Too much sex causes the genitals of beetles to change shape, according to a new study.

The study provides new evidence that conflict over how often mating takes place can lead to male burying beetles evolving longer penis-like organs and females larger 'claws' on their genitalia.

Genital shape varies enormously across the animal kingdom compared, for instance, to body shape.

One reason for this may be that the shapes of male and female genitalia co-evolve as a result of sexual conflict, according to scientists at the University of Exeter.
Co-author Dr Megan Head said: "It takes two to tango, so when changes in shape in one sex lead to corresponding changes in the other sex this is known as co-evolution."

Sexual conflict over mating occurs because, while having lots of sex is usually good for a male - as it increases the number of offspring he is likely to produce - it is not so good for a female because she only needs to mate a few times to fertilise all her eggs.

In addition, too much sex can be costly for female burying beetles as it reduces their ability to provide parental care.

In order to test whether sexual conflict could lead to co-evolutionary changes in the shape of genitals the researchers artificially selected pairs of burying beetles for either high mating rates or low rates for 10 generations.

The research found that this artificial selection resulted in changes in the shapes of both male and female genitalia.

It also found that changes in one sex were reflected by changes in the shape of the other sex, showing there was co-evolution.

The greatest changes in shape occurred in beetles selected for high mating rates, where sexual conflict was greatest: males evolved to have longer intromittent organs and females responded by evolving more pronounced 'claws' on their genitalia.

Dr Paul Hopwood added: "Although we don't know the ins and outs of how these genital structures relate to the reproductive success of each sex, our results show that sexual conflict over mating can lead to co-evolutionary changes in the shape of the genitals of burying beetles.

"Our research demonstrates the general importance of conflicts of interest between males and females in helping to generate some of the biodiversity that we see in the natural world.

"It's fascinating how genital evolution can happen so fast - in 10 generations - showing how rapidly evolutionary changes can occur."

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