HOW COME? Why bats sleep upside down

only bats can fly. And these flying mammals have bodies designed for an inverted lifestyle.

In fact, bats know some flying tricks that the average bird couldn't dream of mastering. Bats have four large flying muscles to a bird's two, and specially-adapted lightweight, slender legs. And bat knees -- in fact, a bat's hind legs -- are attached backward. This joint rotation helps bats maneuver easily during its acrobatic flights, and even land upside down (say, on a barn rafter or branch).

Besides its rear-facing knees, a bat has strong feet tipped by long, curved claws. And bat feet have tendons designed to firmly clamp bats to their roosts. How it works: As a bat swoops up to a rafter, landing upside down, the weight of his hanging body tugs sharply on his feet. Thanks to the special tendons, a bat's feet go into "auto-lock," clamping on tight.

Since the feet lock all by themselves, a resting bat doesn't need to hold on. The tendon locks work so well that some bats even give birth upside-down, catching the falling pups in their wings. And, of course, the bat can safely drift off to sleep, no worries about falling mid-dream. The feet's locked tendons are so strong that a bat often remains hanging from a branch even after he's dead.

But not all bats sleep hanging by their toes. Disk-winged bats snooze upright, in hiding places like curled-up banana leaves. Meanwhile, Hardwicke's Woolly bat, a species living in Asia, prefers to slumber in meat-eating plants. Such carnivorous plant species trap and digest insects and small animals.

Why would a bat snuggle up to a killer plant? Scientists think the woolly bat evolved a special relationship with the pitcher plants it calls home. Each bat is just the right size to snugly fit halfway down the plant's feeding tube, without slipping into the corrosive digestive juices below. It turns out that the plants get something in return for providing a handy hotel for bats -- a steady supply of bat feces, especially rich in nitrogen.