Male flies sharpen eyesight to avoid being tricked by females

In an evolutionary battle of the sexes, male fruit flies have sharpened their eyesight to avoid being tricked by females seeking food gifts while pretending to be fertile.

To increase the chance that their genes will be passed on male flies look to mate with females that appear to have the most eggs, and are, thus, most fertile.

However, female flies try to take advantage of this by changing their looks so that they appear to be more fertile than they are in order to get food from males.

“We see that the females have different characteristics and tricks to imitate being fecund,” said Stockholm University researcher Axel Wiberg in a statement.

“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,” said Wiberg.

Males bring food gifts before mating

When seeking a mate, a male fly brings a food gift, usually a dead insect, which he gives to the female. Without such a gift, the male is usually not allowed to mate.

The female needs the gift because she rarely hunts on her own and thus relies on the protein from the insect to produce her eggs.

By capturing male flies and measuring their eye facets—the identical eye units seen in many insects—researchers discovered the facets on the top of the eyes were larger than those on the underside in males of some species.

The male fly approaches the female from below during mating and researchers believe that their eyes have evolved to determine whether the female is truly full of eggs or just pretending to be fertile in order to receive food gifts.

“For generations, male flies with the largest facets have been favored in mating and their genes are passed on” said Luc Bussière, evolutionary biologist at the University of Gothenburg.

“We see this as an evolutionary system where the development of different traits in male and female flies has been alternating,” said Bussière.

Scientists expect that in an evolutionary arms race the female will evolve features to respond to the male’s ability to see their trickery.

“Perhaps the female develops new traits that make it harder for the male to judge her size,” said Wiberg.

“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 if helps the female to see an approaching male more quickly and thus find an advantageous position in the swarm,” said Wiberg.

Reference

Wiberg, R. A. W., Murray, R. L., Herridge, E., Pärssinen, V., Gwynne, D. T., & Bussière, L. F. (2024). Sexually antagonistic coevolution can explain female display signals and male sensory adaptations. Evolution, 78(12), 2006–2016. https://doi.org/10.1093/evolut/qpae133