Field of Science

Homosexuality is catholic in the animal kingdom

ResearchBlogging.orgBeetles, bisons, black swans, bonobos, dolphins, elephants, flamingos, fruit bats, fruit flies, giraffes, lions, lizards, macaques, orangutans, ostriches, penguins, sheep. What do these animals have in common?

They are all homosexual. In fact, the list is much, much longer. Here is what Petter Bøckman has to say about it:
No species has been found in which homosexual behaviour has not been shown to exist, with the exception of species that never have sex at all
We can thus safely conclude that the argument against human same-sex marriage - that homosexuality is not natural - is invalid. If we wish to be informed by nature in this matter, proposition 8 should not have been passed (but of course, those who are against same-sex marriage are either homophobic or religious - they don't really care if it's natural or not).

In fact, I personally find this little story quite touching:
Roy and Silo, two male chinstrap penguins at New York's Central Park Zoo have been inseparable for six years now. They display classic pair-bonding behavior—entwining of necks, mutual preening, flipper flapping, and the rest. They also have sex, while ignoring potential female mates.
The question then is why animals are homosexual at all. As reproduction really only does occur between male and female, it would seem evolutionarily disadvantageous to spend any time sexing up to members of your own sex. Lost effort. So why do it?

There are a number of hypotheses trying to explain the phenomenon. It could be to establish social dominance. I have heard this is the case with orangutans, where the dominant male will take the loser after a fight. It could be practice for prom night. Could even be that sperm is deposited on the other male, who then injects it into the next female he gets frisky with. Social cohesion is another possibility (which is ironic, because religion has also been posited to boost social cohesion). This is most probably the case in bonobos: 75 percent of bonobo sex is nonreproductive and that nearly all bonobos are bisexual (source).

These are all adaptive reasons for homosexuality. But there are non-adaptive possibilities too. Generally, it could be a by-product of another trait that is adaptive. It could be a developmental anomaly, such as an enlarged sexually dimorphic nucleus in the ovine medial preoptic area (source).

Last month a paper came out in the Journal of Evolutionary Biology (reference below) in which the authors suggest that homosexual copulations may be a behavioural mechanism that allow males to expel older, potentially low-quality sperm (reader-friendly report in National Geographic). The authors investigated the "dominance", "practice", and "sperm translocation" hypotheses mentioned above in flour beetles (Tribolium castaneum), but found no evidence for the first two, and only little for the third. Instead, based on direct observation of flour beetle sperm ejaculated upon another male, the researchers conclude that the beetles seem to use homosexual copulatory behavior to get rid of inferior spermatophores.

I find this hypothesis intriguing, but I really don't think there is enough evidence to conclude that flour beetle homosexuality serves such a mundane purpose. Without having to invest any time in such research, I would bet that this behavior is maintained in evolution because it feels good, which sex most probably often does for adaptive reasons. And as long as the males that indulge with other males also do it with females, the behavior need not be lost in evolution.

If the males are only so good at telling males and females apart, the fittest males may be those who don't think twice about who they get behind. Imagine this algorithm:

1) Find another beetle.
2) Mate with other beetle.
3) Check gender of other beetle.
4) Goto 1.

Depending on how easy step 3 is, that might be way more adaptive than this one:

1) Find another beetle.
2) Check gender of other beetle.
3) If other beetle is female, copulate.
4) Goto 1.

K. E. LEVAN, 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 DOI: 10.1111/j.1420-9101.2008.01616.x


  1. Great post. You're right that there's nothing necessarily maladaptive about bisexuality, your flour beetle explanation is great, and I think highly likely. How do we explain pure homosexuality though?

    I like the explanation that developmental anomalies could lead to homosexuality. It does seem like a lot of sexual dimorphism is assigned developmentally, and could therefore be easily disrupted. Since in many species both sexes usually have most of the genes necessary to confer sexually dimorphic traits I can see how it could be easier to allow for some homosexuality than to eliminate it.

    I recently wrote a much less informed post about a half baked evolutionary psychology article in the Economist at

  2. I think the developmental anomaly idea could explain pure homosexuality. I wouldn't venture a more detailed description, though. I would be lovely with a solid theory, because if one could show that it is an effect that has nothing to do with choice or some such, then it would really shut up those homophobes who think homosexuals can be "cured" with prayer, etc.

    Note that the word anomaly isn't to be taken as "wrong", but merely to indicate that it is something outside of the norm. And while the majority of, say, humans are heterosexual, homosexuals number close to 10%, so it isn't that small a minority. I realize you didn't take it as that anyway, but many other people automatically think of homosexuality akin to a disease.

  3. One thing I have wondered at is what the losers in sexual combat do in order to relieve themselves after a battle. Stags have neither hands nor baby oil, and if they don't get to mate how would they ejaculate? I am curious as to whether homosexual behavior may also serve to keep the non-successful males satisfied until the next rutting season, and the next attempt at winning the right to copulate with a female.

    It seems apparent that bisexual and homosexual behavior is not maladaptive for a population, because it could prevent the losers from "ganging" up and killing the "victor." That would be a positive adaptation for homosexual behavior.

  4. I'm not a scientist by any mean, so would be curious as to what more knowledgeable folks here would say about this:

    I read this blog post, and kept reading, looking for a theory which mentioned population, but never saw it.

    It was either in college (85-89) or reading something shortly thereafter which postulated population as a possible explanation for homosexuality. That theory has always stuck with me as something that might make sense, and expanding on this, I've been curious if male-to-female ratios could be factored in as a partial explanation, in addition to or instead of (over)population factors.

    I suppose that this would be more of an environmental adaptation explanation, as opposed to genetic, which intuitively I would find hard to imagine is not at least a partial factor.

    But what do I know? I'm not a scientist. But curious.

  5. Mike, your relief-theory is not such a bad idea. Wanting to have sex is so obviously the means by which e.g. humans reproduce (as opposed to a drive to have children). But when we don't get any, there are other means, which species without hands don't have access to. It's like being programmed to ejaculate at all costs, and then hoping that some of it ends up in contact with an egg.

    Anonymous, curiosity is key. I wish you could give us some more details on the population-idea. Do you mean that over-population is hypothesized to select for homosexuality? Also, whether there is a genetic basis for homosexuality is debated (I recall there is some evidence, but I could be wrong), but whether homosexuality is adaptive in any way is perhaps even more uncertain. There need not be, just as there probably isn't any advantage to male pattern baldness, or the production of snot when we have a cold (phenotypic by-products are very common).

  6. However, Mike's hypothesis is supported by a "species selection" argument. Homosexual behaviour must increase the fitness of the homosexual individual to be “fixed” by natural selection.

    Moreover, actually hands are not needed to “sperm renovation”: Galapago's iguanas can masturbate themselves "without hands" (Wikelski and Bäurle 1996. Proc. R. Soc. Lond. B 263: 439-444).

  7. Bjorn,
    While i amply agree with you that homosexuality is natural and that there are probably individuals of most species that are, indeed, more homosexual or exclusively homosexual than bisexual or straight, i don't see how you can make a sound argument against Prop 8 based on that. Prop 8 (which i agree is hateful and bigoted) is about marriage. Marriage is a strictly human cultural activity. Just as taking photographs and shopping are strictly human cultural activities. They serve several, sometimes contradictory purposes and human beings have not always had such institutions. One can certainly choose to be in a monogamous relationship and have children even if marriage did not exist. Many feminists even argue that marriage has historically, for most of the world, been an institution through which women are conceptualized as property to the families they marry into.

  8. What I did was counter-argue against those who were for prop 8 saying that "homosexuality is unnatural." I heard this several times before the election, and I wrote this post as a response.

    We can thus safely conclude that the argument against human same-sex marriage - that homosexuality is not natural - is invalid. If we wish to be informed by nature in this matter, proposition 8 should not have been passed.

  9. If every individual in a species was to successfully engage in heterosexual behavior the species would quickly over produce their ecological niche and go extinct. (research says that only 10-2o% of a opulation needs to successfully breed to maintain existence of a species.) I truly believe the change in developmental and in-utero environments effecting the percentage of non-procreative sexual behavior IS adaptive for the species groups as a whole in the long run not for the individual's win. Therefore, bisexual-homosexual behavior IS beneficial in keeping fecund populations from over breeding.

  10. I truly believe the change in developmental and in-utero environments effecting the percentage of non-procreative sexual behavior IS adaptive for the species groups as a whole in the long run not for the individual's win.

    It is well-known, though, that this doesn't work like you describe. There is no incentive for any individual or group/population to breed less for the species as a whole. Other populations of the same species that do not have this restriction on their reproduction will outcompete the population that does, so the trait you are talking about will never fix. It is not adaptive.

    Besides, there are plenty of examples of species that "overbreed", and yet haven't gone extinct. It isn't really possible for a species to become so big that it goes extinct (unless we are talking about strange scenarios where the species crashes the resources it feeds on, but this is not likely, as the population size will decrease prior to the resource vanishing completely).


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