Although most seasonally breeding animals display high levels of breeding aggression, some species are equally or more aggressive during the non-breeding season, despite gonadal regression and reduced circulating steroid levels. Thus, it is becoming increasingly clear that alternative neuroendocrine mechanisms, which are independent of gonadal steroids, are critical in allowing animals to maintain high levels of aggression year-round. While we are starting to learn more about how these alternative neuroendocrine processes regulate aggressive behavior, little is known about how the hormonal correlates of environmental cues regulate these mechanisms.

For my dissertation, I investigated the role of the hormone melatonin, which serves as the body’s biochemical cue for photoperiod (i.e., day length), in regulating the neuroendocrine mechanisms underlying seasonal aggression in Siberian hamsters (Phodopus sungorus), a species in which both males and females display increased aggression during the short-day (SD) photoperiods of the non-breeding season (reviewed in Munley et al. 2022, Journal of Experimental Zoology Part A; Munley et al. 2018, Frontiers in Endocrinology). I demonstrated that melatonin treatment and SDs produce similar changes in aggressive behavior and circulating and neural steroids levels in male hamsters, including a reduction in DHEA, T, and estradiol levels in brain regions associated with aggression (Munley et al. 2020, Hormones and Behavior; Munley et al. 2021, Journal of Neuroendocrinology). In addition, I showed that the activity of 3β-hydroxysteroid dehydrogenase/∆5-∆4 isomerase, an enzyme that catalyzes DHEA synthesis and metabolism, varies in a melatonin-dependent and sex-specific manner in the adrenal glands and brain (Munley et al., in revision). Finally, I found evidence that adrenal MT₁ melatonin receptors regulate seasonal variation in social behavior in male hamsters, including aggression, using a molecular genetic approach (Munley et al. 2022, Hormones and Behavior).

Currently, I am conducting additional studies to further investigate how the actions of melatonin on hormones and aggression vary between male and female hamsters. Specifically, I am comparing the neural expression of steroid-related genes and arginine vasopressin in males and females following treatment with melatonin or SDs.