May 2013 Archives

More Creatures' Secrets - A Moth with Amazing Ears

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From New Scientist:

The greater wax moth is flapping gently through the dark, looking for a mate, when it suddenly hears a high-pitched click. The sound is well outside the range of human hearing, but the moth has no problem picking it up. It swerves to the right - and escapes the jaws of a predatory bat by inches.

Greater wax moths have hearing like no other animal we know of. They can hear sounds that are so high-pitched that no known bat can produce them.

In the evolutionary race for survival, this moth has a head start.

Put simply, greater wax moths are a pest. Their larvae often live inside honeybee nests, where they survive on a diet of little more than beeswax. They have a particular taste for the brood combs where the bee larvae live, and can quickly trash the nest.

Adult moths only leave the hive to mate. Males gather on nearby trees and,shortly after sunsetbegin making calls to females, at frequencies above the range of human hearing.

Higher-pitched still are the calls of their predators: bats. While the male moth's calls range from 90,000 to 95,000 hertz, bats echolocate using sounds often closer to 110,000 Hz.

Evolution has pushed moths to keep up, so although they can't produce calls in the same range as bats, they can hear them coming. One North American moth can hear sounds up to 150,000 Hz - good, but not good enough to escape all bats, whose calls can reach 212,000 Hz.

I can hear you!

Enter the greater wax moth. Hannah Moir and colleagues at the University of Strathclyde in Glasgow, UK tested the hearing of 20 adult moths by playing them a wide range of sounds and measuring both the vibration of their tympanal membrane - the moths' "eardrum" - and whether a signal was transmitted down their auditory nerve.

Each sound was at 90 decibels, which is about as noisy as heavy traffic but not quite as loud as most bat echolocation calls. All the moths' tympanal membranes vibrated strongly to sounds at a frequency of 300,000 Hz, and 15 out of the 20 also showed strong neural signals.

Incredibly, that means the moths can hear a sound that no known animal makes.

Moir says there are two possible explanations. "It could be that the bats are producing higher frequencies than can be recorded," she says. Microphones struggle to record sounds higher than 150,000 Hz, and bat calls can be particularly difficult to capture.

Alternatively, the moths' sensitivity to high-pitched sounds could be an evolutionary accident, Moir says. The moths need to detect bat calls quickly to take evasive action. The physics of sound means that any sensor that can pick up a wide range of frequencies will also have a fast response time. So their sensitivity to a wide range of frequencies could be a fringe benefit of the ability to pick up the bats' calls faster.

Bats may eventually evolve higher-pitched echolocation calls to help them hunt those moths that can listen in on their existing calls. But this won't help them catch the greater wax moth: it is several steps ahead of them.

Despite their outstanding abilities, there doesn't seem to be anything special about the greater wax moth's ears. Moir says its tympanal membranes look pretty much like those of other insects, the only difference being that they are unusually thin - which would make them more sensitive to high-pitched sounds. So, as incredible as it seems, the moths' hearing may not be unique, says Moir.

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