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Well-Tuned Bats: These animals are what they hear

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10:46am, June 9, 2004
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Bats living side-by-side may, in effect, be in different worlds. Two studies of the animals' beeps find that their hearing is differently tuned in ways likely to affect their mating and hunting.

On two Indonesian islands, three groups of the large-eared horseshoe bat hunt in the same forests but rely on different sound frequencies to detect insect prey, says Tigga Kingston of Boston University. The three groups of Rhinolophus philippinensis appear to be splitting into new species, she and Stephen Rossiter of Queen Mary College in London report in the June 10 Nature. If so, this would be the first mammalian example of a species dividing within a habitat, Kingston says.

In the same journal issue, other researchers compare five species of bats in Germany, all in the Myotis genus. Although these bats naturally forage in similar settings, they performed differently in a test of insect detection. The bats have corresponding differences in their calls and in details of their hunting grounds, say Björn Siemers and Hans-Ulrich Schnitzler of the University of Tübingen.

Most bats hunt by echolocation. To monitor their world, they send out beeps and listen for variations in the echoes.

Tests showed that, for echolocation, each subspecies emits a different multiple, or harmonic, of the same fundamental frequency, say the researchers. The bats' ears have an exquisite sensitivity to their own harmonic frequency. The researchers calculated that such differences in echolocation render prey of particular sizes or at certain distances invisible to one subspecies but not to another.

Bats within each group seem to communicate with each other via their own echolocation frequency, says Kingston, so individuals would find mates with similarly tuned hearing—a recipe for species splitting. The researchers' analysis of bat DNA suggests that the subspecies are beginning to diverge.

Menno Schilthuizen, an evolutionary biologist at the University of Malaysia Sabah in Kota Kinabalu, says that he'd like to see direct tests of whether call frequencies affect mating. "However, overall, I think it is a wonderful study," he says.

The bats in Germany don't use a constant tone, as the Indonesian bats do, but instead emit bursts of sound that slide down frequencies. The extent of the slide varies with species.

Siemers and Schnitzler dangled a mealworm at various distances in front of a vertical, nubby carpet that created a background of confusing echoes. The five species differed in how well they could capture the mealworm between 5 and 10 centimeters from the carpet, the researchers report.

The bats that nabbed insects closest to the acoustic clutter have calls with the greatest frequency slide and naturally forage in tree canopies rather than over rivers.

This study "is the first to provide empirical evidence that seemingly minor differences in call design can have real behavioral consequences," say Brock Fenton of the University of Western Ontario in London and John Ratcliffe of the University of Toronto at Mississauga in the same issue of Nature.

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