Driven batty, katydids change tune.

Driven batty, katydids change tune

Female katydids are all ears when it comes to finding a mate. The cricket-like insects locate potential mates by moving toward the sound of male katydids rubbing their forewings together. In tropical regions populated by insect-eating bats, however, a male's chirping may attract more than the bug bargained for. Indeed, bats are quite good at locating tasty katydids by their mating calls, and the resulting selective pressure has brought about a variety of changes in katydid mating behavior, new research suggests.

Jacqueline J. Belwood of the University of Florida in Gainesville and Glenn K. Morris of the University of Toronto in Mississauga, Canada, report in the Oct. 2 SCIENCE that Panamanian katydids living in bat-infested areas adjust their mating calls in ways that minimize the possibility of being overheard by bats. Katydids in bat-free neighborhoods have conspicuous calls, characterized by frequent repetitions of chirpings over a broad band of frequencies. In contrast, the researchers find, katydids in bat-infested areas have higher pitched, purer tones that are more difficult to locate, and they tend to spend a lot more time simply being quiet.

The altered behavior makes sense, Morris told SCIENCE NEWS, "given the fact that from the point of view of the insect, every time you open your mouth--so to speak--or every time you make a noise, you're attracting predators.'

To evaluate the actual risk of katydid talkativeness, the researchers put individual katydids in large, screened flight cages with insect-eating bats. Among the males, infrequent chirpers (less than one call per minute) survived an average of 34 minutes before becoming bat food, while frequent chirpers (60 calls per minute) survived an average of 26 seconds. Totally silent females went completely unnoticed.

The problem, of course, is that the less chirping that male katydids do, the more difficult it is for a female katydid to find herself a mate. This difficulty is addressed, the researchers suggest, by another mode of katydid behavior that has been observed for some time but is only now becoming well understood. Male katydids supplement their shortened songs with complex, vigorous body vibrations called tremulations. These are inaudible to bats, but female katydids on the same plant stem or twig can recognize the vibration characteristic of their own species and can follow the vibration to its romantic source.

"It turns out that the number of katydids that are sending signals by bouncing up and down on vegetation is very large,' Morris says. Depending on the plant she's resting on, a female may be able to feel and respond to the vibrations from several meters away. "So the systems are sort of integrated,' he says, "with the sound being more of a long-range thing, and the vibrations helping out at shorter ranges.'

One final mystery remains, however. Of the katydid species examined in the current research, only males exhibited any chirping behavior--leading the researchers to expect that bats would find-- and eat--more males than females. The researchers decided to confirm that theory by examining "leftovers' in the nests of katydid-eating bats. It seems that bats pluck the wings off of katydids before eating ("From the bat's point of view it's like throwing away peanut shells,' Morris says), and male wings are easily differentiated from female wings in the bottom of the nest.

Much to their surprise, the researchers found that approximately half of the bats' diet consists of female katydids, even though the females do not chirp. They are unable to explain this finding. But it's possible, they say, that females may get caught while making their way toward the sporadically calling males.