Wild Animal Sex

New research in birds, reptiles, and insects is redefining “normal” sexual behavior, revealing that gender-bending, promiscuous, and dangerous sex isn’t limited to humans.

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The Australian splendid fairy-wren has a peculiar way of passing on its genetic material. It starts off in a manner that might seem familiar to anyone who’s seen a 1950s family sitcom: Boy meets girl, boy partners with girl for life, boy and girl raise family together. But that’s where the similarities end. After the baby wrens grow up, they don’t pair up with other wrens right away; instead, they help their parents raise the next brood. Except that next brood is likely not the true genetic offspring of the “father” of this family.

You see, while splendid fairy-wrens do pair up in family units for life, they rarely mate with their original partners. Instead, both males and females like to get together for dangerous sex with other wrens, who in turn may be socially paired with other wrens. And calling this sex “dangerous” may be only a slight exaggeration. Science writer Rob Mitchum blogged last week about research showing that these tiny wrens not only mate primarily outside the family unit, they may be more interested in mating in the presence of one of their primary predators, the butcherbird.

Bright blue male splendid fairy-wrens, instead of fleeing in the face of danger, sing a distinctive song when a butcherbird is in the area. The researchers, Emma Greig and Stephen Pruett-Jones, say that the wren’s song responds to the butcherbird’s song, so that the two are almost singing a duet. The team played a variety of recorded songs to female butcherbirds in the wild, and found that females were significantly more likely to look in the direction of the songs and respond with their own songs when they heard the combined butcherbird–male wren duets.

The next obvious question is, why? This research doesn’t tell us. The classic Darwinian explanation might be that “braver” males produce offspring that are fitter and more likely to survive. But Greig says that the wrens might not be in any real danger; when the butcherbird is singing it’s probably not interested in hunting. What’s more, there are no obvious differences in male wrens’ singing behavior based on other physical traits such as health and color. Future studies could examine whether females that are more attracted to the dangerous duets produce more or fitter offspring. Greig is also studying other bird species in the area to see if they exhibit similar behaviors. The research was published in Behavioral Ecology.

But the sexual behavior of blue fairy wrens is not the only lifestyle you wouldn’t expect to see on Leave it to Beaver or Father Knows Best. In 2006, here at Seed, Jonah Lehrer took a controversial look at the work of Joan Roughgarden, who has catalogued hundreds of examples of homosexuality in animals, including lesbian bonobos, gay big horn sheep, and bluegill sunfish threesomes. Last week, evolutionary biology graduate student Jeremy Yoder revisited Roughgarden’s work and the peer-reviewed research that forms its basis.

Yoder cites Roughgarden’s example of white-throated sparrows as an example of an animal that flouts the usual pattern of sexual behavior. These sparrows come in two different color schemes, possessing either a white facial stripe, or a tan stripe. But unlike many birds where the coloration varies by gender, both males and females come in both types. White-striped sparrows tend to be aggressive, while tan-striped birds are more submissive. The most successful sparrow couples are mixed pairs, but it doesn’t matter whether the male or female is the aggressive one.

Whiptail lizards go even further. The females don’t actually need males to reproduce; they can lay fertile eggs without mating at all. However, these females still pair—with other female lizards. The lesbian lizard couples do have sex, and although intercourse is not needed to produce fertile eggs, sex in female pairs stimulates them to lay eggs. These female-female couples are more stable than heterosexual couplings in related lizard species.

So should this research be used to argue that homosexuality is morally justifiable? Not unless you want to also argue, among other things, that men should break their genitals and leave them inside women after mating, just like the wasp spider. But it certainly demonstrates that there is a wide range of natural sexual behaviors, all of which can enable a species to thrive. While most evolutionary biologists, including P.Z. Myers, believe that Darwinian sexual selection based on male-female pairing can still explain many features of animal evolution, Roughgarden and her colleagues have shown that for lots of animals, the traditional sexual model doesn’t cut it. Perhaps father doesn’t always know best.

Dave Munger is editor of ResearchBlogging.org, where you can find thousands of blog posts on this and myriad other topics. Each week, he writes about recent posts on peer-reviewed research from across the blogosphere. See previous Research Blogging columns »

Originally published January 26, 2011
Credit: Flickr user jmcgross Research Blogging / by Dave Munger /

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