Large female dance flies may be more attractive to males because they appear to be full of eggs – enhancing males’ opportunity to father offspring – and during each mating process, the male “gifts” food to the female.
The scientists wanted to understand the significance of each of the female’s sexual ornaments and, in a bid to gain a greater insight into the preferences of males, they used artificial physical models of females in the wild, combining varying sizes of abdominal and leg ornaments. The team were then able to count how much interest each artificial model attracted from wild males.
“We found that males like both large bodies and hairy legs, but were much more attracted to large bodies than hairy legs,” Dr Bussière said. “Our findings suggest that these ornaments may be deceptive rather than honestly indicating female value to males. In other words, the females are seducing the males by inflating their abdominal sacs and positioning their hairy legs in such a way to fool the males into believing they are full of eggs. In turn, the female is rewarded with food.”
This work may explain why the focal species has multiple female ornaments, while close relatives have one or none. The scientists concluded that sexual conflict – females deceiving the males so as to increase their chances of obtaining food from mates, and males trying to adapt to protect themselves from being duped by skinnier-than-advertised females – creates diversity among closely related species.
“The angle about deception is the most intriguing one from our perspective,” Dr Bussiere continued. “One might have expected the two ornaments work together to create a better signal than either on its own, but the asymmetric importance of the traits to attraction is best explained by conflicts of interest between the sexes.
“We suggest that cycles of evolution could produce a series of ornamental traits in females, as males evolve to resist the deceptive adornments each in turn. Such an explanation would simultaneously explain why females are able to exhibit such elaborate displays – otherwise mostly absent elsewhere in the animal kingdom – and why, in several dance fly species, there are multiple ornaments.”
He added: “In order to explain diversity, we have to challenge our understanding of rare cases like this example of female ornamentation, which at first seems to contradict what we know about evolution.”
Dr Jill Wheeler and Professor Darryl Gwynne, both from the University of Toronto, collaborated on the research, Sexual selection on multiple female ornaments in dance flies, which is published in Proceedings of the Royal Society B.
The study was funded by the Natural Sciences and Engineering Research Council of Canada and the University of Stirling.