I’ve seen this paper about strong homosexual pair bonding in zebra finches pop up in a couple of places around the web, but it only really caught my eye when I read Carin Bondar’s somewhat breathless report on the matter entitled “The astounding strength of homosexual bonds in Zebra Finches: Ladies need not apply…”. In essence, the researchers discovered that when you manipulate the sex ratio of zebra finch groups to be male-biased, male pair bonds form that display all of the same behaviours that female-male pair bonds do, and that when females are later reintroduced to these homosexual pair bonds, the male pair-bonds don’t break up.
It’s an interesting paper, and the findings are, well, pretty cool. But I have to disagree – respectfully – with Dr. Bondar’s assessment of the startling nature of these results. First, given that zebra finches tend to mate for life anyways, the finding that male-male pair bonds are strong shouldn’t come as a surprise if you think mechanistically. In fact, I think it would have been a lot more surprising if the male-male bonds had been of a different quality; if you think through the evolutionary implications, having a different mechanism for male-male as opposed to male-female bonds would imply that selective pressures on these types of bonds was different for some reason, and would really beg the question of why. Instead of a single ‘mating’ mechanism (a combination of hormones and neurobiology among other things, which I’ll touch on again in a moment), this ‘conditional’ pair bonding would require either a single mechanism with an unintended consequence, or two separate mechanisms, one for male-male bonds and one for male-female. That’s not out of the question, certainly; many potential explanations for same-sex sexual behaviour in animals imply such mechanisms. But to me, having a life long pair bond with females and then an entirely separate short-term pair bonding mechanism for male-male interactions would need explanation. Indeed, as Dr. Bondar’s own blog post notes, “homosexual couples both COURTED and COPULATED with each other”; even if homosexual pairings are adaptive (as they might well be!), it seems odd to waste even a little energy on copulation and suggests a single mechanism or set of mechanisms at work.
Second, it was already established that there are both hormonal and social / developmental mechanisms affecting same-sex preferences in zebra finches; in particular, Elizabeth Adkins-Regan did a lot of work from the late 90s onwards on both of these mechanisms (and her student, James Goodson, has done a lot of great follow-up work on mapping the endocrinology and neurobiology of social behaviour in estrildid finches). The finding in the article by Elie et al. that biased sex ratios promote homosexual pair bonds is interesting, but I wonder how different it is from the social deprivation work by Adkins-Regan and her collaborators.
Don’t get me wrong: this is a cool article. It deserved to be published, and it seems to make a couple of important contributions- exploring and quantifying the strength of these bonds was a worthwhile task, and the evidence it provides for the “social partner hypothesis” is worth looking at. But the media has, as per usual, gotten most of the story wrong here (for instance, the BBC Nature story made it sound like the paper was the first to establish same-sex bonds in zebra finches – <sigh>), and while I share Dr. Bondar’s interest in the results, I don’t think that they’re nearly as shocking as she does.
As a postscript to this: at the end of her article, Dr. Bondar says:
However, long term studies will shed light on whether males will seek out females for the sole purpose of genetic propagation outside of their homosexual partnerships. For the sake of their evolutionary future I hope they do
I’m not aware of anyone having tested this specifically. But it’s been known for a long time that zebra finches engage in a fair amount of extra-pair copulating (i.e. they’re socially monogamous, but not sexually monogamous), so I would expect that the males are stepping out to enhance their reproductive fitness.