Sometimes being ugly pays off

Bats use echolocation (biological sonar) to navigate and to find prey. This involves emitting ultrasonic waves which bounce off objects in the bat’s environment. The bat detects these reverberations, which are translated into an auditory image in its brain.

Whereas most species of bats emit the pulses of ultrasound from their mouths, some, such as the horseshoe bat (Rhinolophus ferrumequinum), do so from their noses. This species has intricate, wrinkled protuberances, called noseleaves, around its nostrils. It was widely believed that these structures are involved in echolocation, but, until now, exactly how was not clear. Computational physicists Rolf Müller and Qiao Zhuang from Shandong University in Jinan, China have used three-dimensional X-ray scanning to generate a computer model (below left) of the horseshoe bat’s noseleaves, and how they interact with the emitted ultrasound pulses. They show that the structures are used to focus the emitted beams.

Here’s the abstract to the paper in Physical Review Letters in which the findings are reported:

Horseshoe bats emit their ultrasonic biosonar pulses through nostrils surrounded by intricately shaped protuberances (noseleaves). While these noseleaves have been hypothesized to affect the sonar beam, their physical function has never been analyzed. Using numerical methods, we show that conspicuous furrows in the noseleaf act as resonance cavities shaping the sonar beam. This demonstrates that (a) animals can use resonances in external, half-open cavities to direct sound emissions, (b) structural detail in the faces of bats can have acoustic effects even if it is not adjacent to the emission sites, and (c) specializations in the biosonar system of horseshoe bats allow for differential processing of subbands of the pulse in the acoustic domain.

The pulses emitted by the horseshoe bat begin at a frequency of 60 kiloHertz (kHz, thousands of cycles per second), then quickly increase to 80 kHz before dropping back down to 60 kHz towards the end of the pulse. The computer model generated by Müller and Zhuang shows that two horizontal furrows in the noseleaves resonate strongly in response to, and enhance, the 60 kHz pulses which occur at the beginning and the end of the ultrasound beams emitted by the bat.

Essentially, the furrows, which are open-ended cavities, are “beam-shaping devices”. The higher frequency beams emitted by the bat are focused in an oval-shaped spot directly ahead of the bat, whereas the lower frequency beams are focued into a wider spot, part of which is aimed above the bat’s head. When the grooves in the computer model were filled, the simulation showed that low frequency beams would instead be focused in the same way as higher frequency beams. The noseleaves therefore cause different frequencies of sonar beams to be emitted in different spatial patterns, enabling the bat to put to best use the limited energy it has available for echolocation. Müller speculates that these mechanisms enable the bat to simultaneously use echolocation for different tasks, such as navigating through a complex environment and detecting flying insects.

“If you would like to think of the bat looking at the world with an ultrasonic flashlight,” says Müller, the study suggests that the noseleaves produce “an entire array of flashlights, each shining a spotlight of different size, shape, and position on its surroundings.” The findings open up new avenues of research into detailed analyses of how bats echolocate – all facial structures found in bats, says Müller, are now candidate beam-shaping devices – and could also lead to improvements in sonar and radio technology.

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