Sex-specific endocrine regulation of seasonal aggression in Siberian hamsters

Brain transcriptomics of a social challenge and maternal aggression in incubating female tree swallows

Aggressive behavior is ubiquitous across many contexts, including defense of territories, mates, and offspring. For decades, researchers have detailed the effect of aggressive behavior on physiology, but our understanding of these mechanisms in females lags behind that of males, despite the fact that female aggression is widespread, particularly in the context of maternal defense of eggs or offspring (i.e., maternal aggression). Here, we measured effects of a social challenge on brain gene expression in free-living incubating females. We hypothesized that the social challenge would generate at least one of three transcriptomic effects: (1) sensitizing the brain to otherwise low levels of sex steroids, (2) changing other neuroendocrine signaling pathways associated with social behavior (e.g., dopamine), or (3) broad shifts related to metabolism or immune function. We tested these hypotheses in incubating female tree swallows (Tachycineta bicolor), exposing 10 females to a 30-min simulated territorial intrusion, which elicited maternal aggression. After this challenge, we measured neural gene expression via RNA-seq and compared gene expression to 10 unchallenged controls. We saw no global treatment effect on gene expression. However, within the experimental group, more maternal aggression was correlated with upregulation of genes associated with immune activation and downregulation of genes associated with synaptic plasticity. Though more research is needed to understand the downstream effects of these transcriptional differences, our findings generate key questions about how the brain responds to social challenges across different contexts.

Sex differences in responses to aggressive encounters among California mice

Despite how widespread female aggression is across the animal kingdom, there remains much unknown about its neuroendocrine mechanisms, especially in females that engage in aggression outside the peripartum period. Although the impact of aggressive experience on steroid hormone responses have been described, little is known about the impact of these experiences on female behavior or the subsequent neuropeptide responses to performing aggression. In this study, we compared behavioral responses in both male and female adult California mice based on if they had 0, 1, or 3 aggressive encounters using a resident intruder paradigm. We measured how arginine vasopressin and oxytocin cells in the paraventricular nucleus responded to aggression using c-fos immunohistochemistry. We saw that both sexes disengaged from intruders with repeated aggressive encounters, but that on the final day of testing females were more likely to freeze when they encountered intruders compared to no aggression controls - which was not significant in males. Finally, we saw that percent of arginine vasopressin and c-fos co-localizations in the posterior region of the paraventricular nucleus increased in males who fought compared to no aggression controls. No difference was observed in females. Overall, there is evidence that engaging in aggression induces stress responses in both sexes, and that females may be more sensitive to the effects of fighting.