Same-sex sexual behavior documented in many mammals: does it mean that similar behavior in humans is “natural”?

The Naturalistic Fallacy, which most of you surely know, it the erroneous equation of what does exist with what should exist.  Discussed extensively by Hume, it is the false equation of “is” with “ought”. In biology, it takes the form of observing some behavior in animals that is similar to a behavior in humans, and then justifying or saying the human behavior “natural”  or “good” because we see it in other species.

But this is a bad argument, for it cuts both ways. After all, animals show a lot of behavior that would be considered reprehensible or even immoral in humans.  In fact, Joan Roughgarden wrote a book, Evolution’s Rainbow, which describes sex and gender diversity in nature as an explicit way of justifying similar behaviors in humans as good—because they are natural. I reviewed the book for TLS and wrote this bit (review no longer online but I can send a copy).

Coyne, J. A.  2004.  Charm schools. (Review of Evolution’s Rainbow, by Joan Roughgarden). Times Literary Supplement, London. July 30, 2004 (No. 5287), p. 5.

But regardless of the truth of Darwin’s theory, should we consult nature to determine which of our behaviours are to be considered normal or moral? Homosexuality may indeed occur in species other than our own, but so do infanticide, robbery and extra-pair copulation.  If the gay cause is somehow boosted by parallels from nature, then so are the causes of child-killers, thieves and adulterers. And given the cultural milieu in which human sexuality and gender are expressed, how closely can we compare ourselves to other species? In what sense does a fish who changes sex resemble a transgendered person? The fish presumably experiences neither distressing feelings about inhabiting the wrong body, nor ostracism by other fish. In some baboons, the only males who show homosexual behaviour are those denied access to females by more dominant males. How can this possibly be equated to human homosexuality?

The step from “natural” to “ethical” is even riskier. As the philosopher G. E. Moore argued, identifying what is good or right by using any natural property is committing the “naturalistic fallacy”: there is no valid way to deduce “ought” from “is”. If no animals showed homosexual behaviour, would discrimination against gay humans be more justified? Certainly not. Roughgarden’s philosophical strategy is as problematic as her biological one.

Now a new paper in Nature Communications has appeared that had the potential to demonstrate the same fallacy, but fortunately the authors went to great lengths to avoid that  The same, however, is not true of ZME Science, which gave a précis of the paper and stepped on the Fallacy’s tail.

First the Nature paper itself, which you can access by clicking on the article below, or by reading the pdf here.

It’s a good paper on the evolution and phylogeny of “same-sex sexual behavior” in mammals, which they define as “transient courtship or mating interactions between members of the same sex“.  

Note that it’s “transient,” which explicitly excludes homosexuality, most notably in humans, which is a persistent sexual attraction to members of one’s own biological sex.  This form of transient sexual interaction is surprisingly common—a conservative estimate is 4% of all animal species, and, as the authors say, [includes] “all main groups from invertebrates such as insects, spiders, echinoderms, and nematodes, to vertebrates such as fish, amphibians, reptiles, birds, and mammals.”

Now there are two ways to explain a behavior that seems on its face maladaptive. Why would you engage in sexual behavior that doesn’t involve passing on your genes? One hypothesis is that it’s just a nonadaptive byproduct of other behaviors: a general drive to mate when the appropriate mates aren’t available, or simply mistaken identity.  But the authors investigate two hypotheses that it is adaptive, and give some tentative evidence for that.

First, the results:

• The authors did a comprehensive survey of same-sex behavior (defined above) in 2546 species of mammals, and superimposed species with and without such behavior on their phylogenetic tree. The object was to see how many times the behavior evolved independently, and whether it was present in the common ancestor of a group (and thus could be passed along to its descendants). Here’s one of those phylogenies with the caption. (You needn’t worry about the details or summary, as I’ll give it below).

A summary:

•  The behavior was reported in 261 mammalian species
• Same-sex sexual behavior appears to be equally common in males and females, and the behavior in both sexes tends to be correlated across groups. That is, male and female same-sex behavior is more likely to both appear in the same species than if it either were distributed randomly among groups.
• It was not possible tell, using phylogenetic analysis, whether same-sex behavior was likely to be a trait in the ancestor of all mammals, but was NOT likely to be a trait in the ancestor of all placental mammals.
• The behavior seems to have evolved independently in many lineages, so same-sex sexual behavior seems to be a case of “convergent evolution.”
• The behavior is correlated with whether or not a species is social. If it is social, there’s a significantly higher probability of same-sex sexual behavior. (Remember that this is a correlation and doesn’t imply that sociality prompts the evolution of such behavior. The behavior could simply result from iondividuals in social species being closer to other individuals than those in non-social species.)
• The common ancestor of all primates does seem to have possessed same-sex sexual behavior.

The association of same-sex sexual behavior with sociality leads the authors to conclude that the behavior evolved by natural selection as a way to enforce inter-individual harmony required by sociality. They mention two such advantages:

1.) Same-sex sexual behavior is a way of creating and maintaining social bonds between individuals in a group; it’s a bonding mechanism.
2.) The behavior could also help prevent or resolve conflicts between members of a group, allowing a hierarchy to develop without injury of death to group members.

The authors mention that these effects have been demonstrated in some species like bottlenose dolphins and American bison, but I’m not familiar with this work, and such conclusions seem to me to be extraordinarily difficult to arrive at. However, I’ll take the authors’ word for it.

The authors are, to be sure, careful in their conclusions. First, they note that nonadaptive hypotheses, like “mistaken identity” could also contribute to the behavior.

Second, and the big one, they note that the behavior they studied is not the same as homosexual behavior like we find in humans.  They do add, however, that it humans do show same-sex sexual behavior in humans (I presume they’re referring to “nonbinary” people who have sex with both males and females). From the paper:

However, same-sex sexual behaviour is operationally defined here as any temporary sexual contact between members of the same sex². This behaviour should be distinguished from homosexuality as a more permanent same sex preference, as found in humans. For this reason, our findings cannot be used to infer the evolution of sexual orientation, identity, and preference or the prevalence of homosexuality as categories of sexual beings Nevertheless, even taking into account this cautionary note, by using phylogenetic inference, our study may provide a potential explanation on the evolutionary history of the occurrence of same-sex sexual behaviour in humans.

They may be right, but I think they should have added that even if same-sex sexual behavior was rare or nonexistent in mammals, its existence in humans is not made “ethical” or “natural” in our species. That would be an example of the naturalistic fallacy, and I emphasize that they do not commit it.  I’d would also emphasize, as I did above, that any sexual behavior between consenting human adults is not for us to judge, regardless of whether or not other species show it, and that such behaviors are fine so long as they’re legal. We don’t need to justify same-sex sexual behavior in humans by seeing it elsewhere in nature. But perhaps this stuff doesn’t belong in a scientific paper. But I want to emphasize it here, as I did in my review of Roughgarden’s book.

As I said, the authors don’t commit the naturalistic fallacy, but the ZME Science paper below comes close to it. Click headline to read:

Up until the end, this article is okay, but then it can’t resist diving into our own species (bolding is mine).

However, the researchers distinguish between SSSB and sexual orientation. While SSSB involves occasional same-sex interactions, sexual orientation encompasses consistent patterns of attraction and identity, particularly prominent in humans.

While SSSB in animals supports the naturalness of such behaviors, human experiences of sexuality include layers of identity, culture, and personal meaning that go beyond biological explanations. Homosexuality in humans often involves stable sexual orientations and relationships, distinct from the transient or context-dependent SSSB observed in some animal species.

Ultimately, the widespread occurrence of SSSB in mammals, especially primates, strongly suggests that such behaviors are natural and adaptive. Normalizing same-sex behavior as a part of this spectrum aligns with both biological evidence and a broader understanding of human social and emotional complexity.

The last paragraph explicitly says that the results show that homosexuality (one of “such behaviors”) is “natural and adaptive”, as are all “same-sex behaviors” in humans.  The Nature paper says nothing of the sort.  The authors of the Nature paper explicitly exclude homosexuality as not a behavior they studied, but ZME Science lumps it in with other same-sex sexual behaviors, dwspite homosexuality being very different from SSSB.

Again, you do NOT need to justify same-sex sexual behavior, whether it be transient or permanent, by finding examples in the natural world. If we didn’t find any other species with homosexual behavior, would that make it wrong or bad in humans? Of course not! “Is” does not equal “ought,” and I’ll add the corollary that “not is” does not equal “not ought”. The Nature paper is valuable it looking at the evolution of a behavior and testing hypotheses about its adaptiveness, but of course adaptiveness or evolution has nothing to do with the ethics of behaviors between consenting human adults.