Abstract
BackgroundMale members of primate species that form multi-male groups typically invest considerable effort into attaining and maintaining high dominance rank. Aggressive behaviors are frequently employed to acquire and maintain dominance status, and testosterone has been considered the quintessential physiological moderator of such behaviors. Testosterone can alter both neurological and musculoskeletal functions that may potentiate pre-existing patterns of aggression. However, elevated testosterone levels impose several costs, including increased metabolic rates and immunosuppression. Cortisol also limits immune and reproductive functions.MethodsTo improve understanding of the relationships between dominance rank, hormones and infection status in nonhuman primates, we collected and analyzed 67 fecal samples from 22 wild adult male chimpanzees (Pan troglodytes schweinfurthii) at Ngogo, Kibale National Park, Uganda. Samples were analyzed for cortisol and testosterone levels as well as intestinal parasite prevalence and richness. 1,700 hours of observation data were used to determine dominance rank of each animal. We hypothesized that dominance rank would be directly associated with fecal testosterone and cortisol levels and intestinal parasite burden.ResultsFecal testosterone (but not cortisol) levels were directly associated with dominance rank, and both testosterone and cortisol were directly associated with intestinal parasite richness (number of unique species recovered). Dominance rank was directly associated with helminth (but not protozoan) parasite richness, so that high ranking animals had higher testosterone levels and greater helminth burden.ConclusionsOne preliminary interpretation is that the antagonist pleiotropic effects of androgens and glucocorticoids place a cost on attaining and maintaining high dominance rank in this species. Because of the costs associated with elevated steroid levels, dominance status may be an honest signal of survivorship against helminth parasites.
Highlights
Male members of primate species that form multi-male groups typically invest considerable effort into attaining and maintaining high dominance rank
The intestinal parasite fauna recovered from all adult and adolescent males is described in detail elsewhere [62]
Our results are consistent with the supposition that high ranking male chimpanzees have higher testosterone levels and increased intestinal helminth burden compared to lower ranking animals, and that neuroendocrine mechanisms may account for rankrelated differences in susceptibility to infection
Summary
Male members of primate species that form multi-male groups typically invest considerable effort into attaining and maintaining high dominance rank. Aggressive behaviors are frequently employed to acquire and maintain dominance status, and testosterone has been considered the quintessential physiological moderator of such behaviors. Testosterone can alter both neurological and musculoskeletal functions that may potentiate preexisting patterns of aggression. Male dominance hierarchies occur in many primate species, and males typically invest considerable effort into attaining and maintaining high dominance rank. Males compete for mating opportunities and form dominance hierarchies, and most males invest considerable effort into striving for high rank [7]. Genetic data from several chimpanzee study sites indicate that alpha males and others who attain high rank generally achieve disproportionately high reproductive success, reproductive skew is only moderate in communities with more than a few adult males [8,9]
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