Saturday, December 30, 2006

 

New Research on the Co-evolution of Bats and Moths

Current understanding of the co-evolution* of bats and moths has been thrown into question following new research reported in Current Biology.

Dr James Windmill from the University of Bristol has shown how the Yellow Underwing moth changes its sensitivity to a bat's calls when the moth is being chased. And in case there is another attack, the moth's ear remain tuned in for several minutes after the calls stop.

Dr Windmill said: "Because the moth cleverly tunes its ear to enhance its detection of bats, we must now question whether the bat in turn modifies its calls to avoid detection by the moth. In view of the vast diversity of bat calls, this is only to be expected.

"To date, this phenomenon has not been reported for insects or, in fact, for any other hearing system in the animal kingdom. These findings change our understanding of the co-evolution of bats and moths and have implications for the hearing of many other animals."

It has been known for over 50 years that moths can hear the ultrasonic hunting calls of their nocturnal predator, the bat. Previously it was thought that these ears were only partially sensitive to the sound frequencies commonly used by bats and that bats would make their hunting calls inaudible to moths.

But now it appears that even though moth ears are among the simplest in the insect world - they have only two or four vibration sensitive cells attached to a small eardrum - moths are not as deaf as previously thought.

As a bat gets closer to the moth, both the loudness and frequency (pitch) of the bat's calls increase. Surprisingly, the sensitivity of the moth's ear to the bat's calls also increases. This occurs because the moth's ear dynamically becomes more sensitive to the frequencies that many bats use when attacking moths.

This multidisciplinary work involved engineers, biologists and physicists; biological measurements are accompanied by a mathematical model explaining the basis for the unconventional behaviour of the moth's ear.

Original Press Release ("How to avoid a bat" - 19th December 2006) available via this link.

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Based on the paper:

Keeping up with Bats: Dynamic Auditory Tuning in a Moth

James Frederick Charles Windmill1, Joseph Curt Jackson, Elizabeth Jane Tuck and Daniel Robert

Abstract

Many night-flying insects evolved ultrasound sensitive ears in response to acoustic predation by echolocating bats. Noctuid moths are most sensitive to frequencies at 20-40 kHz, the lower range of bat ultrasound. This may disadvantage the moth because noctuid-hunting bats in particular echolocate at higher frequencies shortly before prey capture and thus improve their echolocation and reduce their acoustic conspicuousness. Yet, moth hearing is not simple; the ear's nonlinear dynamic response shifts its mechanical sensitivity up to high frequencies. Dependent on incident sound intensity, the moth's ear mechanically tunes up and anticipates the high frequencies used by hunting bats. Surprisingly, this tuning is hysteretic, keeping the ear tuned up for the bat's possible return. A mathematical model is constructed for predicting a linear relationship between the ear's mechanical stiffness and sound intensity. This nonlinear mechanical response is a parametric amplitude dependence that may constitute a feature common to other sensory systems. Adding another twist to the coevolutionary arms race between moths and bats, these results reveal unexpected sophistication in one of the simplest ears known and a novel perspective for interpreting bat echolocation calls.

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*Info on co-evolution:

In biology, co-evolution is the mutual evolutionary influence between two species. Each party in a co-evolutionary relationship exerts selective pressures on the other, thereby affecting each others' evolution. Co-evolution includes the evolution of a host species and its parasites, in examples of mutualism evolving through time. Few perfectly isolated examples of evolution can be identified. Evolution in response to abiotic factors, such as climate change, is not coevolution (since climate is not alive and does not undergo biological evolution). Evolution in a one-on-one interaction, such as that between a specialized host-symbiont or host-parasite pair, is coevolution. But many cases are less clearcut: a species may evolve in response to a number of other species, each of which is also evolving in response to a set of species. This situation has been referred to as "diffuse coevolution". And, certainly, for many organisms, the biotic (living) environment is the most prominent selective pressure, resulting in evolutionary change.

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