"Torture" Phalluses Give Beetles Reproductive Edge
National Geographic News
|February 25, 2009|
It's no pain, no gain in the cutthroat world of seed beetle sex: Males with the longest and spiniest genitalia propagate their genes better than their less endowed rivals, a new study says.
The males' sexual organs have barbs and spikes that resemble medieval torture instruments, said study co-author Göran Arnqvist, an evolutionary biologist at Sweden's Uppsala University.
"They literally injure females internally in their copulatory duct. They're pretty mean," Arnqvist said.
(View pictures of these bizarre beetle genitalia.)
Several species, especially among insects, are known to physically harm their mates in reproduction, but scientists aren't sure why these traits evolved.
The new research offers the first proof that dangerous genitalia in males can represent a reproductive advantage.
The resulting wounds in the females, however, are likely just an "unfortunate side effect" of the strategy, Arnqvist said, and do not provide a reproductive benefit.
(Related: "Beetles Are Thirsty for Sex.")
Arnqvist and Uppsala colleague Cosima Hotzy drew from 13 distinct populations of beetles—from California to Yemen—and one control group from Nigeria, the males of which were sterilized.
Virgin females were mated with both virgin males and sterilized males from the control group.
After examining the males' genitalia under a microscope, the researchers discovered that the males with the most damaging sexual organs fertilized more eggs, the scientists will report in the March 10 issue of the journal Current Biology.
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.
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