Australasian Science: Australia's authority on science since 1938

Costly Copulation

By Magdeline Lum

Wasps and bats upsize their meals when they catch prey that are in the act of mating.

In 2010 Australian plague locusts (Chortoicetes terminifera) descended on farming land and communities throughout Queensland, New South Wales, South Australia and Victoria after a decade of drought had been broken by rainfall. While the locusts were feeding and damaging grazing areas and gardens in agricultural areas, Dr Darrell Kemp from the Department of Biological Sciences at Macquarie University was observing parasitoid digger wasps (Sphex cognatus) entombing mating pairs of locusts.

Increased vulnerability to predation during mating is often mentioned as a cost of reproducing, but there are limited data showing this. However, in December 2010 Dr Kemp observed female digger wasps capturing and burying paralysed locusts in south-eastern Australia. He saw that the wasps caught 43 individual female locust and 19 mating pairs. The female digger wasp’s catch of locusts (30.6% were mating pairs) was far greater than the relative availability of mating pairs in the hunting environment (less than 3.0%).

The data show that there is a cost involved with copulation that is most likely due to being more visible to predators and the reduced ability to escape. Dr Kemp noted that it was only the female locust that was stung and paralysed before burial. The male locust was never stung but, unable to detach himself, was dragged along and buried alive with his mate.

It’s not just digger wasps in Australia benefitting from mating pairs. PhD student Stefan Greif from the Max Planck Institute for Ornithology knew that the European Natterer’s bat (Myotis nattereri) feeding on the flies in cow sheds. What didn’t make sense was that the bats were catching insects that they should not be able to find.

Bats hunt and navigate using sonar, releasing high-pitched sounds that echo back to paint a picture of the world around them. But echoes from the rough ceiling of the cow sheds bouncing off and mask the echoes from flies on the roof. The flies are invisible to the bats but it was clear that the bats were feeding on flies.

Greif filmed 8896 flies walking on the ceiling of a cowshed over 4 years, and not one of them was attacked by bats. However, as soon as two flies engaged in copulation, the risk of being attacked by bats dramatically increased. Over the 4 years of observation, 26% of copulating flies were attacked. The bats were not always successful in their attack, with a 59% success rate, but their misses were by only millimetres.

In successful attacks, the bats almost always swallowed two partners. There were only two instances where one of the two flies escaped the jaws of the bats.

Greif tested whether it was the larger silhouette that led to the demise of the copulating flies. In 2005 he stuck 35 pairs of dead flies in the act of copulation on the ceiling of the cow shed to see whether they returned a different echo to the bats. Not one of the pairs of flies attracted the attention of the bats, so it was not the increase in size that drew their attention.

The breakthrough came when the research team obtained experimental evidence that the bats were eavesdropping on sounds made during fly copulation. During mating, the flies emit a burst of low frequency click-like signals, most likely from the fluttering of the male’s wings. These signals are audible to the human ear as low frequency buzzing.

Greif played this sound through speakers and triggered attack responses from the bats. The sounds of mating made by the flies were making them visible to the bats from a distance.