Vertebrates are animals with backbones. Reptiles, amphibians, birds, and mammals are all vertebrates. Marsupials are families of mammals, such as kangaroos and opossums, whose females carry their young in an external pouch. Nocturnal animals, such as owls, are active at night. Diurnal animals, such as squirrels, are awake during the day.
Pinnipeds are aquatic mammals with flippers, like seals and walruses.
Quadrupeds are animals with four feet, such as cows. Primates include humans and their closest mammalian relatives. The many species of apes, monkeys, and lemurs are among the primates. Cetaceans are ocean mammals, including whales and dolphins. Rodents , like squirrels and gerbils, have large front teeth for gnawing and cheek teeth for chewing.