Female Flies Expel Sperm and Eat It

picture-winged fly
Expelling and eating sperm may be the female fly's way of choosing the father of her babies. (Image credit: Elliotte Rusty Harold | Shutterstock)

Female Ulidiid flies have a kinky habit: After mating with a male, they expel his sperm and eat it.

The odd behavior may help these lady flies choose which guy flies will father their young, researchers reported online today (April 11) in the journal Behavioral Ecology and Sociobiology.

Studying a species of "picture-winged fly," known as Euxesta bilimeki, researchers found that 100 percent of the 74 fly couples they studied spewed out ejaculate after mating. Further study revealed that 25 percent of females harbored no sperm afterward. The findings suggest the fly dames were able to control how much sperm to expel in order to select which males fertilized their eggs.

And these fly gals are no romantics — long periods of courtship before mating made females more likely to expel all the ejaculate, results showed. The researchers suggest that the female may simply be giving in to a determined male in order to stave off future advances, but dispensing with his sperm before he can father her babies.

Another possibility is that the sperm provides nutrition for the female flies when food is scarce. To test this theory, the researchers fed female flies a diet of: protein, sugar and water; sugar and water; just water; or nothing (fasting flies). Then they put the flies with males that could ejaculate or males that could not.

Flies that were fed nutrients (sugar or proteins) or plain water seemed to derive no benefit from eating the sperm. But the fasting flies that ate sperm lived longer than those that were prevented from eating it, suggesting the ejaculate provided a needed source of fluids. These flies live in dry regions where evolution may have favored such behavior.

But studies reveal that females of many insect species receive no nutritional benefit from such "nuptial gifts," the researchers say. The sperm-eating practice may simply be part of the mating effort.

Many questions remain, and the researchers note that more experiments are needed before making conclusive statements about the function of the sperm-spewing-and-scarfing behavior.

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Tanya Lewis
Staff Writer
Tanya was a staff writer for Live Science from 2013 to 2015, covering a wide array of topics, ranging from neuroscience to robotics to strange/cute animals. She received a graduate certificate in science communication from the University of California, Santa Cruz, and a bachelor of science in biomedical engineering from Brown University. She has previously written for Science News, Wired, The Santa Cruz Sentinel, the radio show Big Picture Science and other places. Tanya has lived on a tropical island, witnessed volcanic eruptions and flown in zero gravity (without losing her lunch!). To find out what her latest project is, you can visit her website.