12 June 2007

Sex wars fought with spiky genitals

By
Cosmos Online
For seed beetles, the battle of the sexes is not a psychological game, it's a deadly serious evolutionary arms race, with males packing spectacularly harmful penises covered in sharp spikes.
Sex wars fought with spiky genitals

The penis of a seed beetle - reminiscent of a sinister French tickler - is armed with sharp spikes that cause injuries to females during mating. However, females have evolved resistance to these intimidating structures. Credit: Johanna Rönn

CANBERRA: Male seed beetles have spectacularly harmful penises covered in sharp spikes. These help the male’s chances of fertilizing the eggs by providing an anchor, but can also pierce the female during sex, causing injury.

For seed beetles – a group of insects consisting of many species that infest beans or seeds – the battle of the sexes is not a psychological game played out in the home, it’s a deadly serious evolutionary arms race, according to a new study.

Seemingly harmful genitalia have evolved in many animals, helping males compete among themselves to father offspring. This sort of sexual conflict – known as sexually antagonistic co-evolution – is thought to result in rapid steps of evolutionary tit-for-tat between the sexes, but it is poorly understood. So far, little experimental evidence has supported the theory.

Fearsome and elaborate

To verify that this arms race can lead to the co-evolution of reproductive organs, a team of evolutionary biologists led by Johanna Rönn from the University of Uppsala in Sweden studied seven species of seed beetle, known to have fearsome and elaborate genitalia.

According to her theory, as males evolve more effective and spiny genitalia, female beetles evolve tougher integument to resist collateral damage. By studying the differences between the species and a series of mating experiments, the researchers have exposed a clear scenario of offence and resistance between males and females.

“Our results unveil a co-evolutionary arms race between the sexes and are consistent with a proposed link between sexual conflict [and the survival success of species],” report the experts this week in the U.S. journal Proceedings of the National Academy of Sciences.

As part of their research, Rönn and her team attempted to study the relative harmfulness of male genitalia and amount of scarring in the female reproductive duct in the beetles. Measuring these features is not as straightforward as it may first seem, however, because “adaptation in one sex is obscured by counter-adaptation in the other,” said the researchers. They have discovered female seed beetles have evolved tough padding to protect against the spikes.

So the researchers traced the outline of the male genitalia from scanning electron micrographs, counted the number of spines present in each species, and came up with a mathematical representation of the relative harmfulness of the penis of each species. In addition, they also gave pictures of the genitalia to other biologists, who were not experts in the area, and asked them to rank the genitalia in terms of seeming-harmfulness.

Risky strategy

They found that in species where males have the spikiest penises, the females had more padding in their reproductive tracts. According to the researchers, the spiky male genitalia are less damaging to females with more padding, which results in those females surviving and producing more offspring.

The unusual male adaptation is even beneficial to the females, as it helps ensure their own male offspring are more likely to successfully mate and pass on their genes. However, it’s a risky strategy, said the researchers, as even minor imbalances in this system can be very costly for females.

“These results are exciting,” because they fit with predictions that have been made about evolutionary arms races between the sexes, commented biologist Martin Edvardsson, from the University of Exeter in England.

“Some reproductive characteristics, such as the shape of male genitalia, are among the most rapidly evolving animal traits we know, often varying greatly between species,” he said. “Sexually antagonistic co-evolution is now a popular explanation as to why this is the case.”

The research has provided little solace to female seed beetles, however.

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