Fly mating choices may help explain variation across species

Scientists at the University of Stirling have shed new light on the impact of sexual selection on species diversity after studying the mating rituals of dance flies.

The team, from the Faculty of Natural Sciences, examined the unique mating behaviour of Rhamphomyia longicauda -- the long-tailed dance fly -- in a bid to understand how species evolve.

Some female dance flies exhibit extravagant sexual ornaments -- characteristics to help attract mating partners -- which is unusual because, in other animals, such traits are typically observed in males. Examples include peacocks, which use their tail feathers to advertise their sexual and physical fitness and male black grouse, which attract partners by singing, dancing and showing their tail fans.

In dance flies, some females use their hairy legs and inflatable abdominal sacs to make themselves appear bigger -- and more attractive -- to potential partners.

Dr Luc Bussière, a Lecturer in Biological and Environmental Sciences, worked on the research alongside PhD student Rosalind Murray and colleagues at the University of Toronto.

"Generally speaking, females do not have any sexual ornaments at all -- they are typically a male characteristic," explained Dr Bussière. "Our study looked at this species and considered why females had ornaments and what exactly attracted the males to their partners.

"Interestingly, the research found that females often deceive males by attempting to make themselves appear bigger -- and ultimately more attractive -- than they actually are. In contrast, the males are constantly evolving in an attempt to avoid being duped.

"This work supports our understanding of how sexual selection creates diversity and, in turn, can help us to provide an answer to one of the biggest questions in biology: why are there so many different species?"

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."

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