Predation pressure pushes bushcrickets to evolve new mating strategies
Imagine yourself at a crowded dinner party, with multiple conversations flying at cross-purposes to each other — greetings between friends, flirtation, heated debate. How do we pick out individual voices across the room, let alone carry the thread of our own conversation?
The “cocktail party problem” described above is faced by many species — including humans — but can have additional challenging layers for some. Animals must communicate and locate potential mates while evading predators and filtering out other noisy species. A new study by researchers at Hyderabad Central University and the Indian Institute of Science, Bangalore has found a species of bushcricket with an unusual solution to this quandary. In response to the male call, females of Onomarchus uninotatus—hidden by jackfruit leaves — stay silent, and may not actively seek the male. Instead, the females send covert vibrations through the tree, guiding the male while leaving eavesdropping predators unaware.
Bushcrickets, or katydids, use camouflage that is difficult to spot, but their evening chorus is unmistakable across the globe. In most species of bushcricket, the female locates the calling male and travels towards him. But that road is perilous: even if the travelling female avoids predators, the male might not. “For many species of cricket and bushcricket, broadcasting an acoustic call comes with disadvantages. Both predators and parasites can eavesdrop on these calls,” says Kaveri Rajaraman, faculty at Hyderabad Central University and co-author of the study.
These predators include bats, well adapted to pick up on the high-frequency courtship calls of their insect prey. But earlier work by Rajaraman and colleagues found that O. uninotatus has evolved an unusually low frequency call, potentially reducing male predation pressure. Theoretically, the balance of predation pressure between sexes should influence who can afford risky mate-seeking behaviour during courtship. Thus a shift in pressure, away from males, could allow a species to evolve novel communication strategies.
Indeed, the researchers found that wild-caught O. uninotatus females did not always search for the source of male song — an often fatal part of the process, as evidenced by the ease with which echo-locating bats catch flying females. A third of the females did not seek out the source of male song, and only about half successfully found the playback speaker.
What females universally did, however, was vibrate in place on their jackfruit tree branch, out of phase with the male chirps. Male bushcrickets in turn were consistently able to follow female vibrations to their source. Curiously, however, males only started searching for the female if they detected both the female’s vibration and a male courtship call — either their own or that of another caller. The authors suggest that this may indicate a “satellite strategy” on the part of the males, piggybacking on the communications efforts of others in order to disperse predatory risk.
Vibratory communication combined with sound is unusual in insects. It can be likened, in the noisy party described above, to a woman responding to a man’s advances by tapping in Morse code — a strategy that will keep her message discreet, but comes with its own limitations. Vibrations require that bushcrickets be near one another, and are more energy-intensive than acoustic calls.
The researchers predict, therefore, that this is just one mating strategy amongst many: when the bushcrickets are farther apart, more conventional strategies may apply.
All that [this second loop] does is create a possibility for the male to walk towards the female, when the female is on the same tree,” says Rajaraman. This finding opens up many more avenues of research, to see how predation might have caused that possibility to evolve.