Leptin ameliorates the immunity, but not reproduction, trade-off with endurance in lizards

Life-history trade-offs result from allocation of limited energetic resources to particular traits at the expense of others. When resources are scarce, some traits will take priority over others in the degree of their expression. For example, the current reproduction may be sacrificed to enhance survival. Although intuitive from an evolutionary perspective, such priorities must be based on proximate mechanisms that respond to the current conditions. The hormone leptin serves as a signal of energy availability in vertebrates, and has been proposed as a mediator of energy allocation between reproduction and traits that enhance survival, such as the immune system. However, since leptin affects reproduction and immunity in a similar way, it remains unclear which takes priority when energy availability is low. Green anole lizards (Anolis carolinensis) with increased activity, via exercise training, have a marked decrease in immune function as well as reproduction, especially when calories are restricted. We hypothesized that endurance training and calorie restriction would lower immune and reproductive function due to energy limitation, and supplemental leptin would 'rescue' either immune function or reproduction (or both) due to the hormonal signal that energetic resources are available. We found that supplementary leptin rescued immune function in calorie-restricted, trained lizards, but reproduction was not rescued in males or females. This suggests that immune function and reproduction have different sensitivities to leptin in both sexes, or that reproduction is more energy limited and takes low priority even when the signal of energy availability is present.