Those sexy flies and their endearing ways

GOING INTERNATIONAL: Ms Nalini's research was displayed at an exhibition on the diversity of mating behavior among animals in the Zoological Museum in Zurich. Make love, not war. That is the way to go for the black scavenger flies, scientifically named Sepsidae, when it comes to mating. The male, rather than forcing himself onto the female, curls its hind legs and does a sensual tango dance to arouse his prospective partner.

The stimulatory mating behaviour in Sepsidae provides evidence that females control reproductive success in a study by Ms Nalini Puniamoorthy, NUS Department of Biological Sciences; Assoc Prof Rudolf Meier, NUS Department of Biological Sciences; and Asst Prof Krista Ingram, Colgate University's Department of Biology. The study, which was initially started out as Ms Nalini's undergraduate research under the University Scholars Programme, will be published in the Biological Journal of the Linnean Society. Her research was recently prominently displayed at an exhibition on the diversity of mating behaviour among animals in the Zoological Museum in Zurich. She also presented a talk on her research at the opening of the exhibition.

Ms Nalini observed the sexual behaviour of male and female flies under the microscope in a lab-controlled environment. She found the majority of the males performed non-contact foreplay. With the freedom to choose whether or not to respond to the male's dance, the females control sexual selection, a mating pattern that falls under the "female choice" theory.

"Their first pair of legs holds on to the female, while the mid and hind legs are free to do basically whatever they want. Some males curl their hind legs like swimmers doing the butterfly strokes. They are basically trying to impress the females rather than overcoming them. Hence, it would be a female choice rather than sexual conflict," said Ms Nalini.


"Sexual conflict" theory has attracted a lot of interest in recent years, said Ms Nalini. Researchers have, in fact, suggested this as a possible theory to explain human rape. Insect models are useful in testing these hypotheses, she added. In Sepsidae, the "sexual conflict" theory suggests that the male will overcome the female's attempt to ward him off in mating, thus handing the control of reproductive success to the male. But this does not seem to apply for Sepsidae's sexual selection as only a small number of the males were observed to be forcing themselves onto the female.

Ms Nalini emphasised the sexual behaviour observed is an innate preference rather than a learned behaviour as virgin flies (those which have no contact with the opposite gender the moment they were born) were bred for the experiment. Hence whatever behaviour they displayed under microscope would be in-built rather than learnt.

Also lending weight to the "female choice" theory is the absence of co-evolution of "attacking mechanism" in the male and "defence mechanism" in the female. Ms Nalini said sexual conflict theory implies that the male needs to develop some kind of "weapon" to overcome female. The female, in response, would evolve some kind of "defence". Hence there should exist a co-evolution between male's "weapon" and female's "defence" mechanisms if sexual conflict exists.

Ms Nalini explained: "By observing the body structure of the Sepsidae, we found that the male's structures have evolved very differently. If there was sexual conflict, the female's structures should correspondingly be different to try to ward off the evolving male's weapon mechanism. For instance, the males have different fore legs, one without spine, one with a spine and one with two spines. If sexual conflict existed, the female counterpart should have a wing with an extra thickened spot to ward off this extra spine. But we found out that all the females' structures look identical, so there is no co-evolution of weapons and defences. Hence we can conclude that the male is trying to convince the female rather than overcome the female."

Sexual behaviour can also be used to differentiate between the different species of Sepsidae. Each species of Sepsidae is different in looks. For example, two specimens caught in the wild would be classified as two different species -- as the first specimen has an extra bump and the second specimen does not. But different species also have different sexual behaviour. By observing the sexual behaviour of virgin flies bred in lab-controlled environment, Ms Nalini found that species which look morphologically similar, display different behaviour. As virgin flies are used, the behaviour can be concluded to be species-specific.

Ms Nalini hopes to do her PhD in Sexual Selection in Zurich and to teach in NUS one day.