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First published online March 21, 2005
Journal of Experimental Biology 208, i (2005)
Copyright © 2005 The Company of Biologists Limited
doi: 10.1242/jeb.01561
Inside JEB |
BATS DON'T BREAK A LEG
yfke{at}biologists.com
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With their puny legs and graceful wings, it's clear that bats evolved to fly, not walk. Yet some bats are surprisingly good at scurrying along the ground in pursuit of their prey. Vampire bats are particularly agile crawlers and have chunkier legs than other bats. This has led to the suggestion that other bats are poor walkers because their hind limbs are too weak to cope with the forces associated with pounding the ground; their fragile legs would simply snap if they tried to support their body weight. Armed with a specially designed walkway for bats, Daniel Riskin at Cornell University set out to test this hindlimb-strength hypothesis (p. 1309).
Riskin reasoned that if bats' crawling ability is limited by hindlimb strength, and vampire bats are better walkers because their legs are stronger than other bats' legs, then vampire bats' legs should generate larger forces during walking than the delicate legs of other species. To see if this is true, he decided to measure the forces acting on bats' hind legs as they crawled along.
Force plate expert John Bertram helped Riskin design and build a bat walkway consisting of an elongated box with an ultra-sensitive force plate bottom to measure the delicate pressures exerted by bats' hindquarters as they clambered around. Satisfied that the device was small enough to take into the field, Riskin and John Hermanson boarded a plane to Trinidad to find some bats. Unfortunately, Riskin says, `our contraption looked like a bomb, so it took ages to get it through airport security.' Having convinced the officials that they were not a threat to national security, the pair made it safely to Trinidad.
Using a tempting herd of cattle and coop of chickens to entice the bats, Riskin and Hermanson soon had enough `volunteers' – two species of vampire bat and a particularly clumsy insectivorous bat to compare with the expert walkers. Placing bats in the box, Riskin blew on them to persuade the creatures to shuffle away. He recalls that the vampires were true to form, striding deftly across the force plate walkway. But the ungainly insectivores struggled with the task, `lifting their bodies up and thrashing manically with their legs until they collapsed forwards.' Viewing digital recordings of the bats' locomotor attempts, Riskin and Hermanson identified brief periods during which only the creature's hind limbs touched the force plate. When they compared the peak forces produced by the vampires with those of the insectivores during these periods, they found that the poor crawlers actually exerted larger forces with their hind limbs than the vampires! The insectivores' spindly legs are obviously strong enough for crawling. The pair also found that the vertical component of the peak force (which supports the animal's body weight) was the same for all three species, suggesting that the legs of all three species support the same proportion of their body weights. Finally, they modelled bending stress to compare hindlimb support capabilities among 50 bat species and found that vampire bats' hindquarters really aren't more robust than those of other bats.
Clearly, the hindlimb-strength hypothesis is wrong; the frail legs of non-terrestrial bats can easily withstand the forces produced during walking. Riskin concludes that some other factor must limit the crawling ability of most bats. He admits that he's not sure what this is, so bat locomotion researchers now have their work cut out for them.
References
Riskin, D. K., Bertram, J. E. A. and Hermanson, J. W.
(2005). Testing the hindlimb-strength hypothesis: non-aerial
locomotion by Chiroptera is not constrained by the dimensions of the femur or
tibia. J. Exp. Biol.
208,1309
-1319.
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