Deadly weapon: The penises of seed beetles are covered in vicious barbs giving them the appearance of a medieval club.
Credit: Göran Arnqvist
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.
