for National Geographic News
Disguises used by female damselflies to avoid unwanted sexual advances can cause males to seek out their own sex, a new study suggests.
Belgian researchers investigated why male damselflies often try to mate with each other. The scientists say the reason could lie with females that adopt a range of appearances to throw potential mates off their scent. In an evolutionary battle of the sexes, males become attracted to a range of different looks, with some actually preferring a more masculine appearance.
The study, published recently in the journal Biology Letters, says such evolutionary selection pressures could also explain homosexual behavior seen in males of other animals whose females assume a variety of guises. Such "polymorphic" species are seen in dragonflies, butterflies, hummingbirds, and lizards.
Female blue-tailed damselflies (Ischnura elegans) assume different color forms, or morphs, in adulthood: green-brown, yellow-brown, and blue. The blue form closely matches the male in both body coloration and pattern.
The study team found the sexual preference of male damselflies was influenced by the company they keep. Males that were housed together before being introduced to females tended to seek out their own gender afterward. But males kept in mixed-sex living quarters later preferred all three female forms when choosing a mate.
This suggests male damselflies are likely to become attracted to other males only when females are absent or scarce. Yet a minority of males still showed an innate preference for male mates.
The team's findings were reflected in mating behavior observed in the wild, says study author Hans Van Gossum, an evolutionary biologist at the University of Antwerp, Belgium.
Van Gossum says around 17 percent of males in wild populations appear to favor same-sex pairings, while about one in six males in the lab experiments showed the same tendency despite exposure to females. "This behavior can be considered homosexual," he said.
But why should any males choose to mate with each other instead of with females? Such behavior apparently goes against theories of sexual selection, which predicts the optimization of reproductive success. Homosexual damselflies, however, aren't going to sire too many babies.
Homosexuality has been recorded in a wide range of animals, including beetles, sheep, fruit bats, dolphins, and monkeys. In many such cases explanations have been put forward to explain this behavior.
Consider, for example, the beetle known as the sugarcane rootstalk borer weevil. U.S. and Israeli researchers suggest the reason why females of the species mount other females is that the behavior attracts big males with good genes. Puny males seem to shy away from such antics.
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