Damselfly Mating Game Turns Some Males Gay
for National Geographic News
|June 21, 2005|
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.
Female Japanese macaques also engage in intimate sexual acts with one another. U.S. primatologist Amy Parish and other researchers say female macaques may enhance their social position and form alliance partners through such intimacy which in turn can boost breeding success.
Van Gossum and his colleagues propose a new explanation for homosexuality in animals like the blue-tailed damselfly. When males face strong evolutionary pressures to be flexible about their idea of what a female should look like, males may end up also fancying their own sex.
Males damselflies need to be adaptable because their female counterparts are adaptable. Numbers of the three main female color forms, or morphs, fluctuate over time.
Van Gossum, the study author, says most researchers agree such polymorphism most likely results from sexual conflict, with females evolving traits to avoid excessive harassment. While plenty of sex might suit male damselflies, this isn't the case for females.
Joan Roughgarden is a professor of biological sciences at Stanford University in California. She writes, "Copulation ranges from over one hour to over six hours, averaging three hours. While a long copulation might seem like great fun, this can waste a whole day and be too much of a good thing, especially if carried out day after day over a life span that is only a few days long."
Roughgarden adds that female damselflies collect all the sperm they need to reproduce from a single mating.
"Males go for quantity and females for quality," Van Gossum said. "As a consequence, females may wish to avoid excessive male attention. One way of doing so is by looking different from what a male thinks a female to be."
The blue female form may accomplish this by mimicking the appearance of males. But Van Gossum says an alternative theory is that male harassment also leads to other morphs.
"The minority female morph in a population"whether blue or another form"is the one that benefits, by receiving less male harassment," he added.
In turn, it's likely that males have developed a flexible "search-image" that matches the majority female fashion of the day. This boosts a male's chances of finding a mate.
"Males with a search-image that can be changed if the minority female morph becomes the majority morph are probably out-competing males that are less flexible," Van Gossum said.
Such flexibility may also lead to genuinely homosexual damselflies.
Van Gossum says such behavior could arise when a male is still young. A preference developed in male damselflies before reaching maturity, he says, is probably less prone to change in later life.
The Belgian researcher adds that evolutionary pressures that shape damselfly mating behavior may also explain homosexuality seen in other male animals, including butterflies and hummingbirds, whose females similarly adopt a range of colorful guises.
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