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Can you hear me now?

  • Large groups of bats can 'jam' each others calls when they are in a roost
  • Bats produce longer and more intense calls in busy environments
  • They also use a slightly higher frequency to help detect their own echoes 
  • This is comparable to humans shouting to be heard at a noisy party
By RICHARD GRAY FOR MAILONLINE PUBLISHED: 11:00 EST, 23 December 2015

For an animal that uses sound to find its prey and navigate while in flight, living in large groups of other noisy animals should be a disadvantage.

But bats have a strategy to overcome the problem of hearing the faint echoes of their own calls amid the cacophony of their neighbours – they 'shout'.

Researchers have discovered the flying mammals, which navigate using echolocation, employ a technique also used by humans at noisy parties.
When faced with interference from other bats - known as jamming - they produced calls that were longer and more intense.

These calls were similar to those used by bats when they are flying in wide open environments where it may take longer for sound to be reflected back to them.

The researchers from Tel-Aviv University said this approach may help them pick out their own calls from the noise of others when they come in to land on busy roosts.

Writing in the journal Proceedings of the Royal Society B, Eran Amichai, a zoologist from the university and his colleagues, said: 'Active-sensing systems such as echolocation provide animals with distinct advantages in dark environments.

THE SECRET OF HOW BATS STAY AIRBORNE AT LOW SPEEDS

Bats are among the most manoeuvrable animals on the planet in mid-air, but now scientists may have unravelled how they manage it.

Scientists from the Chinese Academy of Sciences have found bats change their wingspan to enhance lift at low speeds.

The extra lift is mainly generated by increased vortices around the leading-edge of the wing.

Dr Guowei He, who led the research at the Institute of Mechanics in the Chinese Academy of Sciences, said this mechanism could be used to produce new flapping micro-vehicles.

Writing in the Journal of the Royal Society Interface, Dr He and his colleagues said: 'Dynamically changing wingspan leads to the higher efficiency in terms of generating lift for a given amount of the mechanical energy consumed in flight.'

'For social animals, however, like many bat species, active-sensing can present problems as well - when many individuals emit bio-sonar calls simultaneously, detecting and recognising the faint echoes generated by one's own calls amid the general cacophony of the group becomes challenging.

'This problem is often termed "jamming" and bats have been hypothesised to solve it by shifting the spectral content of their calls to decrease the overlap with the jamming signals.

'We tested bats' response in situations of extreme interference mimicking a high density of bats.

'Under severe interference, bats emitted calls of higher intensity and longer duration, and called more often.

'Slight spectral shifts were observed but they did not decrease the spectral overlap with jamming signals.

'We also found that pre-existing inter-individual spectral differences could allow self-call recognition.'

The researchers trained four Kuhl pipistrelle bats to land on an elevated wooden platform and played sounds to mimic the calls of other bats.

In some cases they played sounds aimed at mimicking the noise produced by a busy roost filled with other bats, while in another they replicated the noise of a quiet roost.

They found that when attempting to land on the quiet roost, the bats produced brief calls to help them navigate their landing.

However, when the roost was busier, the bats produced calls almost continuously at a much higher intensity.

The bats only slightly shifted the frequency of their calls upwards. The researchers also tested how the animals would cope with an environment that was cluttered with obstacles.

They added: 'It is already well known that bats will shorten their calls when closing in on a target or in a cluttered environment.

'We show that acoustic masking will induce the opposite effect of increasing call duration.

'The bats which we jammed in the cluttered environment increased call duration making it more similar to that of calls they emitted in the uncluttered environment.

'When approaching the landing platform, bats used longer calls as if they were delaying the initiation of their approach phase.'

BATS HAVE HEAVY WINGS TO HELP THEM PERFORM MID-AIR AEROBATICS

They are perhaps the most manoeuvrable creatures in the animal kingdom, able to perform mid-air aerobatics to snatch insects and flip upside-down to hang off cave ceilings or branches.

But bats agility in the air may be down to something that other flying animals have attempted to avoid – heavy wings.

While most birds and insects have evolved lightweight wings to help them stay aloft, the long extensions of skin, tendons and bone in bats are heavy compared to their body size.

However, it appears these could be the mammal's greatest assets, as scientists have found they use the extra weight in very precise ways to help them turn.

By throwing their wings in different directions as they fly, they can generate inertia that helps them twist in the air, much like Olympic divers do to perform flips before they hit the water.

Bats can often congregate and roost in large numbers (like the Mexican freetail bats pictured) which can make it difficult for them to distinguish the echoes of their own calls from those of others. The new research has helped to unpick how they are able to do this with a remarkably simple approach 

The researchers found that when approaching a roost with a low density of competing calls, the bats tended to produce short and relatively weak calls (pictured top) but when they approached a roost with high density of other bats they produced much longer and far more intense calls (pictured bottom)

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