Main->Readings->4th Grade Readings->Table of Contents->Part 6: Mammals
Vocabulary |
Around two hundred million years ago, a new type of vertebrate animal appeared on the scene. It was a very small animal which had evolved from reptiles. While the huge dinosaurs ruled the land, these tiny animals were underfoot. But when the dinosaurs became extinct, these animals were still there. They were the first mammals.
Like reptiles and bird, mammals have lungs. Mammals are warm-blooded like the birds (and maybe the dinosaurs). They can live and stay warm in cold places, like the birds. What makes mammals different from other vertebrates? For one thing, they have fur or hair. This helps keep them warm. They also take even better care of their young than birds can, because they feed their young with milk. Mammals have large brain sizes compared to their bodies, and they have strong teeth. They are very good at surviving on the earth. There are only about 5,000 species of mammals, compared to 22,000 species of fish, but they live everywhere.
Birds protect their young and feed them after they are hatched, but mammals go one step farther. They grow their young inside their bodies. Female mammals have eggs, but the male fertilizes the eggs inside the female's body as with birds. Unlike bird eggs, fertilized mammal eggs grow and change inside the mother's uterus instead of being hatched outside in a nest. When mammal young are born, the mother continues to protect them, feeding them milk from her body. Only mammals have milk.
There are three groups of mammals. One of the groups is very rare, another has more members, and the third one is very large and includes most mammals.
The first group is the monotremes (MAHN-o-treemz). The only living monotremes are the duck-billed platypus and the spiny anteater. Monotremes lay eggs.
The second group is the marsupials (mar-SOOP-ee-uhls). Although marsupials once lived all over the world, most marsupials now live in the area of Australia and New Zealand. Some, like the opossum, still live in other places such as Haverford, Pennsylvania. Marsupials give live birth very early after fertilization. The tiny infant then climbs into a pouch on its mother's body, where it drinks milk and continues growing. Kangaroos, wallabies, and koala bears belong to this group.
The last group is the placentals (plah-SEN-tuhls). Most living mammals are placentals. The young of placentals stay in the uterus until they are ready to be born. Horses, mice, bats, dogs, whales, and humans belong to this group.
Fossils of a group of mammals called primates (PRY-mates) appeared late in the history of animals, about 30 million years ago. These animals were the ancestors of human beings, chimpanzees, orangutans, and spider monkeys. Early "hominids" (pre-humans) appeared only about 2 million years ago. And some time between 100,000 years ago and 500,000 years ago, not very long ago in the Earth's history, true humans appeared.
Scientists continue to wonder where and how human beings first turned up. Some believe they first evolved in Africa, while others think they evolved from early hominids who had left Africa. However, all human beings are very closely related. Humans are all one species.
Human are classified as mammals for many reasons: They are many-celled, move around, and do not make their own food, so they are animals. They have backbones, so they are classed as vertebrates. They breathe with lungs, so they are land vertebrates. They are warm-blooded, have hair, give live birth, and feed milk to their young, so they are mammals.
Or, as a scientist would say it more exactly, humans are:
Questions: For your first assignment of the week, answer these questions in complete sentences on a sheet of loose-leaf paper, with a proper header:
Notes: For your second assignment of the week, in your journal on the next clean page, write the vocabulary words from this section and their definitions.
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This page last modified August 15, 2002
Copyright ©2000 Delia Marshall Turner. All rights reserved.
Questions? Send me a note at dturner@haverford.org