Mammals evolved from a group of reptiles called therapsids. The earliest true mammals, appearing over 200 million years ago, were only 5 cm (2 in) long and probably spent at least some of their lives in trees. These tiny mammals resembled shrews with four roughly equal short legs and sharp claws. Since that time mammals have evolved an extraordinary variety of body shapes and sizes. Of the approximately 4600 species of mammals alive today, most live on land, and most still move about on all four legs. But modern mammals also include animals that hop on two legs, ones that live permanently in water, and ones that can fly. These swimming and flying species include the world’s largest mammal—the blue whale, which can grow over 30 m (100 ft) long—and also the smallest, the Kitti’s hog-nosed bat.
Discovered in 1973 in the forests of Thailand, this tiny bat is about the size of a bumblebee, and weighs just 2 g (0.07 oz).
The life spans of mammals vary as enormously as their sizes. Most shrews survive for less than a year, exhausting themselves in a life of almost ceaseless activity. By contrast, horses can live about 20 years, chimpanzees can live to be over 50, and elephants can survive into their 60s. Humans have the longest life span of any mammal, with a few individuals living over 110 years. Mammals have adapted to some of the most extreme habitats on earth.
They are warm-blooded, or endothermic, meaning that they maintain their body temperature within a narrow range despite changes in the environment. Polar bears survive on Arctic ice, while Arctic foxes can sleep on open snow in temperatures as low as –68° C (-90° F). Camels and kangaroo rats live in deserts, and can tolerate blazing temperatures that would kill many animals from cooler habitats.
Mammals can also tolerate the thin air of the highest mountains as well as the crushing pressures of the ocean depths. Yaks, for example, forage for food on mountain slopes at
altitudes of up to 6100 m (20,000 ft), while sperm whales can dive to depths of at least 2100 m (7000 ft), holding their breath for over an hour. Mammals also include some of the world’s greatest animal migrants. During its yearly migration from Arctic waters to the coast of Mexico and back, a gray whale may travel 20,000 km (12,500 mi).
WHERE DO MAMMALS LIVE?
Mammals can live in all kinds of places. They can live in hot places, cold places, and places in between. Mammals are warm-blooded. They keep their bodies at about the same temperature wherever they are. Mammals use food energy to keep their bodies warm when it is cold outside. Mammals sweat to get rid of body heat when it is hot outside.
Mammals adapted (changed) over a long time to live in very different places. Polar bears, for example, have thick fur and a layer of fat that keeps them warm. These features let them live in; freezing cold near the North Pole. Their fur is white to blend in with the ice and snow.
Camels are mammals adapted to live in hot, dry deserts. Camels can go for a long time without drinking water. Otters, dolphins, whales, and seals have adaptations that let them live in the water.
Humans are mammals too.They can live almost anywhere on Earth. Humans learn how to adjust. You can live in the cold of the Arctic or the heat of a jungle by wearing the proper clothing. You can even live underwater in a submarine! You can breathe underwater with scuba tanks.
HOW MANY MAMMALS DO WE HAVE
There are about 4,600 species (kinds) of mammals. Most mammals live on land and move about on four legs. There are also mammals that walk or hop on two legs, mammals that
live in water, and mammals that can fly.
The world’s largest mammal is the blue whale. The blue whale lives in the ocean and can be more than 80 feet (24 meters) long. The smallest mammal is the Kitti’s hog-nosed bat.
Scientists discovered this tiny bat in 1973 in the forests of Thailand. It is about the size of a bumblebee.
Some mammals live a long time. Horses can live about 20 years. Chimpanzees can live about 50 years, and elephants can live about 60 years. Humans live longer than any other mammal. A few humans have lived to be more than 110 years old. Shrews are mouse like mammals that live very short lives. Most shrews live less than one year.
WHAT ARE THE MAIN KINDS OF MAMMALS?
Scientists put mammals into three main groups called monotremes, marsupials, and placentals. Monotremes and most marsupials live in Australia and on islands near Australia.
Monotremes are mammals that lay eggs. The duck-billed platypus and two kinds of spiny anteaters arethe only monotremes.
Kangaroos, koalas, and wombats are marsupials. Marsupials give birth to babies that are not fully developed. The newborns climb into a pouch on their mother’s stomach. They stay
in the pouch and drink mother’s milk until they finish growing. There are 250 kinds of marsupials.
All other mammals are placentals. You are a placental mammal. Like all placentals, you stayed inside your mother’s body until you were ready to be born. You were warm and safe attached to an organ called the placenta. The placenta takes food from the mother’s blood and gives it to the unborn baby.
Rats, mice, squirrels, and other rodents are the largest group of placental mammals. There are about 1,500 species of rodents. Bats, the only flying mammals, make up another big group of placentals.
There are about 1,000 species of bats. The smartest mammals are primates. Scientists group such animals as apes, monkeys, lemurs, and humans as primates. Humans are the smartest primates.
WHICH MAMMALS ARE PLANT EATERS?
Some mammals eat only plants. Cows, horses, pigs, and goats are plant-eating farm animals. Another name for plant eater is herbivore. Many wild plant-eating mammals live on plains and grasslands. Buffalo, deer, and antelope are wild plant-eaters.
WHICH MAMMALS ARE MEAT EATERS?
Wolves and dogs and lions, tigers, and other cats are some of the mammals that eat only meat.Most mammals that live in water are also meat eaters.
Another name for meat-eating animals is carnivore. Carnivores usually hunt for their food.
Hyenas are carnivores that do not hunt. They eat animals that are already dead.
The smallest carnivores eat worms and insects. Shrews and hedgehogs are small carnivores.
WHICH MAMMALS EAT MEAT AND PLANTS?
Some mammals eat both animals and plants. Another name for animals that eat both animals and plants is omnivore. The brown rat and the opossum are examples of omnivores. Humans are omnivores. They eat meat, fish, vegetables, and fruit. Scientists group bears with carnivores, but most bears also eat nuts, leaves, and berries.
HOW DO MAMMALS MOVE?
Most mammals that live on land have four legs. They use all four legs to walk or run. The cheetah is the fastest mammal. It can reach speeds of 58 miles (93 kilometers) per hour
while chasing prey.
Mammals that live in water have flippers instead of legs. Seals, whales, and dolphins use their flippers to swim.