WHY FEMALE FLIES EAT UP 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.

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.
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.

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.

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.