Immune activity suppresses reproduction in food-limited female tree lizards Urosaurus ornatus

1. When resources are scarce, organisms are faced with critical challenges trying to optimize competing functions. Reproduction and immune function are both resource intensive and important to most species. Most studies have reported a down-regulation of immune function during reproduction. Conversely, it is unclear whether mounting an immune response can affect the reproductive process and few studies have examined competition for resources between these two processes. 2. Here we report evidence for direct competition for resources between the reproductive and immune systems in a vertebrate model system, the tree lizard (Urosaurus ornatus). We manipulated food intake in reproductive female tree lizards, where half of the females were undergoing cutaneous wound healing and the other half were not. We then measured reproductive investment in all animals and wound healing rate in all wounded animals. 3. We found that animals with unlimited access to food sustained both reproduction and immune function whereas animals maintained on a restricted diet were not. Specifically, when food was unlimited, females were able to invest in reproduction and heal their wounds. However, when food was limited, females forced to heal a cutaneous wound had significantly smaller follicles than their non-wounded counterparts. Under extreme food limitation (i.e. no food) both reproductive investment and wound healing were suppressed. 4. These results clearly demonstrate resource competition between the reproductive and immune systems, whereby physiological trade-offs between the two systems only arise when resources are limiting. Furthermore, this type of facultative regulation is adjustable and allows animals to respond to changing environmental conditions.