Group structure, but not dominance rank, predicts fecal androgen metabolite concentrations of wild male mountain gorillas (Gorilla beringei beringei)

Abstract

Androgens are important mediators of male-male competition in many primate species. Male gorillas' morphology is consistent with a reproductive strategy that relies heavily on androgen-dependent traits (e.g., extreme size and muscle mass).Despite possessing characteristics typical of species with an exclusively single-male group structure, multimale groups with strong dominance hierarchies are common in mountain gorillas. Theory predicts that androgens should mediate their dominance hierarchies, and potentially vary with the type of group males live in. We validated the use of a testosterone enzyme immunoassay (T-EIA R156/7, CJ Munro, UC-Davis) for use with mountain gorilla fecal material by (1) examining individuallevel androgen responses to competitive events, and (2) isolating assay-specific hormone metabolites via high-performance liquid chromatography. Males had large (2.6-and 6.5-fold), temporary increases in fecal androgen metabolite (FAM) after competitive events, and most captured metabolites were testosterone or 5αdihydrotestosterone-like androgens. We then examined the relationship between males' dominance ranks, group type, and FAM concentrations. Males in single-male groups had higher FAM concentrations than males in multimale groups, and a small pool of samples from solitary males suggested they may have lower FAM than group-living peers. However, data from two different time periods (n = 1610 samples) indicated there was no clear relationship between rank and FAM concentrations, confirming results from the larger of two prior studies that measured urinary androgens. These findings highlight the need for additional research to clarify the surprising lack of a dominance hierarchy/androgen relationship in mountain gorillas.

Facial and genital color ornamentation, testosterone, and reproductive output in high-ranking male rhesus macaques

“…We measured immunoreactive testosterone in the resulting 234 fecal extracts ( = 11 samples/male) using enzyme immunoassays (EIAs) at the University of New Mexico, with reagents and protocols provided by the Clinical Endocrinology Laboratory at the University of California at Davis (Antibody R156/7). This protocol is the most widely used assay for the determination of testosterone in mammalian feces, having been validated across a wide range of taxa, including in primates ( Mandrillus sphinx 39 ; Colobus vellerosus 66 ; Gorilla beringei beringei 67 ). An alternative assay for epiandrosterone has been preferred for macaques based on an in vivo experiment that found negligible concentrations of excreted testosterone in the feces of a single long-tailed macaque 68 .…”