Urban grasshoppers change their tune for females

CHORTHIPPUS BIGUTTULUS Bow-winged grasshoppers alter the frequency of their calls in noisy areas

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Grasshoppers that live in noisy urban environments are having to change their song, a study has found.

Researchers suggest that high levels of background noise may affect the grasshoppers' mating process.

They say the insects are forced to increase the volume of the low-frequency sections of their call.

Results of the study, by scientists from the University of Bielefeld, Germany, are published in the journal Functional Ecology.

The research, which shows traffic noise could upset bow-winged grasshoppers' (Chorthippus biguttulus) mating system, is the first of its kind, according to lead researcher Ulrike Lampe.


Crickets and critters

"Effects of man-made noise on acoustic communication has only been studied with vertebrates, so far," said Ms Lampe, a PhD student at the University of Bielefeld's Department of Evolutionary Biology.

The scientists caught 188 male bow-winged grasshoppers from noisy roadsides and quiet rural locations.

According to Ms Lampe: "Bow-winged grasshoppers are a good model organism to study sexual selection because females can respond to male courtship songs with their own low-frequency acoustic signal, if they are attracted to a male song."

The grasshoppers produce their mating call by rubbing a toothed file on their hind-legs against a protruding vein that is located on their front wings.

The male's song consists of short phrases of two to three seconds that increase in amplitude towards the end. The first part of the call comprises slower ticking sounds that increase in speed and amplitude, leading to a buzzing sound towards the end of the phrase.

Testing times

In order to stimulate the males to begin mating calls, scientists exposed the males to a female and recorded the results in the laboratory.

The team then analysed the differences between the results of each group of grasshoppers.

Results showed that compared to males from rural locations, urban grasshoppers "shift the frequency peak of the lower part of their spectrum upwards," Ms Lampe explained.

This would make sense to avoid low-frequency noise, as traffic noise could mask signals in that part of the frequency spectrum.

The team's findings demonstrate that traffic noise could be upsetting the grasshopper's mating system.

"Increased noise levels could affect grasshopper courtship in several ways," commented Ms Lampe.

"It could prevent females from hearing male courtship songs properly, prevent females from recognising males of their own species, or impair females' ability to estimate how attractive a male is from his song."

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