Zoologger is our weekly column highlighting extraordinary animals – and occasionally other organisms – from around the world

Don’t mind if I do (Image: Odette Brunel)

Species: Euxesta bilimeki

Habitat: Chilling out on agave plants on the high plains of Mexico

There’s no getting away from it, sex is great. All organisms feel the urge to reproduce, and thus pass their genes on to another generation.

Thirst, on the other hand, is terrible. There’s a popular myth that humans can survive only three days without water. In reality we can go a bit longer than that, but we suffer when parched, and go in desperate search of a drink.


Living in a dry climate, female Euxesta bilimeki flies are regularly faced with deadly thirst. But they have an unusual solution: they regularly eat sperm from the males of their species. It has little nutritional value, but does contain plenty of water.

Come-hither wings

E. bilimeki is one member of the picture-winged flies, a family which gets its name from the conspicuous patterns on members’ wings. E. bilimeki‘s wings have dark stripes.

It lives on the high plains of Mexico, where it spends its lives on Agave atrovirens plants – a close relative of the agave used to make mescal and tequila.

The flies are active year-round, and males constantly harass females for sex, displaying their wings to try and draw attention to themselves. But a successful copulation doesn’t guarantee that the male has succeeded in passing on his genes – within seconds, the female pops the ejaculate right back out, and often proceeds to eat it.

“Expelling the ejaculate is not that uncommon,” says Juan Rull of the Institute of Ecology in Xalapa, Mexico. Plenty of species do that. “What is uncommon is consuming it.”

Dietary test

To find out why the females chow down on the sperm, Rull and his colleagues put females on a range of diets, allowed them to mate and then monitored the effect of eating the ejaculate.

Only females deprived of both food and water benefited from the sperm drink, living longer than those on the same starvation diet but without access to sperm – because the males in their enclosure had had their genitals tweezered off. Whether or not the flies had access to ejaculate made no difference if they were well-nourished or well-watered flies – suggesting it is the fluid in the sperm that is beneficial.

Rull says this strategy could help the females survive the dry winter. “Perhaps they do so by ‘milking’ the males,” he says.

But not all sperm gets ejected and gobbled up, so what becomes of the sperm that females deign to keep in their bodies? When the researchers dissected the females, they found that they were more likely to retain sperm if they had mated with a large male, or if the courtship had been long. This suggests that the females were choosing the best males to fertilise their eggs.

So despite the constant male harassment, the females seem very much in control. Not only are they are very selective about who fertilises their eggs, but they manage to use the unsuccessful males as mobile drinking fountains.

Journal reference: Behavioral Ecology and Sociobiology, doi.org/k9v