A form of superfast muscle, not previously known to exist in mammals, endows bats with their echlocating abilities.
Insect-eating bats hunt and navigate by firing off salvos of high-pitched soundwaves. The echoes as these waves bounce back to the bat lead the animal to its lunch. But the rate at which the bats emit these sounds isn't constant. As they home in on their quarry, they ramp up the rate of sonar pip production from 40 pulses per second to almost 200 pulses per second to ensure maximum accuracy.
This is known as the "terminal buzz", but it has always left scientists with a problem - how to explain this rapid rate of sound production and why this is limited to 200 pulses per second? Is this because a faster terminal buzz would make it too tricky to match up calls and echoes, or does the rate at which the muscles are capable of working restrict the maximum buzz frequency?
To answer this question, University of Southern Denmark scientist Coen Elemans and his colleagues recorded bats hunting using an array of 12 regularly-spaced microphones in order to accurately distinguish the timings between calls and echoes. No new call, they found, was issued until the bats' ears had picked up the echo from the previous call, and as the frequency of the calls climbs the animals shorten their duration to prevent any overlaps. The bats could therefore theoretically increase their call rate to about 400 times per second without any ambiguity. This means that echo-processing does not limit the rate of call repetition during the terminal buzz.
Next the team turned their attention to the animals' vocal cords. To make each sound pulse, the bat needs to tense and relax the muscles linked to its vocal folds. But the fastest mammalian muscles are known to contract is about 40 times per second, a far cry from the 200 the bats must be capable of to produce their terminal buzz.
To discover how the bats were bypassing this problem, Elemans and his colleagues electrically stimulated individual muscle fibres while simultaneously recordingthe force generated. Surprisingly, the muscles appeared to be able to work superfast, at rates of up to 200 contractions per second, sufficient to create the rapid pulses of sound needed for the buzz. Although muscles this fast had been seen before in toadfish and some songbirds, they'd never been described in mammals.
What makes these muscles work so quickly remains to be see. But together with flight, this evolutionary advance, say the researchers in their paper in Science this week, is the crucial linchpin that endowed bats with the ability to exploit the previously unrealised foraging niche of night-active insects...