Hang Time

Image courtesy Ann Hudgins/USFWS Mariana fruit bat hanging around

A common bat-related question is, "Why do they hang upside down?" First, it puts them in an ideal takeoff position. Unlike birds, bats can't take off from the ground. Their wings simply don't produce enough lift to take off from a dead stop. And, their hind legs are so small and underdeveloped that they can't run to build up the necessary takeoff speed. Instead, they use their front claws to climb to a high spot, and then fall into flight. By sleeping upside down in a high location, they're all set to take off at any time.

Hanging upside down is also a great way to avoid danger. During the hours when most predators are active (particularly hawks and falcons), bats congregate where few animals would think to look and most can't reach -- they're hidden away until night comes again. Another perk is the open real estate market; there's little competition for these roosting spots, as other flying animals aren't able to hang upside down.

Bats have a special physiological adaptation that enables them to hang around this way. For you to clench your fist around an object, you contract several muscles in your arm, which are connected to your fingers by tendons; as one muscle contracts, it pulls a tendon, which pulls one of your fingers closed. A bat's talons close in the same way, except that their tendons are connected only to the upper body, not to a muscle.

To hang upside down, a bat flies into position, opens its claws and finds a surface to grip. To get the talons to grab hold of the surface, the bat simply lets its body relax. The weight of the upper body pulls down on the tendons connected to the talons, causing them to clench. Since it is gravity that keeps the talons closed, instead of a contracted muscle, the bat doesn't have to exert any energy to hang upside down. In fact, a bat will continue to hang upside down if it dies in that position. To release the surface it is gripping, the bat flexes other muscles that pull its talons open.

Bats to the Rescue Most bat species are not only harmless to humans, but actually beneficial. Insect-eating bats are by far the best bug-killers on the planet. The little brown bat, one of the most common North American bat species, can catch and eat as many as 1,200 mosquitoes in one hour. The famous colony of Mexican free-tail bats that lives underneath the Congress Avenue Bridge in Austin, Texas, will eat up to 30,000 pounds of insects in a single night. These bats, and many other species, feed on insects that destroy crops, providing an invaluable service to farmers. Bats are also beneficial as plant pollinators. Many species, particularly in the tropical rainforest, feed on plant nectar, gathering pollen on their bodies as they feed. When they fly away, they spread the pollen, helping the plant disperse its seed. Bats are major pollinators of many plants used by humans, including bananas, figs, mangoes and cashews.