Bats (left) find it easier to echolocate houseflies when the insects are having sex

July 23: Scientists peeping into the night lives of bats and houseflies have generated the first evidence for an idea debated for decades by evolutionary biologists — sex may prove lethal when predators are around.

Bat biologist Stefan Greif and his colleagues at the Max Planck Institute for Ornithology in southern Germany have found that bats are drawn by the sounds of copulating houseflies — and lucky bats get a “double meal” through a single strike.

The scientists who studied a colony of bats and hordes of houseflies in a cowshed for four years observed that bats could not echolocate the houseflies sitting or crawling along its ceilings or walls. A faint echo of the insect picked up by the bats' auditory system is likely to be camouflaged by the echoes from the walls of the cowshed or, when in the wild, from vegetation.

But houseflies are the most common meal of the bats under study. So the researchers set up video equipment to record nocturnal events in the cowshed, capturing images over a four-year period. They found that among 1,105 copulating houseflies, 59 pairs were attacked by bats. “This is a strikingly higher rate than the attack rate on the crawling flies which were never attacked.” Greif told The Telegraph.

The findings appear tomorrow in the research journal Current Biology. The scientists believe their findings for the first time demonstrate the mechanism underlying increased predation on copulating animals.

The mating flies produce click-like sounds that scientists believe emerge from the male fly’s fluttering wings. The clicks occurring at a repetition rate of about 122 per second are audible as a low frequency buzz even to the human ear.

“Through the sounds, the copulating flies become more conspicuous to their predators, the bats,” Greif said. “Evolutionary biologists have wondered about the dangers of copulation since the 1920s.” The first paper on the risks of copulation to insects was published by the late entomologist from Oxford, Owain W Richards, in 1927.

“This could be among the first studies to have looked into this aspect of the predator-prey interactions experimentally,” said Anindita Bhadra, an insect biologist specialist at the Indian Institute of Science Education and Research, near Calcutta, who was not connected with the study.

Bat biologist Stefan Greif

Greif and his colleagues observed that the bats that attacked the copulating houseflies did not always pick them up and at times missed them by only a few millimetres.

Among the 39 pairs attacked, only in two cases did one of the two flies escape. “Therefore keying in on copulating flies typically afforded the bats with a double meal,” the scientists wrote in their paper.

“The five per cent attack rate (59 out of 1,105) may appear rather low, but this may be a balancing strategy in nature — it works for the flies and it works for the bats,” Bhadra told The Telegraph.

Too many flies being eaten would threaten the species, and too little would deny the bats their meals. Scientists say the findings provide insights into evolutionary pres- sures.

Copulation carries risks, Greif said, because the copulating animals may not pay attention to predators, they may not be able to get away quickly enough and they may become more conspicuous to predators.

“The evolutionary reward of copulation is reproduction, but the question is how long should animals copulate when predators are around.”