Physiological Trade-Offs in Lizards: Costs for Individuals and Populations

The immune system is a critical component of health and fitness, whereby organisms must maintain sufficient health to survive to reproduce. Because of the key role of immunity in an organism's fitness, the use of immunological indices is widespread. However, there is a paucity of empirical support for the best way to interpret immunological data, and the internal energetic state of the organism, as well as the external environmental pressures it faces, are often not considered concurrently. A stronger immune response is not always beneficial to the organism; a more attenuated response may ultimately lead to improved fitness if the animal incurs fewer performance costs on competing systems, especially reproduction. Additionally, the external pressures animals encounter (such as anthropogenic disturbance) must be considered along with the animal's internal state. A synthesis of results addressing resource allocation between the immune and reproductive systems is presented using a well-studied organism, the side-blotched lizard, from a combination of field and laboratory studies under varying environmental conditions. Specifically, experiments involving specific immune, reproductive, metabolic, and performance costs in a laboratory setting are discussed, as well as associated demographic trade-offs between survival and reproductive success, demonstrating essential links between immunity and the population.