Normally territorial behavior in mammals is limited to the breeding season and is tightly coupled to circulating gonadal steroid levels.  In contrast, both male and female red squirrels also defend territories during the non-breeding season. A pro-hormone – dehydroepiandrosterone (DHEA) – produced by the adrenals has been linked to aggression. We found the levels are very high, particularly when red squirrels are not breeding and when density was high. Our study is the first examination this pro-hormone in a wild rodent and the first field experiment on the regulation of plasma DHEA in any wild mammal.