Over the past few decades, American society has increased its tolerance and acceptance of differing sexualities. Those that voice opposition to acceptance of homosexuality on religious grounds often consider homosexuality to be “unnatural.” However, homosexual behavior is widespread across the animal kingdom. In addition to well-known examples such as in mammals and birds, homosexual behaviors occur in reptiles, amphibians, fish, and invertebrates. Among the primate order, homosexual behavior is most frequently observed in bonobos. However, it also occurs in other species, such as Japanese macaques and capuchin monkeys. Recent observations of homosexual behavior in male spider monkeys adds to our knowledge of these behaviors and may help us answer questions about the evolutionary functions homosexual behaviors may play, as well as allow us to consider if other animals have sexual orientations similar to the identities that humans construct.
Within the protected reserve of Otoch Ma’ax Yetel Kooh, Mexico, primatologists have been continuously collecting data for over 20 years on two social groups of Geoffroy’s spider monkeys (Ateles geoffroyi). Over the years, they’ve documented surprising behaviors such as territorial raids and lethal aggression. From April-December 2014, Laura Busia and colleagues observed three separate instances of homosexual behavior between male spider monkeys. This is particularly notable, given that observing any sexual behavior in spider monkeys is typically rare. Spider monkeys are a challenge to study because they tend to be high up in the tree canopy and can move very quickly through the trees. They also live in ‘fission-fusion’ communities, in which monkeys spend time frequently changing subgroups, and may go off by themselves. When females are ovulating, male and female pairs frequently go off to form “consortships,” in which they quietly leave the troop in order to spend time by themselves. All of these factors make observing sexual activity very rare. In fact, in about 20 months of research over a period of seven years at my own field site, I’ve only been able to observe a consorting pair once.
All three cases that Busia documented involved the same resident adult male, TU. In the first case, TU first copulated with a female, then left her to travel with another adult male. After grooming and grappling, TU initiated copulation with the other male twice. In the second case, while in a subgroup with both males and females, TU copulated with a lower-ranking peripheral male. In the third case, TU copulated with another resident adult male. The authors were able to describe these encounters in great detail. For example, here is an excerpt from case 3:
At 12:16, TU touched the base of EG’s tail with his foot, first on the dorsal side and then underneath near EG’s genitals. EG and TU lay on their sides, face to face, and each put one arm around the other’s shoulders. TU then slightly shifted and inserted its erect penis into EG’s anus, while in a ventroventral position.
Although it is difficult to draw conclusions from such few observations, Busia and colleagues consider how these behaviors fit with two competing hypotheses for homosexual behavior: the 1) regulation of social relationships hypothesis and 2) the tension-regulation hypothesis. Such hypotheses, derived from research on baboons and bonobos, suggest that homosexual behaviors may either be a means of strengthening alliances or defusing a tense situation. However, distinguishing between these two functions is difficult, particularly with such few observations. In bonobos, where genito-genital rubbing frequently occurs between females, there is some support for both hypotheses.
As with consortships with females, all three episodes occurred in the absence of other group members. However, unlike copulations with females, the male-male copulations were very short. The two studies with the best documentation of sexual behavior in spider monkeys both report very long copulatory periods. Previous studies of spider monkey sexual behavior, report copulations lasting an average duration of 14-17 minutes. In contrast, the male-male copulations that Busia and colleagues observed lasted less than 30 seconds. These differences suggest that male-male sexual behavior is not completely analogous to male-female copulations.
However, the question this study raises for me is the possibility that other animals may potentially vary in sexual orientations, the way that humans do. All of the observations that Busia documented involved a single male, TU. Does this mean TU is gay or bisexual? I don’t think we can assign that sort of terminology to him. Human sexual identities are channeled through human sociocultural constructions. Thus, it is uncertain whether these social identities can be accurately ascribed to other animals. Nonetheless, it is possible that other animals vary in their desire for homosexual versus heterosexual behaviors and partners. Most studies of homosexual behavior in animals suggest that it is a flexible response to social or demographic conditions, rather than emerging from individual orientations. However, in Japanese macaques, some females exhibit preferences for certain female partners over male partners. Additionally, some male sheep exhibit preferences for other male sheep, corresponding to differences in their brains. The fact that TU left consorting with a female for another male suggests that perhaps TU prefers males. While poor TU may be a bit isolated in his preference for males among his spider monkey brethren, at least he does not experience the prejudices that gay humans experience. Thus far, discrimination against sexual minorities appears to be a uniquely human trait.