Social rank can profoundly affect many aspects of mammalian reproduction and stress physiology, but little is known about how immune function is affected by rank and other socio-ecological factors in free-living animals.In this study we examine the effects of sex, social rank, and reproductive status on immune function in long-lived carnivores that are routinely exposed to a plethora of pathogens, yet rarely show signs of disease.Here we show that two types of immune defenses, complement-mediated bacterial killing capacity (BKC) and total IgM, are positively correlated with social rank in wild hyenas, but that a third type, total IgG, does not vary with rank.Female spotted hyenas, which are socially dominant to males in this species, have higher BKC, and higher IgG and IgM concentrations, than do males.Immune defenses are lower in lactating than pregnant females, suggesting the immune defenses may be energetically costly.Serum cortisol and testosterone concentrations are not reliable predictors of basic immune defenses in wild female spotted hyenas.These results suggest that immune defenses are costly and multiple socioecological variables are important determinants of basic immune defenses among wild hyenas. Effects of these variables should be accounted for when attempting to understand disease ecology and immune function.
