Timed melatonin administration increases territorial but not non-territorial aggression in female Siberian hamsters

Many animals exhibit seasonal changes in physiology and behavior, including reproductive and non-reproductive social behaviors. Some seasonally-breeding animals display increases in territorial aggression during the short days (i.e., winter), despite gonadal regression and reduced circulating steroid hormones. Previous research suggests that melatonin acts via steroid hormones to regulate seasonal aggression in Siberian hamsters (Phodopus sungorus), a species in which males and females exhibit increased territorial aggression during the non-breeding season. Previously, we identified a role for adrenal dehydroepiandrosterone (DHEA) in regulating territorial aggression. Little is known, however, regarding potential seasonal changes in non-territorial aggression in this or other species. Here, we examined how short days and exogenous melatonin influence non-territorial and territorial aggression and explored the neuroendocrine circuits involved in regulating aggression in female hamsters. Specifically, we housed females in long (LD) or short-days (SD), administered timed melatonin (MEL) or control injections, and assessed territorial (i.e., resident-intruder) and non-territorial (i.e., neutral cage) aggression. Further, we quantified serum DHEA and neural activity in specific brain regions by c-Fos immunolabeling. SD and MEL-treated LD animals exhibited reproductive inhibition and increased territorial, but not non-territorial aggression, suggesting the neuroendocrine regulation of seasonal aggression is context-specific. Further, DHEA was elevated following behavioral challenges; c-Fos expression in specific brain regions was both context- and season-dependent. Collectively, these findings highlight the role of melatonin in coordinating a "seasonal switch" in aggression while suggesting the neuroendocrine mechanisms may differ across types of aggression.