The agonistic adrenal: melatonin elicits female aggression via regulation of adrenal androgens

Classic findings have demonstrated an important role for sex steroids as regulators of aggression, but this relationship is lacking within some environmental contexts. In mammals and birds, the adrenal androgen dehydroepiandrosterone (DHEA), a non-gonadal precursor of biologically active steroids, has been linked to aggression. Although females, like males, use aggression when competing for limited resources, the mechanisms underlying female aggression remain understudied. Here, we propose a previously undescribed endocrine mechanism regulating female aggression via direct action of the pineal hormone melatonin on adrenal androgens. We examined this in a solitary hamster species, Phodopus sungorus, in which both sexes are highly territorial across the seasons, and display increased aggression concomitant with decreased serum levels of sex steroids in short 'winter-like' days. Short- but not long-day females had increased adrenal DHEA responsiveness co-occurring with morphological changes in the adrenal gland. Further, serum DHEA and total adrenal DHEA content were elevated in short days. Lastly, melatonin increased DHEA and aggression and stimulated DHEA release from cultured adrenals. Collectively, these findings demonstrate that DHEA is a key peripheral regulator of aggression and that melatonin coordinates a 'seasonal switch' from gonadal to adrenal regulation of aggression by direct action on the adrenal glands.