Endocrine and genomic architecture of life history trade-offs in an avian model of social behavior

Life history trade-offs can drive the evolution of alternative phenotypes, the expression of which is usually under hormonal control. Here, I review the endocrine and genetic bases of a trade-off between parental and competitive behavior in an increasingly popular model of social behavior, the white-throated sparrow (Zonotrichia albicollis). Within a population, approximately half of the individuals of this species exhibit a tan stripe (TS) on the crown and adopt a parental strategy, whereas the other half exhibit a white stripe (WS) and adopt a competitive strategy that manifests as increased territorial aggression and mate finding. We and others have shown evidence that the two morphs differ with respect to HPG function; for example, plasma levels of gonadal steroids differ between the morphs in both sexes. Comparing the morphs with regard to hormone levels gives only limited information about causal mechanisms, however, and preliminary behavioral studies in males suggest that morph differences in plasma androgens do not completely explain morph differences in territorial aggression. The polymorphism segregates with a structural rearrangement of chromosome 2 (ZAL2m), which offers a unique and powerful starting point on which to base a more targeted approach. An ongoing effort to characterize the ZAL2m arrangement using modern genomic techniques has revealed two included inversions that have captured a number of endocrine genes, linking them together as a potential “supergene”. This finding is compelling in light of classic hypotheses regarding the evolution of alternative phenotypes, which predict the involvement of linked genes with pleiotropic and/or antagonistic effects that cause disruptive selection toward alternative optima. Similar predictions apply to the evolution of sex and sex chromosomes, which strongly resemble the ZAL2m system. Overall, the white-throated sparrow represents an ideal model in which to study the genetic and endocrine bases of life history strategies and their evolution.