Current and time‐lagged effects of climate on innate immunity in two sympatric snake species

1. Changing environments result in alterations at all levels of biological organization, from genetics to physiology to demography. The increasing frequency of droughts worldwide is associated with higher temperatures and reduced precipitation that can impact population persistence via effects on individual immune function and survival. 2. We examined the effects of annual climate variation on immunity in two sympatric species of garter snakes from four populations in California over a seven‐year period that included the record‐breaking drought. 3. We examined three indices of innate immunity: bactericidal competence (BC), natural antibodies (NABs), and complement‐mediated lysis (CL). 4. Precipitation was the only climatic variable explaining variation in immune function: spring precipitation of the current year was positively correlated to Thamnophis sirtalis BC and NABs, whereas spring precipitation of the previous year was positively correlated to T. elegans BC and NABs. This suggests that T. elegans experiences a physiological time‐lag in response to reduced precipitation, which may reflect lack of capital for investment in immunity in the year following a dry year. 5. In general, our findings demonstrate compelling evidence that climate can influence wild populations through effects on physiological processes, suggesting that physiological indices such as these may offer valuable opportunities for monitoring the effects of climate.