Photo by Jennifer R. Merritt: White-striped (left) and tan-striped (right) birds
I recently published a piece at SAPIENS.org exploring the question of whether gender is unique to humans. A major point I tried to make in the piece was that plenty of animal species show sex differences in behavior, but that’s not really the same as gender. Human gender is much more complex than most animal sex roles; for example, it includes socially determined sex roles and an internal sense of gender identity—aspects that are missing in most if not all other species. The tl;dr of the piece was that the evidence on the topic is far from conclusive, but suggests it’s not implausible that chimpanzees and bonobos might have something comparable to human gender.
Something I was thinking about as I was working on the piece was how, in humans, gender is extricable from sex, meaning that a person’s sex doesn’t entirely determine their gender. Transgender, agender, and non-binary people demonstrate this. I wanted to explore whether we see any examples of species where the “traditional” sex roles we see in many animals—where males are aggressive and flamboyant whereas females are nurturing—are dissociated from the sexes, as human gender is sometimes dissociated from sex. This led me to consider the white-throated sparrow.
In this species, like many others, some individuals focus their energy on infant care while others focus on aggression and protecting their territory. Also, as in many other species, white-throated sparrows form mating pairs consisting of one aggressive individual and one parental individual. Where white-throated sparrows differ from other species is that these two roles aren’t tied to the two sexes. Instead, a chromosomal inversion (a mutation event where a piece of chromosome—in this case the ZAL2 chromosome—breaks and is reinserted backwards) has resulted in two “morphs” or types of bird, with both males and females of each morph represented: the white-stripe morph is aggressive, whether male or female (although male white-stripes are more so), while the tan-stripe morph is parental (but males are less so). Mating pairs almost always either consist of a white-stripe male and tan-stripe female, or a white-stripe female and tan-stripe male.
For these reasons, some have referred to the white-throated sparrow as the bird with “four sexes”—white-stripe male, tan-stripe male, white-stripe female, tan-stripe female. But just how like sexes (or genders) are these two morphs, really?
I talked to Emory University Psychology Ph.D. Candidate Jenny Merritt, who studies these birds. She explained how a bird’s morph is determined: tan-stripe birds have two normal ZAL2 chromosomes, while white-stripe birds have one normal copy of ZAL2 and one inverted copy, referred to as ZAL2m. In this way, a white-throated sparrow’s morph is determined very similarly to the system of mammalian sex determination we are all familiar with: just as the Y chromosome contains the DNA that codes for male features, the ZAL2m chromosome contains the DNA that codes for the white-stripe appearance and behavior. The critical distinction, again, is that these birds also have two sexes—morph is overlaid on, and somewhat independent of, sex. Perhaps like gender is, in humans?
Merritt is far from convinced. “I don’t think that the white-throated sparrow mating system is analogous to human gender. … I am unaware of any evidence that the sparrows have internal senses of their own morph, or whether that sense could be subject to the same kinds of factors that shape human gender.”
Furthermore, human gender roles are at least partly socially learned, and Merritt says scientists don’t know if there is a learning component to the development of behavioral differences between the two morphs. There is a strong relationship between behavior and genetic morph, and some of Merritt’s own research points to specific way that ZAL2m might influence the brain to increase aggression in white-stripe birds; these observations make it unlikely that morph roles are socially learned like gender roles are in humans.
That said, while human gender is at least partly socially determined, it probably isn’t completely free of genetic influences (although we don’t have a good sense of what those might be). Research on white-throated sparrows by Merritt and others improves our understanding of how genes generally lead to sex differences in behavior, and how sex differences evolve, with potential implications for human sex and gender.
To summarize, although there are some links to be drawn, the ways humans think about gender are almost certainly very different from how sparrows – and most (though perhaps not all) other animals – think about their behavioral roles. The best way to understand the latter might be to approach animal research with the goal of learning more about about animals themselves, absent the bias that often comes with an anthropocentric “animal model” mindset. This is a point that all of us who study nonhuman species would do well to keep in mind: while “is X unique to humans?”-type questions can be alluring, they can also lead us to miss the beauty that comes with understanding nature on its own terms.