Insects learn to choose the right mate

"It is fascinating to see that even small insects can learn these things," says Professor Erik Svensson at the Department of Biology at Lund University.

Erik Svensson and his fellow researchers at Lund University have studied two co-existing species of damselfly (called "demoiselles," belonging to the genus Calopteryx). Damselflies belong to a group of insects called odonates, together with the more familiar dragonflies. The researchers have investigated the mechanisms by which females choose males with whom to mate. The main difference between the two species in terms of appearance is the amount of black on the males' wings. The females therefore have to keep an eye on the wing colour if they are to mate with males of their own species, i.e. the correct mates.

"If a female mates with a male of the wrong species, she essentially throws away her eggs, because mating between species leads to few offspring," says Erik Svensson.

The researchers have studied the mating behaviour of the damselflies at several different locations in southern Sweden. At some of the sites the two species live alongside one another. At these sites, the females reject the males of the other species. However, at other sites, only one of the species is present. There, the females showed much greater interest in males of the other species when they were presented to the females in a field experiment. The females at these sites were clearly not aware of the fact that these novel males were the wrong species when they came into contact with them for the first time.

"It is interesting that the females at the different sites behave very differently, despite the fact that the different sites are not far from one another," says Erik Svensson.

If the choice of mate was only a genetic (inherited) behaviour, the differences between the sites should not be as dramatic, because dispersal of individuals and the resulting gene flow between sites should erase such strong differences in mating behaviour. This prompted the researchers to carry out additional field experiments to investigate whether young and sexually inexperienced females learn to recognise males of their own species.

Newly hatched and sexually inexperienced female damselflies were captured in the field and kept isolated in cages without any contact with males. When these virgin females then came into contact with males of both species for the first time, they showed equal interest in the males of both species. In another experiment, newly hatched females were again kept isolated in cages, but were able to see males of their own species for a while, yet without physical contact. When these females were subsequently exposed to physical contact with the males, they developed a stronger interest in their own species and showed a reduced interest in males of the other species.

"Our experiments clearly show that the choice of mate is learnt and not merely genetic. However, we don't yet understand the learning mechanisms or exactly what happens during the short learning time of just a few hours," says Erik Svensson. "We are planning further experiments in the future to investigate these learning mechanisms."

The research results have been published in the scientific journal Evolution.

The species studied are the banded demoiselle (Calopteryx splendens) and the beautiful demoiselle (Calopteryx virgo).