Photoperiodic mediated changes in aggression in Siberian hamsters

Testosterone (T) or its metabolite, 17b-estradiol (E₂), mediate aggression in a wide range of mammalian species.  Specifically, gonadal steroids and aggression are positively correlated with breeding season, and aggression is expected to be higher in a reproductive condition.  In some photoperiodic species, however, such as Siberian hamsters (Phodopus sungorus), aggressive behavior persists or increases during the non-breeding season, when T and E₂ are basal, suggesting that territorial aggression in the reproductively quiescent conditions may be independent of circulating gonadal steroids. Species that do not display a positive correlation between gonadal steroids and aggression need further investigation, specifically characterizing the role of neuroendocrine factors modulating aggression.  This study attempts to connect steroid levels and steroid sensitivity to neural receptor expression.  Specifically, T or E₂ may mediate aggression despite basal concentrations of these steroids by increasing sensitivity to steroids or elevating expression in specific brain regions.  Serum steroid levels and neural receptor expression were examined across photoperiod and correlated with aggression.  The present study supports the hypothesis that circulating gonadal steroids are not necessary to activate aggression in adult Siberian hamsters.