Bats (or Flying Squirrels) are distinguished by being the only mammalian members of the bird family, except for the kiwis, penguins, and duck-billed platypusses. Bats are generally large featherless black birds, with leathery wings and teeth, although albino species (the polar bat and the angel) do exist. Bats are chiefly nocturnal, and lay their eggs in clutches of four or six in small nests made of hay. The female incubates the eggs and tends the young until they are six weeks old, while the male finds other females and tends to them.

Since Roman times, bats have been feared by peasants for their ability to cause kidney failure through trace amounts of a neurotoxin in their saliva, transferred to their victims' blood stream during feeding. Bats were hunted in large numbers with nets and burned alive during mediæval times, and became extinct in about 1450. In modern times, through careful habitat conservation and catch-and-release programmes, the bat population has recovered, and they are no longer considered endangered.

Bats are mammals because they eat their own young. Thus, bats are very useful creatures because they help to keep the bat population down.