Crickets chirping to attract potential mates responding to the calls may also attract something else: predators. But these crickets are no more in danger of being eaten than the silent ones, according to a new study. In effect, it questions the belief that calling males have a higher chance of becoming a meal.
In some insects, such as moths and spiders, the female produces pheromones and the male approaches her to mate. Among frogs, toads, grasshoppers and most crickets, the male produces signals and the female has to approach him to mate. Sometimes, however, the female also calls instead of moving, leaving it to the male to move towards her. Scientists have argued that these differences are because the male benefits more than the female from higher mating rates, so the more dangerous job of looking for a mate also falls on the male.
This led in turn precipitated theories that predators are generally attracted by male displays of singing to find mates and that the male is more at risk of predation than the female. However, these hypotheses did not account for the fact that when the female responds by returning the male’s call or by moving towards the male, she could also attract predators.
Among insects such as grasshoppers and crickets, the females move towards a chirping male, who is generally stationary. Such insects share the responsibility of searching for a mate. But how the shared responsibility affects the male and the female vis-à-vis becoming prey is not clearly known.
To understand if both calling males and responding females were at risk of predation, Viraj Torsekar and his colleagues at the Indian Institute of Science, Bangaluru, set out to explore crickets’ night lives. They studied a tree cricket (Oecanthus henryi) and its main predator, the green lynx spider (Peucetia viridans), both found in bushes in Chikkaballapur, Karnataka. The spider can detect male tree-crickets’ calls as well as female tree-crickets’ movements in response to the calls.
The team calculated the chances of a cricket becoming prey by combining three probabilities: that a cricket is present in the same bush as a spider, that a cricket and a spider meet and that the cricket is eaten in the encounter.
To determine the first probability, they sampled bushes between 7 pm and 9 pm, locating the insects either by sight or by the chirp of calling males. To figure if the spider and the cricket met, the team released a captured cricket and a captured spider into the same bush and checked if they came within a certain distance of each other. In a third experiment, the researchers released spiders close to crickets to see if the spiders attacked the crickets.
They performed all experiments with both calling and non-calling males and responding and non-responding females.
And the experiments revealed that the chance of a cricket being eaten up by a spider was quite low, and that this risk was the same for calling males/responding females and silent males/stationary females. In short, the chance of being eaten was the same for both male and female crickets irrespective of whether they were looking for a mate.
Torsekar said the finding didn’t surprise him because “our feeling was that responding to signals by navigating to reach males would be costly too. The cricket’s ability to escape rather easily when attacked demonstrates this point and seems to contribute to low predation risk regardless of cricket behaviour.”
In fact, of the 106 times they checked, a spider captured a cricket only six times.
“The predation risk being so low was not unexpected either,” he continued. “A predator failing to capture prey misses on a meal, but a prey failing to escape loses its life. So it is not surprising that generally prey is good at escaping predators.”
In fact, for overturning a prevalent idea, the results seem to have caught few off guard. Klaus Reinhold, who studies acoustic communication in insects at the University of Bielefeld, Germany, said. “My guess would be that phonotaxis” – moving in response to a call – “is more dangerous than singing (this would be different for acoustically oriented parasites or predators), but is shown for a smaller proportion of the time compared to acoustic signalling by males.”
This doesn’t mean the study is insignificant; in fact, it adds to evidence that among animals that split the responsibility of looking for a mate, the risk of predation is also about the same. For example, another study of bush crickets in Greece found that calling males and moving females have about the same mortality. Yet another study of katydids also found that calling males are not at higher risk of predation than responding females.
Torsekar said their study also presents new puzzles. “Our result that overall predation costs of communication per night are low and that a predation event is very rare raises questions on the importance of predation as a major selection pressure on the evolution of communication.”
And theories that argue that males displaying or calling puts them more at risk might warrant another look. “Our result shows that responding to signals can be as risky and should not be ignored” when creating hypotheses about the “evolution of mate-searching strategies,” he added.
Lakshmi Supriya is a freelance science writer based in Bengaluru.