and tigers

have sharp claws and long canine

teeth to seize and kill their

victims. Lions hunt in groups to

bring down large prey such as

zebras and buffalo.

Mammals

A group of warm-blooded animals with a bony skeleton, mammals include

mice and other. RODENTS,. PRIMATES, such as monkeys and humans,

and animals as various as hippos, deer, and cats. The 5,000 or so species

include elephants, the largest creatures on land, and whales, the largest of

all animals on Earth.

WHAT FEATURES DO ALL MAMMALS SHARE?

In almost all mammals, the babies develop inside the

mother before they are born. This process is called

. GESTATION. Once born, baby mammals suckle,
or feed, on their mother’s milk. Most mammals have
hair, and all land mammals have four limbs. However,
in whales, the rear limbs have disappeared.

WHAT DO MAMMALS EAT?

Mammals have become very successful because of

the wide range of foods they eat. Meat-eaters include

cats, hyenas, and dogs. Shrews and hedgehogs eat

insects. Plant-eaters include hoofed animals such as

horses and deer, and also rabbits and rodents. Some

mammals are omnivores, eating both plants and meat.

Umbilical cord

links the embryo

to the placenta

Gorilla embryo

grows for nine

months before

being born

GESTATION

Gestation is the time young mammals spend growing in their

mother’s womb. Most mammals develop in this way, so the mother

gives birth to fully formed young. Many. MARSUPIAL babies, such

as kangaroos, complete their development in their mother’s pouch.

PREGNANT GORILLA 3

This cross-section model shows

how a baby gorilla is carried inside

its mother’s womb before birth.

WHAT HAPPENS DURING GESTATION?

In most mammals, the fertilized egg implants itself in

the mother’s womb. There it develops into an embryo,

which is nourished by the placenta. Marsupials have

no placenta and give birth to tiny, helpless young.

. MONOTREMES, such as the platypus, lay eggs.

HOW LONG DOES GESTATION TAKE?

Gestation takes longer in some mammals than in

others. In rodents, such as hamsters, it takes just two

to three weeks. Larger mammals produce fewer

offspring, which usually take much longer to gestate.

Elephants take the longest time of all – 20 months.

Placenta nourishes and

maintains the embryo through

the umbilical cord, and is

expelled after birth