Evolution of sexuality
We have seen only one side of sexual selection: female choice (male choice also occurs but appears to be less common, because females are more typically the limiting resource—this fascinating concept will be explored soon). The other side of sexual selection is intrasexual selection—competition between members of one's own sex. Again, one kind is more common: male-male competition. These giraffe, impala, and elephant males are practicing for more serious male-to-male combats in the future.
Numerous publications have requested to use the image of the giraffes necking. We agreed as long as the behavior was not described as homosexual behavior, which would have been misleading. Time Magazine, however, published it to our surprise. In spite of a deceptive title, the article is a good description of how such behavior represents youthful sparring in preparation for adult competition. A consultant, the primatologist Frans de Waal, fortunately had explained to the Time Magazine editors that same-sex pairing in nonhuman animals occurs mainly as an isolated activity, not as a sexual lifestyle.
Do we see same-sex pairings in nonhuman animals?
Yes, and here are a few examples:
- Bonobos (our closest kin, along with chimpanzees) engage in recreational sex, often on a daily basis. Females rub genitals in face-to-face encounters, with apparent excitement and pleasure. Youngsters invite mutual sexual stimulation with other youngsters and adults. Playful sex is a part of their normal social life.
- Male dolphins have been seen to copulate briefly with other males of their species and even with turtles, sharks, and eels. The Amazon River Dolphin has even attempted copulation in the blowhole of another!
- An Adelie Penguin male often mounts another.
- Brief same-sex pairings of animals are often seen in zoos.
All of these examples are of short-term pairings. Do any nonhuman animals engage in long-term same-sex pairings? There are hints that this occasionally occurs in birds, but we are just beginning to address this question.