Which is the only mammal that can truly fly?

Bats. Flying squirrels and sugar gliders glide, which means they fall slowly with style and always end lower than they started; a bat generates lift and can climb. Around a fifth of all mammal species are bats, which surprises most people.

The wing is a hand. The bones inside it are the same ones in yours - four enormously elongated fingers with skin stretched between them, and a thumb left free as a claw at the leading edge. That is what the order's name, Chiroptera, means: hand-wing. Because the membrane is skin rather than feathers, a bat can change the shape of its wing continuously in flight, which makes it more manoeuvrable than any bird.

Most bats also hunt by shouting. They emit calls, often far above human hearing, and build a picture from the returning echoes precise enough to catch a moth in the dark - and some moths have counter-evolved, jamming the signal or simply folding their wings and dropping. And bats are remarkably long-lived for their size: a mouse-sized mammal that gets thirty years instead of two is interesting enough that ageing researchers study them closely.