All organisms are constrained by limited energy and thus must allocate resources in such a way to maximize fitness. Due to their importance to survival, immunity and performance are each often favored in selective environments and thus may be tradeoffs of each other. To test this tradeoff hypothesis, we compared performance to three constitutive measures of immunity in a wild hybrid population of water snakes (Nerodia clarkii X Nerodia fasciata): agglutination response to sheep red blood cells (SRBC), heterophil: lymphocyte ratio (HLR), and bactericidal ability (BA). Performance was quantified by exercising snakes up to the point of loss of righting response (LRR) and measuring plasma lactate concentration at this point of fatigue. Support for the tradeoff hypothesis was mixed among the immune assays. Bactericidal ability had a significant positive correlation with LRR (P = 0.003, R2 = 0.263); this was counter to our tradeoff hypothesis and suggests that individuals invest in both traits simultaneously. We failed to detect a significant relationship between SRBC and performance. A significant negative relationship was found between HLR and LRR (P = 0.005, R2 = 0.237). Heterophil: lymphocyte ratio is a balance between innate and acquired immunity that is also indicative of baseline stress. This result showed that snakes with higher HLR are constrained and perform poorly compared to snakes with lower HLR, thus indicating a potential role of stress in affecting performance. Alternatively, this result may denote that internal tradeoffs in the immune system between innate and acquired immunity drive tradeoffs between immunity and performance. Snout–vent length (SVL) was significantly positively correlated with our performance measure (P = 0.012, R2 = 0.192). This result may be explained by selection driving increases in performance and/or ontogenetic development of the locomotor apparatus. Similarly, SVL was an important driver of immunity in snakes. Together, these results clearly show an important link exists between multiple physiological traits that are each vital to survival.

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