Role Reversal: Male Gets Easy Ride In Insect Courtship

Well, University of Melbourne scientists in collaboration with a Swedish colleague have found a tiny voracious water bug where the female of the species lays all this on for their male partner, the first time such nuptial behaviour, at least the food for sex part, has ever been observed in female animals.

Their findings will appear in the next issue of Nature (24 July 2003).

Evolution has generally dictated that it is usually the male that lavishes the female with expensive gifts (usually food) and often risks life and limb to secure the opportunity to pass on his genes to the next generation. Females are typically choosy, picking only the best males while also getting enough food to ensure their offspring are well fed and inherit the quality attributes from their father.

The tiny Zeus bug (Phoreticovelia disparata) is a water skater common along Australia's east coast. Its behaviour, which defies this norm, has scientists baffled.

"All the advantages in this relationship seem to fall to the male with no obvious advantage for the female, yet the female Zeus bug seems a willing partner in this one-sided affair," says paper author, Dr. Mark Elgar, University of Melbourne Department of Zoology.

The male Zeus bug at about 1mm in length and half the size of the female will piggy-back on his female mate, seating himself comfortably in a seemingly custom-made hollow. During the ride, the male sups on an all-you-can-eat protein-packed wax the female secretes from a gland near her head. All that remains for the male is to get down to the serious business of mating with the female.

"The male can ride the female, feeding and mating for up to a week," says Elgar

"The female usually produces the wax feed when a male is riding her and she will continue to produce it for as long as the male remains, yet once deposited, his sperm will allow her to continually produce batches of fertile eggs for up to two weeks," he says.

So why do females let the male stay onboard? What does the female get for continually expending precious energy to feed and carry around the male?

While they can only speculate at this stage, the biologists suggest that by keeping one male for long periods, possibly for the duration of her reproductive life, the female will expend less energy than she would if she tried throwing off the male after he deposited his sperm, only to have another amorous male start harassing her for a free ride.

"A constant stream of suitors wanting to participate in a polygamous free-for-all could possibly lead to greater harassment, leading to the female expending more energy and placing herself at greater risk of harm than if she doted on just the one male," says Elgar.

"For the male, while it seems he may be putting all his eggs in one basket by remaining faithful, by doing so he is ensuring that his sperm rather than his rival's sperm is being used," he says.

"The finding gives us a new perspective on how mating behaviours have evolved and been maintained."

**NB According to legend, Zeus consumed his first wife Metis.

Dr. Mark Elgar and Dr. Therésa Jones (University of Melbourne) collaborated on this project with Göran Arnqvist, University of Uppsala, Sweden.