We found significant interactions between the reproductive system, the immune system, and food availability. Food supplementation and T manipulation interacted to influence immune function in male sagebrush lizards. Consistent with our predictions, food supplementation increased immune response in lizards and this response was more pronounced in T-treated compared with control animals. Furthermore, food supplementation increased plasma T levels in both T-treated and control lizards. In contrast to our predictions, however, T enhanced immunity in food supplemented lizards. These results suggest that T can act differentially to alter immune function, depending on the energetic state of the animal. For example, T may enhance both reproduction and immunity when energy is abundant, but T may also mediate trade-offs between these two physiological systems when resources are limiting. These context-dependent effects of T may reconcile the conflicting evidence for both immunosuppression and immunoenhancement of T in the literature (Muehlenbein and Bribiescas, 2005
). Variability in lab or field conditions or in species requirements of energetic resources may account for these variable effects of T. Trade-offs between reproduction (as measured by T) and immunity may also be due to the indirect effects of energy allocation (Owen-Ashley et al., 2004
), explaining our lack of immunosuppression in food supplemented lizards. Nevertheless, we only considered one area of immunity, namely innate immune function, and may have attained different responses (ie., noted immunosuppression with T) if we had measured acquired or cell-mediated immunity.
Although T-treated individuals had increased courtship behavior when food-limited, T-patch did not produce long-term elevation of circulating levels of T, as plasma T was not significantly elevated at the time of blood sampling. The fact that T-patches increased courtship behavior in food-limited lizards one week after patch administration even without a measured effect on circulating T levels, suggests that some behavioral regulation may be in response to previous hormone levels. Similar to the way in which CORT-patches acted on circulating CORT levels (Knapp and Moore, 1997
), T-patches likely produced a short-term elevation of T. This may have up-regulated androgen receptors, producing a prolonged behavioral and immune response.
One of the most pronounced effects of our study is that food supplementation increased levels of circulating testosterone in both T-patch and control individuals, as well as was involved in the enhancement of immune function. Mechanisms are now unclear but may involve differences in clearance rates or conversion from other precursors. An increase in energy resources provided by an increase in food may have allowed for greater energy distribution to multiple functions, including reproduction and immunity (French et al., 2007b
). The fact that increasing food intake allowed for the elevation of both reproductive and immune processes suggests that these processes are not mutually exclusive and thus trade-offs may exist due to limitations in energy uptake. Female snow skinks in good condition may produce high levels of both reproductive investment and immune function, whereas costs arise in females that do not have the resources to produce high levels of both processes (Olsson et al., 2001
). Tropical pythons with low body condition have decreased immunity (Ujvari and Madsen, 2006
). Food availability may be one factor affecting individual quality. High levels of activity have immunosuppressive effects in female mice on a restricted diet (Schubert et al., 2008
), suggesting energetic requirements are important for allocation towards multiple functions.
In addition to acting internally to increase energy stores, food supplementation may impose other externally-derived motives for increasing circulating testosterone. Increases in food availability in some territories may affect social interactions within those territories. Androgen levels can then be modulated by the social environment (Oliveira et al., 2002
). This may occur due to an increase in density of, and thus behavioral encounters within, these now higher quality territories, promoting elevated levels of T (Beletsky et al., 1992
). Increasing food resources in some territories may also elevate aggression to retain high quality territories. We witnessed one highly aggressive encounter between a focal food supplemented lizard and a conspecific male (pers. obs.). In this case, food supplementation may have elevated T due to the increased potential for aggressive interactions in defense of high food resources. Variation in food abundance causes changes in home ranges and associated social structures of mice (Schradin and Pillay, 2006
) and red squirrels (Wauters et al., 2005
). The role of food availability on social structures has yet to be studied, however, in sagebrush lizards.
Supplemental food items may have also provided a stimulus for increased testosterone. Lizards in food-supplemented sites were supplied with vitamin-dusted crickets and mealworms, which are notably larger than most of the natural prey of these lizards (personal observation). The size of prey items correlates with rump coloration (a signal of condition) in kestrel fledglings (Vergara and Fargallo, 2008
). Similarly, the interaction with larger prey objects, most likely also indicative of greater food intake, may have allowed for an elevation of T due to enhancement of individual quality. Furthermore, providing lizards in supplemented group with vitamin-dusted crickets and mealworms may not only increase the amount of available food but also the quality of the food by providing essential vitamins and both a protein source (crickets) and a fat source (mealworms). In this way, lizards were able to intake their required nutrients without much effort. Diet quality has the potential to influence the direction of trade-offs (Naya et al., 2007
). This may have allowed for the increase of T in these higher quality individuals. Nevertheless, calorie restriction, not diet quality, is associated with suppression of reproduction (including T levels) in other studies (Govic et al., 2008
, Santos et al., 2004
In conclusion, the present findings demonstrate that several key factors may interact to influence important life history functions, such as immunity and reproduction. Here we considered testosterone, a reproductive mediator, in the context of energetic resources. Testosterone was found to influence both immune function and courtship behavior differently depending on the energetic state of the animal. Thus, T is not necessarily immunosuppressive in all species and across all conditions. Rather, the effects of T on immunity appear depend on the environmental condition of the animal. For example, T can be immunosuppressive when food is limited; however, our results also support T-induced immunoenhancement when food availability is high. These findings support the idea of an energetic trade-off between reproduction and immune function, at least when environmental resources are limiting. Furthermore, this study suggests a role for food availability in enhancing immune function (in presence of T) and on reproduction (by elevating circulating T). Further research is needed to determine the mechanism regulating hormone levels by energetic state.