To us, the calls of male bushcrickets are a comforting sound on warm summer evenings. But to a female insect, they are a cacophony of potential mates,each competing for her attention; she has to decide which call is produced by a suitor from her own species. Species differ in the pattern of sound they produce, varying the frequency, length of pulses and intervals between them to produce their distinctive calls. One group, the American coneheads, are puzzling because their calls are repeated at higher rates than the insect's ear can resolve. But if females can't hear the pattern of sound, how do they know which call is the right one? Scientists from the University of Missouri set out to discover how two species of conehead recognise their suitors' calls() and found that different species listen for fundamentally different aspects of the serenade.
Johannes Schul explains that most insect calls repeat pulses between 10 and 80 times a second (Hz). However, above 100 Hz, the insect ear becomes less able to distinguish the individual pulses. Coneheads, named for the horns between their antennae, have extremely fast pulse rates–up to 250 Hz is common. Wanting to know how females distinguish between these high frequency calls, Joshua Deily and Schul studied two species that look almost identical and live alongside each other in eastern north America: Neoconocephalus robustus has regularly spaced pulses, at 200 Hz, whereas Neoconocephalus bivocatus produces pairs of sounds with a longer gap every other pulse.
The team used a clever piece of apparatus called a walking compensator to measure exactly what features of the call lead the lady to her man. They placed a female bushcricket on a moveable ball inside a dark, soundproof chamber, and played her artificial calls, waiting to see which one she responded to most enthusiastically. Using a system of cameras and motors that moved the ball to compensate exactly for her movements, the team were able to record the direction she walked in response to the calls.
So which aspects of their suitors' calls were the females responding to?The team found that, as suspected, a female N. robustus cannot hear the fast pulses, but responds to a continuous sound. And she only responds to what she perceives as a continuous sound; break the call with intervals longer than 4 ms and the females usually ignore it. For N. bivocatus, the story is quite different. They also fail to hear the full pattern of the male's double-pulsed song. Instead, each double pulse becomes one long pulse and they listen out for the longer interval between the merged pulses,responding to the frequency of that interval. Providing the longer intervals'frequency is around 87 Hz, it doesn't matter how long the intervening pulses are or how many the male produces, the female still responds.
Why does the male N. bivocatus toil away making double pulses when it could simply produce a longer single pulse 87 times a second? Schul suspects that the ability to recognise call patterns by pulse rate evolved late in the group. He explains that the ancestral conehead may have had imperceptibly high pulse rates, but N. bivocatus found a way of providing a perceptible pulse rate, by leaving a longer interval between every other pulse. Schul adds that the finding that two closely related species have developed such different call patterns undermines the longstanding dogma that call recognition systems evolved by sexual selection. `The jump in recognition mechanism between two such similar species is the interesting evolutionary phenomenon,' he says.
