LONDON: Male seed beetles have barbed penises that damage females, decreasing their life expectancy. A new study now provides evidence that the curious trait exists to increase chances of fertilisation.
“This is a new piece of the puzzle as in what’s in it for males,” said study co-author Göran Arnqvist. “Harming a mate is clearly not a good idea, so there must be some benefits.”
Genital barbs are found in many insects and some snakes and lizards, and in those species the female reproductive tract can be left scarred and injured.
Earlier research already established that, in seed beetles, these spiky genitals – and female defences to them – have evolved in concert, resulting in a bizarre sexual arms race (see Sex wars fought with spiky genitals).
Scientists have been unsure as to exactly why males are spiking their mates, though. One theory is that injuring a female dissuades her from mating with rivals.
But the latest research shows that females are merely caught in the crossfire of male rivalry, said Arnqvist, an ecologist from Uppsala University in Sweden. The spikes have evolved as a result of male in-fighting as they compete to fertilise eggs, he said.
For the study, Arnqvist and his co-worker, Cosima Hotzy, mated female seed beetles (Callosobruchus maculates) with males that had genital spikes of varying lengths.
Sterlised with radiation
In each mating they paired females up with two males, who mated with her in turn. One of these males, in each case, was previously sterilised with gamma radiation, which didn’t kill his sperm cells, but damaged the DNA inside them. The result being that the irradiated male could fertilise eggs with his sperm, but these eggs wouldn’t hatch.
By counting how many of the females eggs later hatched – or didn’t hatch – the researchers were then able to predict which male had been more successful in fertilising the female.
The results showed that, on average, males with longer barbs fertilised more of the females’ eggs than those males with shorter barbs, despite doing damage to the females along the way.
The study is published in the journal Current Biology.
Collateral damage to the females is an unfortunate side effect, said Arnqvist, who speculates that the spikes help position the penis for the best chance of fertilising the eggs.
“The cost to females is quite small, they don’t get totally ripped-up inside,” he said. “They do get some injuries, but they are not too severe, because they have counter evolved reproductive traits, such as lot of connective tissue there.”
Half the life expectancy
Nevertheless, the lifespan of females who mate most frequently is 40 to 50% less than females who mate infrequently, he added.
“This is the first study that shows a direct link between damaged females and male-male competition,” commented David Hosken, an evolutionary biologist from Exeter University in England.
The findings may be applicable to other animals with genital barbs, and researchers need to concentrate on studying male sperm competition in those species too, he said.
The next step for Arnqvist’s team is to find out precisely how longer spikes allow males to compete more effectively. They plan to use a laser ablation technique to trim the ends of the males’ penis barbs to see if they are less able to anchor themselves to the females.
The study in Current Biology