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Fifty-two things to do with a bat wing

Scientists record a host of nonflight duties that might influence wing structure

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11:37pm, January 7, 2009
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BOSTON — Wings are for swimming. Boxing. Catching a falling baby.

So far, bat biologist Thomas Kunz of Boston University and his colleagues have come up with 52 things bats do with their wings besides fly. Flight may be the main job for wings, but other uses deserve study too, Kunz said January 4 at the annual meeting of the Society for Integrative and Comparative Biology.

Nonflight tasks might have had their own influences on wing evolution, Kunz said, calling the unusual listing project a first step toward exploring any such effects.

Bats manage far more forms of locomotion than flying, and, yes, in an emergency, bats can swim, Kunz said. Insect-eating bats in particular often hunt over water, where a miscalculated swoop ends with a dip. He has videotaped a foraging bat that splashed down in a Boston pond and then flap-stroked its wings to reach shore, climb a cattail and shudder off water droplets like a wet dog.

In tests of nine bat species, Kunz found that all could swim, and at least some could cope in the water as babies. “They can swim before they can fly,” he said.

Bats also need to maneuver themselves onto a perch, and Kunz cited new work by Dan Riskin of Brown University on wing and body acrobatics used for landing on ceilings.

In test setups, three kinds of leaf-nosed bats consistently landed on their feet (upside down) with a twist and tuck of their wings and hind legs, Riskin reported January 6 at the meeting. The bats hit the ceiling lightly, with a force equivalent to less than their body weight.

Lesser dog-faced fruit bats, in contrast, flew up to a four-point landing with a force equivalent to several times their body weight, and on one occasion 11 times their body weight.

Riskin hasn’t tested other species yet to check for widespread patterns, but he notes that the gently landing species roost in caves, where a rough touchdown would mean smacking into stone. Tree branches, where his four-point landers roost, might be more forgiving.

Bat wings prove useful for right-side up movements too, according to Kunz’s list. Vampire bats that feed on mammals typically creep along the ground to reach targets instead of flitting in from above. These bats can walk by “stepping” forward on the wrists of one furled wing at a time.

Bats can also run. Riskin discovered this when he put vampire bats on a treadmill and found that they shifted to a new gait at higher speeds, reaching out both wings at a time to bound forward.

And, “vampire bats are the best climbers,” Kunz said. Some species use their wings to crawl around trees to reach sleeping bird prey.

Kunz has tallied nine kinds of bat locomotion other than flight that require wings. The various ground-based movements and climbing are his first choices for nonflight demands that might have influenced wing evolution, he said. Vampire bats, for example, move readily on the ground and have relatively big wing bones, but don’t fly long distances.

Most of the wing jobs on Kunz’s list, however, have little to do with moving from place to place. Wings help out in such everyday business as foraging, staying warm or cool, and having a family. Male bats defending their harems fight wing to wing. They wrestle in furious wing clinches and throw boxing punches at each other with wristlike wing parts. In testy species, Kunz says, “Harem males have bloody wrists from beating up on each other.”

Male sac-winged bats ferment a mix of spit and a penis secretion in pouches on their wings. Males lick out and replace the brew daily, maintaining just the right scent to fan at females. “Here I am, ladies — don’t I smell good,” is how Kunz translated the message, though he said, to his nose, it smells moldy.

Roosting females wrap their wings around nursing infants, offering both warmth and security. Babies grip with their mouths and big feet, but accidents can happen.

These versatile wings evolved from forelimb bones that shifted proportions and grew forefinger-to-foot swathes of skin “Think of how many things we do with our hands,” Kunz said. “A wing is a hand.”

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