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Page last updated at 12:05 GMT, Thursday, 10 March 2011
Bat navigation explained by robotic moth experiment
By Ella Davies
Earth News reporter

Pteronotus parnellii (c) Brock Fenton
Parnell's moustached bat found the robotic moth with ease

A test with a robotic moth has revealed how bats use echoes to find insects.

By using the moth scientists found that fluttering insects produced "siren-like" echoes, which could be detected by bats with more sensitive hearing.

Bats that emitted continuous radar-like calls approached "Robo-moth" in far greater numbers than bats that produced intermittent calls.

Researchers think that this continuous echolocation may have evolved to help the bats locate fluttering prey.

The robotic moth (c) Louis Lazure
A tiny flag, 'fluttered' by a large motor, simulates a moth's movements

Researchers from the University of Western Ontario, Canada, created a robotic moth to simulate a fluttering insect.

They set up their moth in forest locations in Taiwan and Belize and recorded how different species of bat interacted with it.

"For the eye, it doesn't look at all as an insect, but acoustically, it worked quite well," said student Louis Lazure who helped to test the robotic insect.

The scientists used a bat detector to determine which species of bat approached the simulated insect more often.

The study, which was published in the Journal of Experimental Biology, aimed to test the theory that some bats' style of echolocation is better suited to hunting flying insects than others.


A bat approaches Robo-moth in the dark forests of Taiwan

In previous research, one of the researchers involved in this study, Dr Brock Fenton, identified that bat echolocation can be divided into two different categories.

Rather than relying on their sight, most species of bat broadcast intermittent search calls and wait for the echoes of these calls. They use these to create a sound-based map of their surroundings.

However, 20% of bat species can emit continuous calls because they are able to tell the difference in frequency between the sounds they make and the echoes.

Despite the saying, bats are not blind.
To hunt in the dark however, they emit high-frequency calls and use the echoes of these calls to build a sonic map of their surroundings.
Their calls are largely beyond the range of human hearing.

This ability comes from the bats' advanced hearing - their ears are more sensitive to different frequencies, allowing them to pick out the echoes even when they are emitting calls.

Intermittent-calling bats' reactions are slowed as they must wait for an echo response. Continuous callers, on the other hand, can track their prey's every move.

So Dr Fenton thought that bats that are able to differentiate between frequencies in this way might be particularly adept at detecting flying insects, such as moths.

Wailing siren

To test the theory, Dr Fenton's team created an accurate simulation of a fluttering insect and played recorded bat calls near it.

The reflected calls had an oscillating pitch, like a wailing siren, whereas stationary objects including leaves and branches produced single-toned echoes.

Dr Fenton was convinced that the bats would be able to tell the difference between these two types of echoes. To prove it, he and his team tested "Robo-moth" in the field.

Rhinolophus monoceros (c) Ying-Yi Ho
Rhinolophid bats are very efficient at catching fluttering targets

During their research in Taiwan, the team found almost all of the approaches to the robotic moth were made by continuous-calling bats, including lesser horseshoe bats and great roundleaf bats.

"People had predicted that the bats should be able to detect fluttering targets in clutter... and this is an experimental demonstration that they do just that," said Dr Fenton.

There were however four species of bat that were not continuous callers that still approached Robo-moth. Dr Fenton believes these bats may be "intermediate species" - an evolutionary link between intermittent and continuous callers.

He suggests that bats could have evolved the radar-like echolocation strategy to improve their chances of catching more nutritious insect prey.

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