What are the main groups of mammals and how do they differ?

Answer 1

There are three main groups of mammals-monotremes, marsupials, and placental mammals. The groups differ in how their young develop.

Egg-laying mammals are called monotremes. There are just three species of monotremes-two species of spiny anteaters and the duck-billed platypus. A female spiny anteater lays one to three leathery-shelled eggs directly into a pouch on her belly. After the young hatch, they stay in the pouch for six to eight weeks. There they drink milk that seeps out of pores on the mother's skin. In contrast, the duck-billed platypus lays her eggs in an underground nest. The tiny young feed by lapping at the milk that oozes from slits onto the fur of their mother's belly.

Koalas, kangaroos, and opossums are some of the better-known marsupials.Marsupialsare mammals whose young are born at an early stage of development, and they usually continue to develop in a pouch on their mother's body. Marsupials have a very short gestation period, the length of time between fertilization and birth. For example, opossums have a gestation period of about 13 days. Newborn marsupials are tiny-some opossums are less than 1 centimeter long at birth! When they are born, marsupials are blind, hairless, and pink. They crawl along the wet fur of their mother's belly until they reach her pouch. Once inside, they find one of her nipples and attach to it. They remain in the pouch until they have grown enough to peer out of the pouch opening.

Unlike a monotreme or a marsupial, a placental mammal develops inside its mother's body until its body systems can function independently. The name of this group comes from the placenta, an organ in pregnant female mammals that passes materials between the mother and the developing embryo. Food and oxygen pass from the mother to her young. Wastes pass from the young to the mother, who eliminates them. An umbilical cord connects the young to the mother's placenta. Most mammals, including humans, are placental mammals. Gestation periods of placental mammals are generally longer than those of marsupials. Usually, the larger the placental mammal, the longer the gestation period. The gestation period for an elephant, for example, averages about 21 months, but for a mouse, it's only about 20 days.

Placental mammals are classified into groups on the basis of characteristics such as how they eat and how their bodies move.