Mammal diversity: monotremes, marsupials, bats and rodents

Mammals are divided by structure, reproduction and development of the young into three groups: monotremes, marsupials and placentals. Monotremes (cloacal animals) do not give birth to live young but lay eggs. They have fur and milk glands so they are mammals, but their features are weakly developed: a simple brain, no teats, lower body temperature (about 30–32 °C), and a cloaca – a reptile-like feature. The group includes the platypus and the echidna; they live in Australia and nearby islands (echidnas also in New Guinea). The platypus has a wide horny bill, webbed toes, lives in a riverbank burrow, incubates its eggs, and its young lick milk from the mother’s fur; the male has a venomous spur on the hind leg. The echidna is spiny and carries a single egg in a belly pouch. Marsupials bear live young, have no cloaca and have teats, but the womb is poorly developed, so the young is born tiny and weak and grows in the mother’s pouch: a kangaroo 2 m tall is only 3 cm long at birth. Marsupials live in Australia and nearby islands, and some (opossums) in the Americas; there are over 300 species. The kangaroo hops on long hind legs, and the koala eats eucalyptus leaves. In placentals the foetus develops in the womb through a placenta; they are the most highly organised: from mice to elephants, nearly all domestic animals belong here. Safety: do not touch wild animals, especially the platypus (venomous spur); at a zoo do not cross barriers and do not feed animals.