Abstract:
Sexual selection acts on traits that increase reproductive success. Variation in reproductive success is often higher among males than females. Consequently, sexual selection has been studied extensively in males while its possible role in females has only recently attracted considerable attention. In some cooperatively breeding species females compete intensely for reproductive opportunities and may thereby have evolved 'male-like' traits such as increased intra-sexual aggression and exaggerated secondary sexual traits. The expression of the latter tends to be testosterone-dependent in male vertebrates but whether this is also the case among females remains poorly understood. Here, we compare two cooperatively breeding mole-rat species (Natal, Cryptomys hottentotus natalensis, and Damaraland mole-rats, Fukomys damarensis) in which a single female monopolises reproduction through behavioural and physiological suppression, respectively, to evaluate the effect of female intra-sexual competition. Consistent with the hypothesis that intra-sexual competition has shaped patterns of testosterone (T) secretion among females in these species, we show that (i) female T levels in both species are significantly higher among breeding (BFs) (who may face the highest degree of intra-sexual competition) compared to non-breeding females (NBFs), (ii) that T levels in both species are significantly higher when access to unrelated males can be assumed to be greatest (i.e., wet season), and (iii) that the average female T levels are a full order of magnitude higher in the absence of a physiological mechanism of reproductive suppression. Together, our results suggest a role for intra-sexual competition in shaping patterns of T secretion among females of the social mole-rats and raise the possibility of a modulatory role for the mode of reproductive suppression on competition-related traits in females.
