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With bloated bellies and hairy legs, female flies try to look bigger to get food from courting mates. But male flies, in turn, have sharpened their eyesight to call their bluff. A new study by researchers from the Universities of Gothenburg and Stockholm suggests that this is an ongoing evolution where both sexes try to outsmart each other.
For the first time, researchers have been able to show that also males can develop traits that help them pass on their genes despite the manipulative adaptations of the opposite sex. In different species of dance flies, there is a clear correlation between how richly decorated the female flies are and how large the eye facets are on the males. The researchers interpret this to mean that the males needed to develop better vision to find the optimal female in the swarm.
Dead insect as a nuptial gift
“We almost always see enlarged eye facets in male flies in those species where the female flies have developed hairy legs. Therefore, we dare to assume that there is a connection,” says Luc Bussière, evolutionary biologist at the University of Gothenburg.
There is an intricate interplay that takes place when the dance flies mate. The flies only fly for 1–2 weeks in their life cycle and that is when they have to reproduce by laying eggs that become fly larvae. To increase the chance that their genes will be passed on, male flies look for females that appear to have the most eggs. When mating, the male fly brings a gift, usually a dead insect, which he gives to the female. The female rarely hunts on her own and relies on the protein from the insect to produce eggs. Without an insect for the female, the male is usually not allowed to mate.
So how does a younger female fly without eggs attract male flies to get food supplies?
“We see that the females have different characteristics and tricks to imitate being fecund. They swallow air that fills their abdomen, and to look really big when they swarm, they have developed hair on their legs and their wings are bigger and darker,” says Axel Wiberg, researcher at Stockholm University.
The fact that animals imitate, behave and have appearances to fool the opposite sex is nothing new. Previous research on flies has also shown that male flies prefer a plump female fly to a skinny one when mating. But now scientists have discovered that male flies have sharpened their eyesight to avoid being fooled.
Sexual play evolves the species
By capturing male flies and measuring their eye facets, the researchers discovered that the facets on the top of the eyes were larger than those on the underside in the males of some species. Since the male fly approaches the female from below during mating, the researchers believe that the eyes have evolved to determine whether the female is truly full of eggs, or just faking it.
“This sexual play may force an evolution of the species. For generations, male flies with the largest facets have been favoured in mating and their genes are passed on. We see this as an evolutionary system where the development of different traits in male and female flies has been alternating,” says Luc Bussière.
Female flies of the different species in the study have different characteristics, some have only slightly darker wings, others have both darker wings and hairy legs, and can inflate their abdomen by swallowing air. These traits did not appear all at once, but probably as the male gets better and better at seeing past the bluff.
What will be the female's next move?
“Perhaps the female develops new traits that make it harder for the male to judge her size. Or she may develop entirely new strategies to gain an evolutionary advantage. For example, we see that in the species with the largest facets in the male, the female also has slightly enlarged facets – but on the underside of the eye. We don't know what this means, but perhaps it helps the female to see an approaching male more quickly and thus find an advantageous position in the swarm,” says Axel Wiberg.
Facts: The life of a dance fly
Adult dance flies lay eggs during the summer which then hatch into predatory larvae in the soil. The larvae hunt and eat other organisms and in the northern hemisphere are likely to become pupae before winter. In spring, the pupae complete their development and become flying insects; flies. Flies emerge at different times during the summer, for most species the flight period is only a few weeks at most. During this time, males and females gather in swarms to mate. In almost all of the dance flies from the subfamily included in the study, the males bring nuptial gifts to the mating swarm in the form of insects. During mating, the female feeds on the male's gift while the male inseminates her. Mating is repeated several times with different partners. Once the female has mated and eaten enough meals from the males, she can lay several eggs in the soil that will be the start of the next generation.
Luc Bussière, evolutionary biologist at the Department of Biology and Environmental Sciences at the University of Gothenburg, phone: +46 76-618 55 37, e-mail: luc.bussiere@bioenv.gu.se
Axel Wiberg, researcher at the Department of Zoology at Stockholm University, phone: +46 79-339 53 16, e-mail: axel.wiberg@zoologi.su.se
Scientific article in Evolution: Sexually antagonistic coevolution can explain female display signals and male sensory adaptations https://academic.oup.com/evolut/article/78/12/2006/7777002
Luc Bussière, evolutionary biologist at the University of Gothenburg.
Photo: Olof Lönnehed
Axel Wiberg, researcher at Stockholm University.
Photo: Elina Immonen
The female receives a dead insect as a gift from the male during mating. The gift provides the female with the necessary protein to produce eggs. If the male arrives empty-handed, he is rejected.
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Luc Bussière, evolutionary biologist at the University of Gothenburg.
Photo: Olof Lönnehed
Axel Wiberg, researcher at Stockholm University.
Photo: Elina Immonen
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