Tree crickets face low predatory risks during mate-searching

Field assistant Manjunatha K. Reddy making observations during one of the experiments.Viraj R. TorsekarViraj R. Torsekar  

Love is risky in the natural world especially for males of most species because their serenades and movements to actively seek out females can be picked up by predators. But deviating from the norm, tree crickets — both males and females — face low and similar predation risks, finds a study. The low predation levels also question the popular notion that predation is the major factor influencing the evolution of communication, according to scientists at the Centre for Ecological Studies in Bengaluru’s Indian Institute of Science.

The team including then doctoral scholar Viraj R. Torsekar estimated predation risk in the tree cricket Oecanthus henryi, males of which call loudly. The team first conducted surveys in an unused farmland in Karnataka’s Chikkaballapur to identify the main predators of the cricket. Acoustic playback and field predation experiments revealed that this distinction went to green lynx spiders, which could be picking up acoustic cues and vibrations of crickets’ movements.

The team then studied three spatial scales at which the spiders and crickets interacted. First, they quantified the probability that spiders and crickets occurred on the same bush together. They searched around 120 bushes to locate calling males and counted non-calling males, responding and non-responding females and spiders, to find that all the insects co-occurred in each bush at similar probabilities.

Next, they determined the probability of crickets encountering spiders on a bush. They released a wild-caught cricket (calling and non-calling males, responding and non-responding females) and later, a spider, onto the same bush in an outdoor enclosure; successful or failed attacks and captures by spiders counted as encounters. Almost 200 trials revealed that regardless of whether males were calling or not, and whether females responded or not, crickets encountered spiders at the same probabilities. A similar experiment with 105 trials to determine the probability of a cricket being eaten showed that the spiders captured very few crickets (only six), again, on similar probabilities whether males were calling or not and females responding or not.

The three probabilities together reveal predation risk for crickets to be low, and similar across sexes, in the team's paper published in Evolutionary Ecology. While searching for mates, males are thought to face a higher predation risk compared to females but our paper dispels this surprisingly rarely-tested notion, said Mr. Torsekar (currently with The Hebrew University of Jerusalem, Israel).

An alternative explanation for the shared mate-searching behaviour in tree crickets could be the ‘direct benefits’ that females of some insect species receive from males during mating, added Mr. Torsekar. In O. henryi, a gland on the male’s back releases nutritious secretions that the female drinks during mating, he said.