Gay sex – in beetles, at least – gives males a chance to indirectly fertilise females they may never encounter directly.

Homosexual copulations are common in insects, where they pose the same conundrum as in mammals: what evolutionary advantage, if any, might such apparently fruitless activity provide?

Over the years, biologists have proposed a range of explanations. Homosexual activity might, for example, help males practise for straight sex, or they might offer males a way to assert dominance over one another.

To test the explanations, Sara Lewis, an evolutionary ecologist at Tufts University in Boston and colleagues investigated flour beetles (Tribolium castaneum).

When housed together in laboratory containers, male beetles often mount one another and copulate, even transferring sperm on some occasions. Males are unlikely to be merely mistaking their male mates for females, since they are known to make the more subtle discrimination between virgin and mated females.

Male beetles with homosexual experience proved no more effective than virgin males at siring offspring when later allowed to copulate with a female, the researchers found. This, the team says, rules out the “practise” hypothesis.

Moreover, the mounting male of a homosexual pair was no different in size than the mounted male, and neither male had an advantage when the two were forced to compete for a single female, the team found. This rules out the “dominance” hypothesis.

Instead, male-male matings may offer an unexpected, though subtle, benefit. Some of the sperm transferred in such encounters may later be “accidentally” passed along when the recipient male later mates with a female, the researchers found.

Read the full story from NewScientist.