Class: Mammalia

Blue whale. National Aquarium in Baltimore.

National Aquarium in Baltimore

Mammals are endothermic vertebrates, usually with hair (or fur), but never feathers. Mammals suckle their young with milk produced by mammary glands (thus the name). All mammals use lungs for respiration, though some (like the whales) live entirely aquatic lives.

Mammals are a very diverse group of animals, this variety displayed with just a small sampling: whales, bats, mice (rodents), dogs (canines), cats (felines), rabbits, seals, bears, manatees, horses, rhinoceroses, giraffes, camels, and monkeys (primates).

The class of mammalia is divided into three main groups: The order of monotremata, the order of marsupialia, and the remaining orders put into a group called placentals.

Order: Monotremata

Monotremes are egg-laying mammals. Monotremes are strange animals, and include the achidna (spiny ant-eater) and the platypus. The male platypus has venomous spurs on its hind legs, making it one of perhaps only four species of mammals which produce venom.

Platypus. Dr. Lloyd Glenn Ingles © California
Academy of Sciences.

Achidna (also Echidna). Dr. Lloyd Glenn Ingles © California Academy of Sciences.

Order: Marsupialia

Marsupials have a pouch in which their young mature. The young are born very undeveloped, and then have to crawl through their mother's fur up and into the pouch, where they stay until they have grown enough. Common marsupials are the kangaroo, the koala, and the opossum (the only marsupial living wild in North America).

Koala. Environment Australia, Australian Nature Conservancy.

Opossum. Alden M. Johnson © California
Academy of Sciences.

Placentals

The rest of the orders of mammals are grouped as placentals. Placentals are live-bearing and their young are born at a fairly developed stage, doing away with the need of a pouch. Most mammals, such as those pictured at the top of this page and including humans, are placentals.