Summary Temperature affects nearly all aspects of the physiology of ectotherms, including their ability to mount an immune response. Typically, the ectothermic vertebrate immune system can respond over a wide range of temperatures, but there is a species‐specific temperature at which responses are strongest, with impaired responses above and below this threshold. In long‐lived ectotherms, ageing could also influence the ability to respond to temperature changes as immunosenecence, the functional decrease in immune function with age, is widely reported. This study examined the effects of the interaction between temperature and age on B‐cell function in a long‐lived reptile, the red‐eared slider turtle, Trachemys scripta. B cells in this species have previously been shown to have two main functions, phagocytosis and antibody production. Adult turtles were trapped and blood samples taken. Because sliders grow throughout their lifetime, plastron length was used as a proxy for age. Leucocytes were isolated and used in either an ELISpot assay to examine their ability to produce antibodies spontaneously or when stimulated, or used in a phagocytic assay. The ELISpot and phagocytic assays were conducted over a range of biologically relevant temperatures. We found no interaction between age and temperature on any measure of B‐cell function. In all cases there was a significant effect of temperature, with impaired function at temperatures below 29 °C, but no impairment of function at higher temperatures. We also found little evidence of immunosenescence in any response. This study provides insight into the thermal biology of B‐cell function in sliders and provides an interesting connection between immunology, behaviour and ecology in this long‐lived turtle. A http://onlinelibrary.wiley.com/doi/10.1111/1365-2435.12867/suppinfo is available for this article.

The oversimplification of thermal conditions in both empirical treatments and theoretical applications has hindered our ability to understand how organisms respond to natural temperature variation. This is particularly problematic given that climate change is manifesting as an increase in thermal variability. Consequently, our understanding of how thermally sensitive species, like reptiles with temperaturedependent sex determination (TSD), respond to natural thermal variation is lacking. My dissertation research has focused on identifying the parameters of thermal variation most influential to sex determination and assessing the contribution of maternally derived estrogens to this process. Embryonic sex determination is sensitive to diurnal, seasonal, and stochastic thermal variability, and maternally derived estrogens increase thermal sensitivity of embryos in a seasonal manner. By integrating empirical and modeling approaches, this research demonstrates how the use of more natural variation in incubation conditions can drastically improve predicted thermal responses in the field. These results help resolve how sex is determined in species with TSD in nature, and emphasize the importance of using relevant thermal variation when estimating how temperature affects biological processes in current and predicted climates.

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