Abstract
In vertebrates, reproductive endocrine concentrations are strongly differentiated by sex, with androgen biases typifying males and estrogen biases typifying females. These sex differences can be reduced in female-dominant species; however, even the most masculinised of females have less testosterone (T) than do conspecific males. To test if aggressively dominant, female meerkats (Suricata suricatta) may be hormonally masculinised, we measured serum androstenedione (A4), T and estradiol (E2) in both sexes and social classes, during both ‘baseline’ and reproductive events. Relative to resident males, dominant females had greater A4, equivalent T and greater E2 concentrations. Males, whose endocrine values did not vary by social status, experienced increased T during reproductive forays, linking T to sexual behaviour, but not social status. Moreover, substantial E2 concentrations in male meerkats may facilitate their role as helpers. In females, dominance status and pregnancy magnified the unusual concentrations of measured sex steroids. Lastly, faecal androgen metabolites replicated the findings derived from serum, highlighting the female bias in total androgens. Female meerkats are thus strongly hormonally masculinised, possibly via A4’s bioavailability for conversion to T. These raised androgen concentrations may explain female aggressiveness in this species and give dominant breeders a heritable mechanism for their daughters’ competitive edge.
Highlights
Examination of ‘heterologous’ hormones in animals of both sexes, of both social classes, and in different reproductive phases
In our study of serum endocrine patterns in meerkats, we found the interaction between sex and social status to be strongly predictive of variation outside of reproductive events
The A4 concentrations of dominant females were elevated by comparison to other female mammals[12], including female-dominant species assayed in the same manner as in the present study[16], but were significantly greater than those of conspecific males (LSD: DF vs. DM: t = 5.832, P < 0.001; DF vs. SM: t = 5.984, P < 0.001; Fig. 1a)
Summary
Examination of ‘heterologous’ hormones (i.e., various androgens in females and estrogens in males) in animals of both sexes, of both social classes, and in different reproductive phases. In the best-known case of exceptional female aggressiveness[22] and social dominance[23], displayed by the spotted hyaena (Crocuta crocuta), female behavioural traits are associated with hormonal masculinisation[19]. In previous reproductive endocrine studies of meerkats, researchers identified a status-related difference in T, not in males[34], but in pregnant females, [7]. The generalisability of this difference outside of gestation, remains unknown. Researchers previously identified an increase in the T concentrations of subordinate males associated with roving[35], but values for other sex steroids and for dominant males remain unknown. Because serum sampling was limited for animals during the targeted reproductive events, our final aim was to validate faecal analyses (against our serum analyses) to allow greater depth of monitoring
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