Bat studies to aid roving robots

Detailed studies of the ways bats and dolphins use sound to echo-locate could soon help people with cochlear implants listen in stereo.

The research could also help robots improve how they find their way around hazardous environments.

The studies have revealed some of the tricks echo-locating animals use when catching prey.

Adapting the tricks should produce artificial systems that do a better job of locating static and moving objects.

Sound lesson

Bats and dolphins are well-known for their use of sound but relatively little is known about all the mechanisms that underlie this ability, said Professor Robert Allen from the University of Southampton who has co-ordinated the investigation into echo-location.

The research project, called Biologically Inspired Acoustic Systems (BIAS), has looked at the physiology of echo-locators, particularly bats, and how they structure the sounds they emit to help them navigate and catch prey.

"Colleagues at Leeds University have produced casts of bat heads and we're able to bounce sound off them to see what happens and to try to understand how the head shape affects the echoes received at the ears," said Professor Allen. "These are fundamental studies."

In parallel with these laboratory studies, a small, lightweight backpack has been developed that can be strapped to adult Egyptian fruit bats to listen to the sounds they emit while flying.

It was important to be close to the origin of a signal, said Professor Allen, because some of the subtleties of emitted sounds and their echoes will be lost with a microphone that records bats as they fly past.

The recordings have revealed the complexity of the sounds that some bats emit. The creatures being studied by researchers from the University of Leeds and Stratchclyde emit pulses of sound at fixed and fluctuating frequencies and many typically last only a quarter of a millisecond.

The fixed frequency pulses seem to help work out, via doppler shift, how prey, for example, is moving relative to the bat.

The joint Leeds and Strathclyde work is looking at developing ultrasonic transducers that can be used in small robotic vehicles that can go to places that are too dangerous or small for humans to get at.

"We're currently looking to apply these methods to positioning of robotic vehicles, which are used for structural testing," said Simon Whiteley from the centre for ultrasonic engineering at Strathclyde.

The robots might be able to use the echo-locating techniques to spot cracks in the walls of reactors or containment vessels.

Dr Whiteley and colleagues published their results in the journal Bioinspiration & Biomimetics.

Other pulses are used to help the bat focus on an object, spot very small objects and time their attack.

Chirps, sounds that start at one frequency and slide up or down to another over a short interval, help the bat spot where an insect is. The frequencies a bat uses are related to the size of its prey and the size of the bat.

The researchers also found that overlapping signals let bats spot objects that are smaller than the wavelengths of the sounds they emit. Researchers are pursuing this insight to see how it can help improve the resolution of sound-based imaging systems.

Hearing textures The researchers also looked at the way that dolphins echolocate

"Some bats have a very sensitive region of the cochlear around their call frequency so they can use doppler shift to get information as to whether an insect is flying towards or away from them," said Professor Allen.

For instance, he said, the sophisticated sound signals give a bat clues about the texture of an object. Adapting this could make medical ultrasound systems more sensitive and able to pick out different tissue types beneath the skin.

Research partners are also looking at using this technique to help in probing the ground for oil or other mineral deposits. Work is now going on to bounce the signals off different types of geological materials to determine the signals they reflect.

One application being pursued is adapting the way that bats process sound to improve the location-finding abilities of hearing-impaired people with a hearing aid or a cochlear implant.

'Echoes' in bat and dolphin DNA

Scientists have found a striking similarity in the DNA that enables some bats and dolphins to echolocate.

A key gene that gives their ears the ability to detect high-frequency sound has undergone the exact same changes over time in both creatures.

The researchers report their findings in the journal Current Biology.

It may be the first time that identical genetics has been shown to underpin the evolution of similar characteristics in very different organisms.

Nature is full of cases where the path taken by evolution has resulted in the same traits, or phenotypes, developing independently in diverse animal groups.

Examples would include the tusks displayed by elephants and walruses, or the bioluminescence seen in fireflies and jellyfish.

"It's common on a morphological scale but it's assumed not to occur at a DNA level because there are so many different ways to arrive at the same solution," explained Dr Stephen Rossiter of Queen Mary's School of Biological and Chemical Sciences.

"The fact that we're able to link convergence of the DNA with a phenotype I think is unique, and in such a complex phenotype as hearing as well," he told BBC News.

Animal and human

Many bats and toothed whales like dolphins have exceptional hearing, and are able to track down their prey by emitting high-frequency noises and then listening for the echoes that bounce back.

Critical to echolocation are tiny hairs in the inner ear that move in response to sound.

Their keen performance is driven by a particular protein known as prestin, which in turn is encoded by a gene, also known as prestin.

Those bats that echolocate use high-frequency sound to track small prey

Two studies published this week in Current Biology find that this gene in bats and dolphins has picked up the same mutations over time.

"We've found a whole suite of amino acid changes that are common to these two groups that have evolved in parallel, convergently," Dr Rossiter said.

Both research teams also have evidence showing that these changes to prestin were selected for, suggesting that they must be critical for the animals' echolocation for reasons the researchers do not yet fully understand.

"The results imply that there are very limited ways, if not only one way, for a mammal to hear high-frequency sounds," said Professor Jianzhi Zhang of the University of Michigan, US, who led the other study.

This type of research is a beneficiary of the immense and ongoing effort to understand human genetics, which finds interesting targets for biologists from many fields to follow up.

Mutations in the prestin gene in humans have been shown to be associated with the loss of high-frequency hearing. It was this revelation that initiated the study of prestin's role in echolocation.