Though Arnqvist doesn't know why bigger is better, he suggests longer spines may act as an anchor: "Males can position their genitalia in an optimal way inside the female as the male releases [its] sperm."
A female typically mates with several males in her lifetime, driving competition among her various partners' sperm.
Scott Pitnick, a Syracuse University biologist who studies the evolution of reproductive strategies, said he was "really bowled over" by the study.
For one, it makes a "convincing case" that sperm success is due to the spininess of the genitalia, not to the damage to females, said Pitnick, who was not involved in the research.
"This takes us a giant step forward in understanding the evolutionary basis of male harm to females."
As spinier genitalia have evolved in males, female genitalia have also evolved, in a sort of "arms race," study co-author Arnqvist said.
Females have thick padding on their reproductive tract that's reinforced with strong, elastic connective tissue. After each mating event—about five to ten in their 25- to 30-day lifetime—the wounds heal and leave scar tissue.
Females are also skillful at thwarting males' attempts to mate, Arnqvist added.
This battle could hinder the beetles' longterm viability as a species, since evolving new defenses wastes energy that could be spent elsewhere.
But in this case, the arms race may give rise to so many adaptations—especially in genitalia—that the resulting beetles may represent entirely new species, Arnqvist said.
For instance, 20 to 30 seed beetle species have arisen whose males look nearly identical, but have wildly divergent sexual parts.
"The male copulatory organ is the single [physical] trait that evolves very rapidly across animals," Arnqvist said.
SOURCES AND RELATED WEB SITES