Binary labels, binary stories: Sex and the white-throated sparrow

Abstract Debates about whether sex is binary often conflate gamete size, which is unambiguously binary, with the development of sex-related traits, which is not. Most sex-related behaviors, morphologies, and physiological processes arise from developmental pathways that generate overlapping, multidimensional variation. Here, I summarize two decades of research on a species that has been said to “break” the sex binary: the white-throated sparrow (Zonotrichia albicollis), This species exhibits a chromosomal polymorphism linked to alternative behavioral strategies that cut across gonadal sex. Birds of the white-striped and tan-striped morphs differ in territorial aggression and parental provisioning, behaviors that form opposing components of a life-history trade-off. These phenotypes are associated with differential expression of the genes ESR1 and VIP, which are located within a chromosomal rearrangement. Variation in expression of these genes predicts behavioral variation independently of sex and morph, and the same genes contribute to both aggression and parenting through brain region-specific regulation. These findings demonstrate that traits often treated as sex-related can be reorganized along dimensions that cross sex categories. More importantly, they illustrate that physiological variables such as gene expression, hormonal signaling, and neural circuit function can better explain variation than categorical labels alone. Focusing on mechanisms rather than categories allows a better understanding of how phenotypes arise and avoids attributing causal power to binary groupings that obscure biologically meaningful variation.