Lonely beetles make more sperm

There's an interesting strategy organisms use to compensate for losing a battle over a female -- make more sperm. The finding, in male flour beetles, suggests that losing males may have a better chance of fertilizing the next female that comes along, according to a new study in the Royal Society journal Biology Letters.

Image: Kensuke Okada

"These experiments show quite nicely that males can lose the first battle, but potentially still win the war," said Sara Lewis, a professor of evolutionary ecology at Tufts University and not associated with the study. "It really highlights the importance of post-mating processes."

Like most of us, the horned flour beetle (Gnatocerus cornutus) has to compete for mating privileges, grappling with each other for access to available females. The winners get the mates of course, and often establish "harems," mating with the females multiple times. The losers, however, change their behavior entirely.

"The losing males assess that their own fighting ability is too low to get mates with fighting behavior," lead study author Kensuke Okada, from Okayama University's Laboratory of Evolutionary Ecology, said in an email.

After the battle, winning males tend to use their efforts to guard the females they've won. But losing males need to find another strategy, since they now have limited access to the mating females. So rather than jumping back into the arena, the losers instead search out new territories and females, or sneak into winner males' harems. And instead of fighting, the authors found that the losers put more energy into producing extra sperm.

Okada and coworkers looked at male flour beetles that had no previous fighting experience. They let the males fight, and divided them into winners and losers. They then mated both sets with virgin females one day and five days after the fight. They assessed the amount of sperm transferred to the females, and found it was found to be significantly higher per ejaculate from the beaten males that first post-fight day. On the fifth day, the sperm counts went back to their normal, pre-fight levels.

This strategy might help their chances of actually impregnating a female if they can get access, since the losers don't necessarily have to fight to mate. If their new territory contains females with no males guarding them, or they manage to get back into the harem undetected, mating with a female is likely to happen. But since the winner males get to mate multiple times, the loser's one shot with the extra sperm may help them be a bit more competitive in what Okada called the "war of sperms."

In fact, Okada and his team also found that losing made the losers give up fighting entirely, if only temporarily. Both winners and losers were paired up with a new set of males that had not fought before. The loser males did not initiate attacks at all, and always lost when they were attacked. This suggests that the losing males shift their energy investment from physical fighting to sperm making, based on whether they've previously won or lost, Okada said. But after five days the vanquished males perked back up, ready to initiate attacks and become winners again, and take the sexual competition back to the outside of the female's reproductive tract.

This strategy of producing more sperm after losing access to a female has only previously been found in Norwegian rats.

"Sexual selection is responsible for a lot of bizarre and unusual traits. We don't know much about it, especially in the later stages," said Lewis. "[This study] adds to the growing recognition that if we want to understand, we need to figure out what goes on after mating."

K. Okada, et al., "Ejaculatory strategies associated with the experience of losing," Biology Letters, published online April 14, 2010, doi: 10.1098/rsbl.2010.0225.

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