Acoustic benefit of big ears can slow bats down

The bigger a bat’s ears, the better its hearing, is likely to be. But is it, asks Richard Collins

Acoustic benefit of big ears can slow bats down

With monstrous head and sickening cry,

And ears like errant wings,

The devil’s walking parody,

On all four-footed things.

During the short-back-and-sides 1950s, youths with big sticking-out ears weren’t attractive to young ladies. Like Chesterton’s ‘tattered outlaw of the earth’, boys couldn’t hide their ears under long hair. But love-sick Romeos were not alone in their misery; according to researchers in Sweden, big ears also raise problems for bats, though not romantic ones.

Bats send out high-pitched pulses and interpret the echoes reflected from objects in the vicinity. We have no idea what the ‘sound pictures’ they ‘see’ are like, but bats seem to have an accurate grasp of their surroundings. The bigger a bat’s ears, the better its hearing, is likely to be. But is it? The researchers tried to find out.

Detecting insect prey, using an animal equivalent of sonar, is only half the bat’s problem; it has to catch the elusive creatures it detects. To do so, it must be able to manoeuvre efficiently and change direction instantly. Big ears generate air resistance, making movement sluggish and rendering aerobatics difficult. There has to be a trade-off, therefore, between the extra hearing sensitivity large ears provide and the reduced flying ability they entail. A bat ‘can’t have its loaf and eat it’.

Models have been used to simulate the flight patterns of bats. Here, the flying efficiency of live bats was investigated. Two species, one with big ears and one with small ones, were monitored as they flew in a wind-tunnel, lured by hand-outs of food. The brown long-eared bat, a common species in Ireland, was first described by Linnaeus in Sweden in 1758. It has large ears. The smaller-eared species selected, Pallas’s long-tongued bat, is native to South and Central America.

Smoke particles, illuminated by lasers, allowed the researchers to see the movement patterns of the air around the bats in the wind-tunnel. High-speed cameras recorded the animals’ flight behaviour. Analysing the turbulence patterns the bats generated, the energy they expended was calculated, enabling their flying efficiencies to be compared.

The ‘body drag’ of both species proved to be higher than expected. The large-eared bats produced more ‘aerodynamic body power’, but there was a bonus. Large ears, the researchers say, act like an extra pair of wings providing additional ‘lift’.

But big ears, they found, do slow a bat down and this has implications for hunting. “The relatively higher power of the large-eared species results in lower optimal flight speeds and our findings support the notion of a trade-off between the acoustic benefits of large external ears and aerodynamic performance… with large-eared bats generally adopting slow-flight feeding strategies,” they report. One such strategy is ‘foliage gleaning’, where bats hover close to vegetation, picking up spiders moths and beetles from leaves and shrubs.

The Swedish research also helps explain why bats, with a lifestyle similar to that of insect-eating birds, seldom migrate. The ‘external ears act like a pair of brakes’; making flying so energy-intensive that undertaking long journeys isn’t on. The swallows swifts and warblers, coming to Ireland from Africa each summer, don’t have this problem; their ears are tucked inside their heads. Streamlined like jet aircraft, they generate less turbulence. Hibernation is the only option available to their night-shift insect-eating equivalents, the bats.

Are there lessons for us here? Drones carry equipment such as cameras, which generate ‘drag’. Designers might follow the bats’ example by ensuring payloads have aerofoil surfaces to provide additional lift.

  • Jonas Hakansson et al. Body lift, drag and power are relatively higher in large-eared than in small-eared bat species. Journal of the Royal Society Interface. November 2017.

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