Homosexual selection: The power of same-sex liaisons
07 December 2009 by Kate Douglas
Gallery: Nature's homosexual pin-ups
Editorial: Homosexuality in nature is no guide to morality
NOT long ago, the news was full of reports about two male Humboldt penguins at a zoo in Germany that adopted an egg, hatched it and reared the chick together. It seems like every time you turn around, the media spotlight has fallen on another example of same-sex liaisons in the animal kingdom.
In the past few years, the ubiquity of such behaviour has become apparent. This summer evolutionary biologists Marlene Zuk and Nathan Bailey from the University of California, Riverside, published a paper on the subject that included examples from dozens of species ranging from dung flies and woodpeckers to bison and macaques.
Homosexual encounter. Female Laysan albatrosses on the Hawaiian island of Oahu pair off with other females. (Image: Tui De Roy/Minden Pictures/FLPA)
That is just the beginning of the story. The burning question is why same-sex behaviour would evolve at all when it runs counter to evolutionary principles. But does it? In fact there are many good reasons for same-sex sexual behaviour. What's more, Zuk and Bailey suggest that in a species where it is common, it is an important driving force in evolution.
Although terms such as homosexual, gay and transgender are commonly used by the mass media, and even by some ethologists, Bailey and Zuk believe you shouldn't extend these descriptors of human sexuality to animals. "It's not simply that they are burdened with the weight of social, moral and political implications, which can obscure objective scientific study," says Bailey. "The problem is that while we can observe the sexual behaviour of animals, we often have little inkling about what motivates it." Besides, as far as we know animals do not form sexual self-identities in the way humans do, he adds. That is why he and Zuk prefer to use the more objective term "same-sex sexual behaviour", which they define as behaviours found in two animals of the same sex that you would find in opposite-sex pairs during courtship, copulation or parenting.
Same-sex behaviour is not necessarily synonymous with same-sex preferences, which have been observed in only a handful of animals. In 2005, for example, Hans Van Gossum from the University of Antwerp in Belgium and colleagues found that damselflies kept in all-male groups subsequently preferred to court other males rather than females, though this preference could be reversed simply by housing them with females (Biological Letters, vol 1, p 268).
Neither can you necessarily infer anything about sexual orientation from same-sex behaviour. Orientation is tricky to establish because it requires information about the consistency of partner preferences over a long period of time. Examples are thin on the ground, either because they do not exist or because they have yet to be discovered. The most notable include some male bighorn sheep that have been observed to predominantly mount other males throughout their lives, and female Laysan albatrosses - more of which later.
Nevertheless, even narrowing the scope to sexual behaviours rather than preferences or orientation leaves a huge evolutionary puzzle. Why would individuals expend time and energy in activities that fail to increase reproductive success? Could the sheer numbers engaging in same-sex behaviour mean that it has survival benefits after all?
Why expend time and energy in activities that fail to increase reproductive success?
In 2008, Sara Lewis at Tufts University in Medford, Massachusetts, and colleagues decided to address this question (Journal of Evolutionary Biology, vol 22, p 60). Red flour beetles are a scourge of the pantry, but they are up to more in there than just infesting your food. Sexually receptive females locate males by homing in on airborne pheromones released by the males, but these same signals also occasionally attract other males. The mounting male clambers on top of his quarry and extrudes his genitals, sometimes transferring sperm to the hind end of his partner. Might these male-male copulations provide some benefit to the participants? The researchers designed an experiment to test three possibilities: that males establish social dominance by mounting other males, that males who mount other males gain practice for later sexual encounters with females, and finally, that mounting males transfer sperm onto the other males, who then inadvertently inseminate a female with it later on. Only this last idea stood up: they found that a small proportion of offspring were fathered by males who had never mated with the mothers but had mounted another male that had subsequently copulated with the female.
Other research groups have tested the evolutionary underpinnings of same-sex behaviour in different species and come up with a variety of explanations. Zuk and Bailey were intrigued by the idea that there might be common factors in these various theories. Their paper brings all the evidence together for the first time and concludes that there are many evolutionary origins of same-sex sexual behaviour (Trends in Evolution and Ecology, vol 24, p 439).
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