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Why Female Flies Eat Sperm

In an odd twist on the dinner date, some female flies eat sperm—and now scientists know why.

Female Ulidiid flies (Euxesta bilimeki) expel and eat ejaculate because it allows them to decide who will be the best father of their offspring, a new study says. (Also see “Female Flies Put Up a Fight to Keep Sex Short.”)

Just as lovers, partners, and spouses frequently exchange gifts and share meals during courtship and beyond, so do many animal species.

Usually, the male builds a nest or provides a food gift for the female, in order to help convince her that his genes are high quality and that he would make a good father. In these cases, the female is directly choosing her mate, and the offspring benefit both from the good genes of the parents and from the food or housing provided by the male. (Read about weird mating and courtship rituals.)

But for some species of mammals, birds, and insects—in which fertilization is internal—the effects of female choice are less obvious to the male. In a process called cryptic female choice, a female can exert control over her baby daddy by expelling a male’s sperm after copulation.

This method is especially effective in species like E. bilimeki, in which the female can store the male’s sperm before using it to fertilize her eggs, according to the study authors.

The Mating Game

Researchers knew that female E. bilimeki would expel and eat sperm, but didn’t know why.

To find out, Christian Luis Rodriguez-Enriquez and colleagues at the Institute for Ecology in Vera Cruz, Mexico, watched 74 pairs of E. bilimeki court and mate. They found that all of the females expelled and ate at least some of the ejaculate that they’d stored in specialized sperm-storage organs. (See “Sperm Tracked in 3-D—A First.”)

When the team looked closer, they found that one-quarter of the females purged all of the ejaculate from their bodies. This meant that the males with whom they recently mated would have no chance of fathering their offspring, according to the study, published recently in the journal Behavioral Ecology and Sociology.

Since females mate multiple times, the amount of ejaculate she expels can help increase or decrease the likelihood that a male will be the father of her offspring. The more sperm she expels, the less likely the male will pass on his genes.

So what makes an undesirable suitor? A male fly that’s too aggressive, the study found: The longer a male pursued a female before mating made the female more likely to expel and consume the ejaculate.

Rodriguez-Enriquez and colleagues hypothesize that this may be because the female grows tired of evading the male and copulates simply to avoid his amorous pursuits. Expelling the sperm means that she doesn’t have to worry about making babies with males who don’t know the meaning of “back off!”

Plus, she just might get some nutritious liquid in exchange for her hassle.

Sperm: It’s Not What’s For Dinner

Ulidiid flies live in the deserts of Mexico and the U.S. Southwest, where water and food are scarce. That made researchers initially suspect that the female flies might be eating the ejaculate for sustenance. (Also see Beetles Are Thirsty for Sex.”)

To test this theory, the team raised females on one of several different diets: a very high-quality diet containing protein, sugar, and water; a high-quality diet containing sugar and water; water only; and neither water nor food.

The females that were given no food or water for two days did live longer if they consumed expelled sperm after mating, but this consumption had no effect on the survival or longevity of the well-fed flies.

But the starved and thirsty flies were no more likely than the well-fed flies to consume the ejaculate—showing that they don’t eat it for food.

Although mysteries remain about the sperm-eating flies, the results show that females continue to play a crucial role in selecting the father of their offspring—even after mating.


  1. Christian
    March 22, 1:48 pm

    Hi EPaul I didn´t use the term Baby Daddy

  2. Paul Mwanamanga
    June 17, 2013, 1:51 am

    Dis site good forever

  3. David Dent
    June 13, 2013, 2:24 am

    …and, it’s good for your complexion too, baby.

    The female does not play any role in selection. The male selects. The male of every species looks for females that are healthy enough, exhibit symmetry, appear to be disease free, and show good genes. If she disagrees, he forces her. End of story. It is the male, and only the male that chooses.

  4. DRidge
    Eugene, OR
    June 1, 2013, 4:36 pm

    Sorry..EPaul is right. As a biology undergrad who wanted to use this for an assignment, I would now feel embarressed handing a copy of this article attached to my paper to my prof.

  5. Amanda
    May 15, 2013, 8:34 pm

    Or was it fly rape?

  6. Rachel
    Anderson, Indiana
    April 25, 2013, 3:43 am

    This was a great article, Ms.Arnold. The heaps of valuable information isn’t diminished by the parodied slang, I promise. Thank you :). Robots don’t understand humor, a new study shows, maybe that can be your next article ;).

  7. EPaul
    Tampa, FL
    April 17, 2013, 4:38 pm

    Did this author really use the term “Baby Daddy”. (shaking my head in utter disappointment)