Life history trade-offs, immune function and the expression of sexual signals in two model groups of birds (Psittaciformes, Charadriiformes)

Ecological immunology is the study of the ecological and evolutionary factors that explain variation in the function of, and investment in, the immune system. Within this field, reproduction-based trade-offs are an important focus, where studies often address the immunological costs associated with the expression of secondary sexual traits used in mate choice. Specifically, the Immunocompetence Handicap Hypothesis (ICHH) links the expression of secondary sexual traits to immune function, stating that testosterone promotes the expression of these traits while suppressing the immune system. In doing so, testosterone may maintain the honest expression of such traits, but also ultimately cause sex differences in immunity because males tend to have high levels of testosterone. I explore aspects of these topics using two model systems: the Psittaciformes (Parrots: Chapter 2) and the Charadriiformes (shorebirds), in particular the sex-role reversed Red phalarope (Phalaropus fulicarius). In Chapter 2, I employed a phylogenetically informed approach to investigate the relationships among immune investment, plumage colouration and longevity in parrots. I found that immune investment was greater in more colourful species, as well as in those with a slower pace-of-life (i.e., longer incubation periods), but not specifically in those with longer lifespans. In Chapter 3, I investigated the role that reproductive behaviours play in determining sex differences in corticosterone levels. One explanation for sex differences in the stress response is that selection favours a reduced response in incubating birds to reduce nest abandonment. I generally found little support for sex differences in corticosterone being driven by behaviours related to incubation. Moreover, in phalaropes, sex differences in corticosterone were already present prior to incubation. In Chapter 4, I found that males have higher levels of testosterone than females, but that females were immunosuppressed compared to males. However, I found evidence that testosterone may regulate immune function in females, but not males. The observation of female-biased immunosuppression is consistent with Bateman’s Principle, and although there was some evidence of a testosterone-mediated handicap acting through immune function, these results attest to a fundamental lability in the relationship between testosterone and immunity. In Chapter 5, I demonstrated that plumage colouration in phalaropes is condition-dependent and so potentially conveys useful information to conspecifics. Yet, the relationship was negative in both sexes such that more colourful individuals had poorer immunocompetence, which was contrary to predictions. In Chapter 6, I demonstrated that phalaropes pair assortatively (positively) based on plumage colouration, but negatively based on aspects of size. I discuss the results of this dissertation in the light of life history theory, as well as in the context of mechanisms maintaining signal honesty.