Population-scale responses of key ecological traits to local environmental conditions provide insight into their adaptive potential. In species with temperature-dependent sex determination (TSD), short-term, individual developmental responses to the incubation environment have long-term consequences for populations. We took a model-based approach to study within- and among-population variation in the physiological components of TSD in 12 populations of painted turtles (Chrysemys picta). We used laboratory and field incubation data to quantify variation in thermal reaction norms at both population and clutch scales, focusing on the pivotal temperature that produces a 1:1 sex ratio (P) and the transitional range of incubation temperatures (TRTs) that produce mixed sex ratios. Defying theoretical expectations, among-population variation in P was not convincingly explained by geography or local thermal conditions. However, within some populations, P varied by '5°C at the clutch scale, indicating that the temperature sensitivity of gonadal differentiation can vary substantially among individual nesting females. In addition, the TRT was wider at lower latitudes, suggesting responsiveness to local incubation conditions. Our results provide a potential explanation for discrepancies observed between constant-temperature experimental results and outcomes of fluctuating incubation conditions experienced in natural nests, exposing important knowledge gaps in our understanding of local adaptation in TSD and identifying shortcomings of traditional laboratory studies. Understanding individual variation and the timing of gonadal differentiation is likely to be far more useful in understanding local adaptation than previously acknowledged. A free Plain Language Summary can be found within the Supporting Information of this article.
Bibliographical noteFunding Information:
This research was supported by National Science Foundation grants DEB-1242510 and IOS-1257857 to FJJ and a Grant in Herpetological Conservation and Research from the Minnesota Herpetological Society and British Ecological Society Small Research Grant to ALC. We would like to thank U.S. Fish and Wildlife Service for collection permits, staff at our study areas and numerous field assistants. We would also like to thank Turk Rhen and Robin Andrews for providing additional raw data, Karen Abbott for assistance with improving the MATLAB code, Rory Telemeco and Marc Girondot for valuable discussions, and members of the Janzen and Bronikowski laboratories at Iowa State University for comments on a previous draft. This study was performed in accordance with the Iowa State University Care and Use of Animals Protocols.
© 2019 The Authors. Functional Ecology © 2019 British Ecological Society
- constant-temperature equivalent (CTE)
- embryonic development
- geographic variation
- painted turtle (Chrysemys picta)
- thermal reaction norm