Mammals

Three Types of Mammals By Cindy Grigg 1 Earth has a large variety of animals living on it. Scientists classify animals into groups based on common characteristics. Mammals are a group of animals (vertebrates) that have backbones and hair or fur. They are warm-blooded (endothermic), and they have four-chambered hearts. They also feed their young with milk from the mother's body. The young of most mammals are born alive. 2 Mammals have about six thousand different species, or kinds, of animals in their group or class. Mammals can be divided into three more groups based on how their babies develop. These three groups are monotremes, marsupials, and the largest group, placental mammals. 3 Monotremes are mammals that lay eggs. The only monotremes that are alive today are the spiny anteater, or echidna, and the platypus. They live in Australia, Tasmania, and New Guinea. These mammals are really different from other mammals. Their body temperature is lower than most warm-blooded animals, a feature that has more in common with reptiles. Their name comes from the fact that they have only one body opening for both wastes and eggs to pass through. Echidnas have sharp spines scattered throughout their hair. They look like a spiky ball. The female anteater lays usually one leathery-shelled egg directly into the pouch on her belly. The egg hatches after only ten or eleven days. The newborn baby is tiny, about the size of a dime. After the baby hatches, it stays in the pouch for several weeks and continues to develop. Babies are fed by their mother's milk that seeps out of pores on her skin. When the babies are six to eight weeks old, they grow spines that begin to irritate the mother anteater. She then scratches the baby out of her pouch. 4 The platypus has webbed feet and a bill. Its body is torpedo-shaped and is covered with soft fur. Platypuses live in the water. They make mazes of tunnels in muddy banks along lakes, small streams, or rivers. The female lays her eggs in an underground nest. After they hatch, the tiny babies drink milk from their mother's body. They feed by lapping up the milk that oozes onto the fur of their mother's belly. The platypus' duck-like bill is not like a bird's beak. A bird's beak opens to reveal its mouth. The platypus' bill is a sensory organ with its mouth on the underside. The platypus is also one of the few mammals that is venomous. The male platypus has a spur on its hind foot that delivers a poison that is very painful to humans. It can kill some small animals. 5 Marsupial mammals give birth to babies that are not completely developed. The babies are very tiny. The babies then crawl up the fur on the mother's belly into a pouch on the outside of the mother's abdomen. The babies drink milk from the mother and continue to develop inside the pouch. Koalas, kangaroos, wallabies, and opossums are some of the better-known marsupials. Today marsupials are found mostly in Australia, New Guinea, and South America. The only marsupial in North America is the opossum. Opossums may give birth to as many as twenty-one babies at one time. However, the mother only has thirteen nipples in her pouch. The first thirteen babies to climb into her pouch and attach to her nipples are the only ones that survive. 6 A placental mammal develops inside its mother's body until its body systems can function on their own. The name of this group comes from the placenta, an organ in pregnant female mammals that passes materials between the mother and the developing baby. Food and oxygen, carried by blood, pass from the mother to the baby through the placenta. Wastes pass from the baby to the mother, where they are eliminated by her body. Most mammals, including humans, are placental mammals.