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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