Researchers have studied small weevils native to Florida, US, and shown how the females that mount each other generally end up entwined with the largest males.

Ally Harari of the Volcani Center, Israel, and Jane Brockmann of the University of Florida, US, say the behaviour they have seen in Diaprepes abbreviatus may also account for the lesbian-like activities seen in many other insect species.

Advantageous in courtship

Right across the living world, there are examples of females behaving like males and vice versa. It has already been shown, for example, that transvestism - adopting the same colour patterns as the opposite sex - can be extremely advantageous in courtship.

But the benefits that come from intra-female mounting have never satisfactorily been explained say the team, who report their findings in the journal Nature.

The biggest males win out

They observed D. abbreviatus females in the sex position for up to 17 minutes at a time. This behaviour would end either with the females going their separate ways or with a male pushing in to start proper mating.

Males and females of D. abbreviatus look very much the same, except that the females are slightly larger. Males are attracted by the scent of a female but have difficulty discriminating males from females, and so often end up mounting another male, or a copulating male-female pair, by mistake.

In the absence of reliable cues, the best strategy for a confused male is to search for large individuals (which are more likely to be female) or for mating couples, because at least one of the pair is likely to be female.

But only the largest males will be successful in breaking up a copulating pair and it is this fact, say Harari and Brockmann, that probably lies at the heart of the apparent lesbianism.

Good genes and resources

They presented a group of males with both small and large female-mounting pairs and saw how the largest males almost invariably headed for the biggest targets.

"By mating preferentially with larger males, females may benefit either through 'good genes' or directly by obtaining resources, as materials are transferred from males to females during mating."