Abstract:
Social status can mediate effects on the immune system, with
profound consequences for individual health; nevertheless,
most investigators of status-related disparities in freeranging
animals have used faecal parasite burdens to
proxy immune function in the males of male-dominant
species. We instead use direct measures of innate immune
function (complement and natural antibodies) to examine
status-related immunocompetence in both sexes of a femaledominant
species. The meerkat is a unique model for such
a study because it is a cooperatively breeding species in
which status-related differences are extreme, evident in
reproductive skew, morphology, behaviour, communication
and physiology, including that dominant females naturally
express the greatest total androgen (androstenedione
plus testosterone) concentrations. We found that, relative
to subordinates, dominant animals had reduced serum
bacteria-killing abilities; also, relative to subordinate females,
dominant females had reduced haemolytic complement
activities. Irrespective of an individual’s sex or social status, androstenedione concentrations (but not body condition, age or reproductive activity) negatively
predicted concurrent immunocompetence. Thus, dominant meerkats of both sexes are
immunocompromised. Moreover, in female meerkats, androstenedione perhaps acting directly
or via local conversion, may exert a double-edged effect of promoting dominance and reproductive
success at the cost of increased parasitism and reduced immune function. Given the prominent
signalling of dominance in female meerkats, these findings may relate to the immunocompetence
handicap hypothesis (ICHH); however, our data would suggest that the endocrine mechanism
underlying the ICHH need not be mediated solely by testosterone and might explain trade-offs in
females, as well as in males.
