Published: 7 May, 2008

Bats' bony wing design not conducive to gliding like birds can

We've heard that bats can glide but not soar, and we've also heard that they can't glide or soar... only fly by flapping. Can you clear this up for us? (We're asking because, in the summertime when we look up, we're not always sure whether we're seeing are swallows or bats at dusk, and this would help.)

Bats are, indeed, the only mammals capable of true flight. There are at least 1000 species of living bats, none of which are particularly good at soaring. In fact, I'd have to say that nearly all of these bats require active, powered wingbeats to fly.

Soaring or gliding is a behavior seen in many birds. The energy necessary to provide lift for these birds primarily comes from rising warm air currents (thermals) or from the lift provided off hills or ocean waves (wind currents deflect upwards off the hill or wave surfaces to lift the birds). Examples include hawks soaring over a warm field in the summer, or albatrosses (with their very large wing spans of nearly 11 feet) that can remain away from land for days.

A main difference between bats and birds is their time of activity. Bats are nocturnal, active at night when winds are calm and when there is no solar energy to heat the atmosphere and to create rising thermals of air. Birds, such as the hawks hunt by day and can take advantage of local thermals to passively gain altitude. North American bats are all nocturnal and have wings well designed for slow speed and highly maneuverable flight, allowing them to seek insects around streetlights or (in a more natural condition) along a forest or stream edge.

Some fast-flying bats may glide briefly as they drop from their roosts. They need this brief glide to gain sufficient airspeed to achieve a "take-off velocity" necessary for their wings to provide sufficient lift. Even the myriad fruit eating bats of South and Central America are nocturnal and need to navigate complex forest understory structure to travel to and from their food sources, as well as to be able to alight on the trees.

Nectar feeding bats are a bit like hummingbirds and have highly constrained wing design allowing them to hover, quite opposite design specifications for a soaring animal. Only a couple of bats, tomb bats found in India, have been reported to soar on rising air currents generated by winds rising against a hilly surface. Although I've never seen them, South Pacific flying foxes fly in some daylight conditions and may glide a bit from tree to tree.

Many scientists believe that feathers are inherently better structures to make a soaring or gliding wing. In contrast, the size of bat wing is dictated by the length of one of their bony fingers. Perhaps bone is too heavy (compared to feathers) to construct a long soaring wing out of. Bat wings are designed like the wings of many small aircraft, with short stubby wings good for maneuverability but inherently not good for gliding should the power shut off.