Food as a supplementary cue triggers seasonal changes in aggression, but not reproduction, in Siberian hamsters.

Animals living in temperate regions prepare for harsh winter conditions by responding to environmental cues that signal resource availability (e.g., food, day length). Siberian hamsters (Phodopus sungorus) breed in long, summer-like days (LD, >14h light), i.e., photoperiods, and undergo robust gonadal regression and become more aggressive when exposed to short, winter-like photoperiods that signal impending limited resources (SD, <10h light). When hamsters are reared within an intermediate photoperiod (ID, 13.5h light), they are reproductively active, but undergo gonadal regression in response to mild food restriction (FR) over 6-12weeks. We hypothesized that short-term (1-2weeks) FR in an ID photoperiod would provide a signal of impending limited resources and initiate the seasonal increase in aggression typical of SD photoperiods, as well as alter reproductive behaviors in advance of gonadal regression. To test this, we housed male and female hamsters in LD or ID photoperiods, with ad libitum (AL) access to food or a 90%-AL ration. We tested aggressive behavior after one week and reproductive behavior after two weeks, and subsequently monitored females for pregnancy and litter production. Both sexes displayed increased aggression in the ID-FR treatment. Untreated male intruders were less likely to ejaculate when paired with ID females during reproductive encounters. ID-FR males were undergoing gonadal regression after two weeks, but were more likely to have ejaculated. Female pregnancy and litter characteristics were unaltered by treatment: females were equally likely to achieve pregnancy and produce comparable litters across treatment groups. Collectively, we demonstrate that a signal of diminishing resources in an ID photoperiod is sufficient to trigger seasonal aggression, but that hamsters are reproductively resilient to inhibitory environmental cues in the short term. Broadly, our findings provide an important context for exploring seasonal changes in behavior and physiology from an ultimate perspective.

Bi-directional actions of dehydroepiandrosterone and aggression in female Siberian hamsters.

There is a well-established positive relationship between gonadal steroids and aggression. In some seasonally breeding species, however, aggression often persists or is increased during short "winter-like" days when the gonads are regressed and circulating levels of gonadal steroids are relatively low. Although the mechanisms underlying short-day increases in aggression are not fully known, the adrenal androgen dehydroepiandrosterone (DHEA) has been suggested as an alternative neuroendocrine mechanism regulating seasonal aggression. We used two complementary experimental approaches to examine the bi-directional actions of DHEA and aggression in female Siberian hamsters, a seasonal rodent that displays increased aggression concomitant with elevated circulating DHEA in short days. In Experiment 1, we examined the effects of aggressive interactions on DHEA concentrations before and after an aggressive encounter in long- and short-day hamsters. Serum DHEA was altered in a photoperiod-dependent manner, with decreased DHEA levels in response to aggression in short- but not long-day hamsters. Next, we experimentally induced adrenal DHEA release via injections of exogenous ACTH and assessed changes in aggressive behavior across photoperiods. We show a robust increase in aggression in short compared with long days during baseline aggression trials; however, aggression was not significantly increased further in response to ACTH in either photoperiod during post-ACTH aggression trials. These findings suggest that DHEA plays a role in the regulation of short-day aggression, while also highlighting the need for additional studies addressing the causal relationship between DHEA and aggression in this and others species.