Physiological trade-offs between the reproductive and immune systems in tree lizards (Urosaurus ornatus)

When resources are scarce, organisms are faced with critical challenges trying to optimize competing vital functions. Key among these competing processes are reproduction and immune function. However, few studies have directly demonstrated competition for resources between these two processes. This dissertation research involved a series of field and laboratory-based experiments in female tree lizards (Urosaurus ornatus), involving immune, reproductive, endocrine, and resource manipulations. The experiments demonstrate resource competition between the reproductive and immune systems, whereby physiological trade-offs between the two systems only arise when resources are limiting.; Specifically the results conclude that (1) experimentally induced restraint stress suppressed immune function, as measured by wound healing rate; (2) immune function was suppressed during energetically costly reproductive stages in the field, but not in the laboratory; (3) limiting resources resulted in immunosuppression, but only during costly reproductive activities; (4) experimentally increasing reproductive investment, via follicle-stimulating hormone injections, when resources were limited resulted in suppressed immunity; (5) experimentally increasing immune investment, via cutaneous wound healing, when resources were limited resulted in suppressed reproduction; and (6) experimentally increasing corticosterone (an energy mobilizing steroid hormone) concentrations via implants, resulted in the suppression of immune function, but only when animals were energetically compromised. Therefore, a balance is maintained between multiple processes, and changes in available resources or immune investment has major consequences for reproduction and vice-versa. It appears that trade-offs between the reproductive and immune system in tree lizards are a facultative response to resource availability and are not an obligate response to sex steroids during reproduction. The regulation is dynamic allowing animals to adjust their energetic output to environmental conditions. When resources are limited wound healing and potentially other immune responses are impaired, thereby increasing organismal susceptibility to infection and parasites. Perhaps most significant is that mounting immune responses under limiting resources can have detrimental consequences for organismal fitness; even if animals survive the immune response they may be unable to reproduce and therefore may be evolutionarily "dead" anyway.