A Jamaican fruit bat (Artibeus jamaicensis) in flight. Photo credit: Jorn Cheney.

A Jamaican fruit bat (Artibeus jamaicensis) in flight. Photo credit: Jorn Cheney.

Watching a bat flit through the night sky, you might think that the delicate membrane stretched between the animal's elbow, long splayed digit bones and down its side to the hind limbs is no more than an inert flexible surface that catches air passively like fabric covering the struts of a kite. Yet, the elegant wing membrane is spanned by thin thread-like muscles. Could the muscles, known as plagiopatagiales proprii, tense and relax the membrane to fine-tune how the structure bows and catches the air as the animals swoop and flutter? Jorn Cheney, Jeremy Rehm, Sharon Swartz and Kenny Breuer from Brown University, USA, decided to find out.

But first, the team need to know how Jamaican fruit bats (Artibeus jamaicensis) fly when they had full use of the wing membrane muscles. Cheney and Rehm tempted the animals with guava juice to fly through still air in a wind tunnel and into a 5 m s−1 head wind, filming the animals with six high-speed cameras to reconstruct the wing shape during their distinctive wingbeats to find out how much the membrane bowed as the animals propelled themselves forward. Then the pair painstakingly paralysed all 25 of the fine muscles in each wing by individually injecting botulism toxin into each, inactivating them temporarily. ‘We use tiny, microliter amounts of dilute Botox, much less than would be used in a cosmetic procedure to minimize a single forehead wrinkle’, says Swartz. Once the animals had recovered, Cheney and Rehm then encouraged the bats to fly again in the still air and headwind, to find out how loss of muscle control affected the animals.

The animals with the paralysed wing membrane muscles struggled to remain aloft in the motionless air. Even though they could still fly at 6.2 m s−1 into the head wind – which was 1 m s−1 slower than they had managed with full use of the wing membrane muscles – the animals struggled when there was no headwind, descending rapidly to the ground as they tried to remain aloft. And, when the team compared the bats’ wing membranes as they flew into the head wind, it was clear that the membranes of the paralysed bats were slacker, bowing 12% more than the wing membranes of the bats that still had the use of their plagiopatagiales proprii, also increasing the amount of air drag on the wing to slow the animals by 16%. However, the bats with the immobilised wing membrane muscles managed to compensate for the increased drag by increasing their wing beat amplitude from ∼60 to ∼75 deg and tipping the wing more to sweep it through a less vertical angle.

‘Reduced forward speed, reduced ability for low-speed flight and changes in kinematics [when the plagiopatagiales were inactive] are consistent with the plagiopatagiales playing a crucial role in reducing drag through active control over membrane tension indirectly controlling camber’, says Cheney, explaining that using the muscles to tension the wing membrane helps bats to fly at higher speeds. However, the team points out that the bats with the paralysed muscles could have taken advantage of their slacker wing membranes to select slower preferred speeds; so why didn't they? They suspect that the animals may not be able to provide the additional flight power required to remain aloft at the lower speeds caused by bowed wing membranes. The researchers conclude, ‘Jamaican fruit bats likely always employ their wing membrane muscles during sustained flight to control camber [bowing] and to enhance flight efficiency’. And, happily, the bats soon recovered full use of their plagiopatagiales muscles after their wing slackening experience to regain full bat mobility.

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J. C.
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Bats actively modulate membrane compliance to control camber and reduce drag
J. Exp. Biol.
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