Homosexual Beetle Activity Offers Reproductive Edge

Matt Kaplan
for National Geographic News
November 4, 2008
Homosexual activity among male flour beetles can actually increase the insects' chances of reproducing, a new study finds.

This behavior in the 0.13-inch-long (three-millimeter-long) Tribolium castaneum, which can be found infesting flour in most temperate areas, has been observed for decades.

From an evolutionary perspective, why homosexuality exists at all is a mystery. In theory, males should focus their energies on reproducing with females.

"We noticed that these male beetles spent quite a lot of time in this seemingly counterproductive behavior and wondered what was going on, so we set up some experiments to find out," said lead author Sara Lewis, an evolutionary ecologist at Tufts University in Medford, Massachusetts.

Her findings appear in the Journal of Evolutionary Biology.

Homosexual behavior has been seen in many animals, including insects, penguins, and primates.

(Related: "Homosexual Activity Among Animals Stirs Debate" [July 23, 2004].)

Some researchers suggest that male animals need to practice breeding as much as they can before meeting females.

Others argue that males need to get rid of old, less effective sperm before they encounter females.

Some scientists have even asserted that homosexual behavior is a method of exerting social dominance over other males.

Tracking Success

Lewis's team marked individual males and females, then tracked their sexual exploits while simultaneously monitoring the paternity of any offspring born in the group.

They found that homosexual encounters did not improve a male's sexual success with females, as measured by the number of offspring carrying his genes.

There was also no connection between homosexual activity and social dominance—male beetles that had many sexual encounters with other male beetles did not earn more attention from females.

What the team did find was that males were dribbling sperm onto each other. This suggested that males might be trying to get rid of old sperm, lining up fresher sperm for their next female encounter.

The team also found that if one male leaked semen on another male and the semen-covered male later bred with a female, the female's eggs could become fertilized with the sperm of the male she had never encountered.

That a male could inseminate a female without directly breeding with her came as a big surprise.

It reveals that the flour beetles' homosexual behavior yields a direct reproductive benefit, allowing males to inseminate females without expending time or energy having sex with them.

"We could not believe these results when we first saw them, so we ran the experiment over and over again to make sure it was actually happening," Lewis said.

A New Perspective

"This is excellent research," said entomologist Klaus Reinhardt at the University of Sheffield, U.K.

"So many papers look at these sorts of behaviors and immediately consider them from a human perspective. This paper has done a remarkable job of not sexing up the homosexuality and [instead] just asking why beetles do this."

Jane Brockmann is a behavioral ecologist at the University of Florida, Gainesville.

"Normally people would just write off this behavior, pitying the bugs as merely confused or overly sexed," she said.

"It is great to see it finally being explored objectively," she said.

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