A review of the literature showed, that across species, there is evidence of males experiencing lower immunocompetence than females. The Immunocompetence handicap hypothesis (ICHH) explains this by the occurrence of a trade-off between reproductive effort (via elevated androgens) and immunocompetence in males, specifically their defence against parasites. According to the ICHH, only high-quality males are able to withstand the immunosuppressive effects of elevated androgens, due to these elevations being energetically costly. The ICHH also proposes that female choice, is therefore, driven by the ability of the males to defend against parasites due to the benefits gained for their offspring. However, support for this hypothesis is inconsistent across and within species, possibly due to the need to include other environmental, social, or biological factors (due to their effect on parasite transmission), which is not always the case.
An alternative hypothesis tried to improve upon the ICHH by proposing the involvement of glucocorticoids (GCs) in the endocrine-parasite relationship. The Stress-Linked ICHH (SL-ICHH) proposes that GCs moderate the relationship between androgens and immunocompetence, however, again support for this hypothesis was inconsistent. Here I tested both the ICHH and SL- ICHH, on a group of free-ranging rhesus macaques, while trying to account for other confounding factors. This includes environmental factors such as rainfall and season, social factors such as rank position, and biological factors such as age (Figure 1).
Overall, there was little support for both the ICHH and SL-ICHH. Androgens alone showed little relationship with male parasitism when accounting for other variables, and in the few cases where an interaction between androgens and GCs was found to affect parasitism, it was in the opposite way to predicted by the SL-ICHH; androgens were only negatively associated with parasitism when GCs were low. There was also more evidence for the role of rank in male mating success than male parasitism, suggesting that female choice is driven by the males’ social status rather than their ability to defend against parasites. This study also highlighted the importance of testing different parasite species separately, as all species included here showed different results.