Naughty boys

Back to beetles: Simmons says that, where a high proportion of males adopt the SF strategy, there will be high levels of sperm competition. Species with a low proportion of sneakers probably invest more of their resources in combat, than in the sperm contest after the main event.

“Not surprisingly, when we looked at testis size relative to body mass, we found species that have a high frequency of sneaky males tend to have larger testes,” he says. “That’s true of both the sneaks and guards.

“We can explore these comparisons statistically, to see what other traits might be correlated with certain mating strategies. There are other factors out there that can influence testes size and the frequency of sneaking males – we can test predictions flowing from sperm-competition theory, but we can only establish associations.”

Simmons and his team have been doing some experimental manipulations of selection by setting up a system in which they can manipulate the mating system of a particular species.

“For example, we allow males and females to mate monogamously, or allow the males to compete to mate with individual females. After 20 generations – about five years, in beetle terms – we compare the size of the testes in the monogamous and polygamous lineages.

“We get evolutionary divergence: males from monogamous lines have smaller testes than those from polygamous lines. Then, if we arrange sperm competitions, males from polygamous lines with larger testes sire more progeny than monogamous males with small testes.”

Simmons says many researchers are studying species with alternative male mating strategies, looking for variation in sperm quantity and sperm quality. Such studies indicate that sneakers generally have higher quality sperm than guarders.

They produce more motile sperm, and despite the sneaky males being at a physical disadvantage in any contest with guarder males that limits their access to females, they tend to out-compete guarder males in terms of numbers of progeny.

“The maintenance of these tactics is an interesting issue, and there’s a lot of theory out there. We’re trying to identify mechanisms that help maintain these alternative tactics.

“Theory predicts that sneaker and guarder males should achieve equal fitness – a sneaker may get an advantage through the sperm contest by out-competing other males, while guarders achieve equal fitness by having access to more females.

“There’s not much empirical data out there, but game theory has developed broad, conceptual predictions.”

Simmons says females are not passive players. “If they don’t mate with multiple males, there’s no sperm competition. Females create competition by mating randomly and through their preferences for certain male behaviours – for example, they strongly prefer males that invest extra effort in courtship. They want the best male to father their offspring, and the tactics of being a sneak or a guard are not necessarily related to the ability to father the fittest offspring.

“Horned males might not have the best genes, so females increase their chances by allowing sneaks to copulate with them.”