The way they don’t do it on the Discovery Channel…
Reflecting on same-sex sexual behavior in animals other than ourselves.
Humans are, by far, not the first, nor the only ones, to be occasionally attracted to the same sex. Homosexual behavior, such as same-sex courtship, pair bonding and copulation, has been documented literally thousands of times in a wide range of species spanning across taxa-- including mammals, birds, reptiles, amphibians, insects, mollusks and nematodes (Bailey and Zuk, 2009). Researchers have attempted to elucidate possible evolutionary origins of these behaviors, as well as reasons for their maintenance in populations. Here, I will touch upon a few particularly intriguing examples of what’s been called an “apparent paradox” that violates the basic “law of nature: that of procreation” (Levan et al., 2008).
Ok… this might be taking it one step further….
Same-sex behavior has been considered as a puzzle that requires a special explanation, and can be compared to behaviors like suicide, or adoption of unrelated infants, all of which don’t lend a direct fitness benefit to the individuals who exhibit them. However, as a behavioral ecologist, I, personally, am pretty hell-bent on finding one! I mean, look how it turned out for the unicorns…
So why do these animals do it? Homosexual behavior defies the long-held popular belief that all sexual behavior in animals is somehow linked to reproduction. However, a look at a particularly promiscuous species, the Bonobo monkey, offers other possible explanations.
Sex in Bonobo monkey societies is, essentially, a free-for-all. Female bonobos only actually reproduce every six years or so, yet they have sex... all the time… no matter if they are pregnant, nursing, very old or very young (Fruth and Hohmann, 2006). Because sex in none of these fours states will yield reproduction, there must be some other advantage to such nymphomanism. Furthermore, they do not restrict themselves to heterosexual activities; females are often found to engage in such behavior as rubbing each other’s genitals (sometimes even successfully…), while males kiss, touch and hump each other to a lesser extent, but frequent nonetheless. Bonobo sex is so prolific (to the extent where it dominates the species’ ethogram, or list of behaviors of a species), it trumps other behaviors such as fighting, and the apes have been dubbed the “Make Love, Not War” primate species. In fact, researchers Fruth and Hohmann hypothesize that sexual behavior might ease social tensions in a hierarchical society such as theirs, as well as facilitate reconciliation among group members that may have had a spat. I think our species may have something to learn from them…
While a fascinating overview of homosexuality in several species, skip to 5:30 for the ultra-sexy story of Bonobos.
Like the majority of bonobos, parthenogenetic whiptail lizards seem to enjoy “meaningless” sex. As a parthenogenetic species, a female needs no male in her life in order to reproduce; her eggs require no fertilization in order to develop. This is surely another example of homosexual behavior lending no direct reproductive benefit… or is it? Studies have found that the females that engage in copulation behavior have more, as well as better quality, eggs than females who didn’t (Crews and Fitzgerald, 1980). Mating behavior, if not actual mating, essentially stimulated good egg production. Now that’s good reason to dry hump!
These frisky reptiles, while appearing to be engaged in a successful copulation, are, in fact, two females.
Another example of animal homosexuality is a touching one. In the Laysan albatross, long-term pairing of unrelated females (ruling out possible kin selection motivation where a female is at least gaining from passing on her genes via related kin) has been well documented (Young et al., 2008). In this case, there is an uneven (female bias) sex ratio in the species, so pairing up with another female to raise offspring is better than not pairing up at all, and fledging no offspring. In fact, both females in the pair were linked genetically to at least one of their mutual young, so both were gaining (fitness wise), by this behavior. Furthermore, same-sex pairing removes excess females from the population that would, under other circumstances, provide pressure for males in opposite-sex pairs to abandon their partner (Young et al., 2008); the same-sex females might essentially be “taking one for the team” for the greater benefit of their species!
Bailey and Zuk, 2009
I’d like to end with a little bit of “food for thought”… while homosexuality is found in over 150 species, homophobia has been found in only one. Homophobia, not homosexuality, may be, then, what is not adaptive!
Bailey, N.W., M. Zuk. 2009. Same-sex sexual behavior and evolution. Trends in Ecology and Evolution 24(8): 439-446.
Crews, D., K.T. Fitzgerald. 1980. “Sexual” behavior in parthenogenetic lizards (Cnemidophorus). Proceedings of the National Academy of Sciences 77(1) 499-502.
Fruth, B. and G. Hohmann. 2006. Social grease for females? Same-sex genital contacts in wild bonobos. Homosexual Behaviour in Animals, Cambridge University Press 294–315.
Levan, K.E., T.Y. Fedina, S.M. Lewis. 2008. Testing multiple hypotheses for the maintenance of male homosexual copulatory behaviour in flour beetles. Journal of Evolutionary Biology 22: 60–70.
Young, L.C., B.J. Zaun, E.A. VanderWerf. 2008. Successful same-sex pairing in Laysan albatross. Biology Letters 4: 323-325.