Changes to insect-seeking calls of horseshoe bats may drive new species formation

06/11/04

BU geographer Tigga Kingston links speciation potential to acoustic differences in calls

(Boston) -- It may not matter whether there is a mountain high enough or a river wide enough to keep members of a species apart. New species may diverge and form because of something as fundamental as a call to dine.

According to new research by Tigga Kingston, a research associate in the Department of Geography at Boston University, and Stephen Rossiter, a National Environment Research Council research fellow in the School of Biological Sciences at Queen Mary, University of London, geographical barriers may not be necessary for speciation. In their study of one species of bat in Southeast Asia, the scientists found that the bats were diverging into exclusive groups primarily because of acoustic differences in the calls they make to locate the insects they eat.

Their finding challenges long-standing theory that geographical barriers are the mechanism by which new species evolve. This new perspective on an old controversy appears in the June 10 issue of Nature.

For centuries, theorists have debated how new species form. Traditional thought holds that speciation occurs over long periods of time as a result of interbreeding among members of a group that are, for one reason or another, isolated from other members of the same population.

If, for example, geologic activity changed an area so that mountains rose and split a region populated by a species of bat, the bat populations on either side of a mountain would no longer be able to breed together. Their genetic information, including changes that lead to physical or behavioral adaptations to the demands of their environments, would no longer be pooled. Future generations of bats found on one side of the mountain would begin to diverge genetically from those on the opposite side. Eventually, the two populations of the bat species would become sufficiently different to qualify as separate species.

In their study, Kingston and Rossiter found that large-eared horseshoe bats (Rhinolophus philippinensis) are diverging to three sizes -- small, medium, and large -- despite their living next to one another. The researchers also discovered the animals' echolocation calls are harmonically distinct. This means the differently sized bats no longer hunt one another's food because they no longer can even "see" one another's food. According to the researchers, when food sources can be partitioned so precisely, it is a short hop to species divergence. They have, in fact, dubbed the divergence process, "harmonic hopping."

The researchers conducted their work in the rainforests of Sulawesi in Southeast Indonesia, a region known as "Wallacea" after Alfred Russell Wallace, a 19th century explorer who, together with Charles Darwin, posited some of the basic ideas that form current evolutionary theory. This region abounds in bat species. The researchers focused on the large-eared horseshoe bat, speculating that the different sizes might indicate a greater difference -- that of species divergence.

When they analyzed the recorded echolocation calls of the bats, Kingston and Rossiter found each size variant calls at a different acoustic variation of one fundamental frequency: 13.5 kHz. They found the large form calls at 27 kHz, the medium one at 40.5 kHz, and the small one at 54 kHz, each frequency a mathematically precise permutation of the basic 13.5-kHz frequency. These harmonic differences, according to the researchers, determine the type of insect any one size of large-eared horseshoe bat can hunt and feed upon. The low harmonic frequency used by large-sized horseshoe bats, for instance, is perfect for detecting large insects over long distances yet completely misses small insects, which are easily scoped out by small-sized horseshoe bats using the higher harmonic frequency.

The researchers speculate that the harmonic differences also can affect other communication among the bat groups, including calls that help them find one another for mating purposes. Harmonic hopping could, therefore, hinder mating between sizes, a key step in speciation. Taken together, the researchers say their findings indicate that sensory ecology may have an important function in speciation and may explain the rapid dissemination of horseshoe bat species in Southeast Asia, where some 30 species have originated in just the past five million years.

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