Abstract
Abstract 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.
Highlights
How organisms respond to changing environments is a guiding question of macroecology
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 Temperature-dependent sex determination (TSD) and identifying shortcomings of traditional laboratory studies
Summary
How organisms respond to changing environments is a guiding question of macroecology. Detecting within and among-population variation in important traits is key for understanding if, and how, species can respond to temporal or spatial environmental shifts. The ideal model trait for examining the intraspecific and interspecific effects of thermal heterogeneity is (1) fundamental to organismal physiology, (2) directly impacted by temperature in at least one measurable way, (3) intrinsically involved with key processes that extend beyond the trait itself and (4) subject to biologically significant, contemporary variation in thermal conditions (Angilletta Jr., 2009; Kingsolver et al, 2013). In species with TSD, incubation temperatures directly impact both embryonic development rates and the outcome of gonadal differentiation at the organism scale These short-term developmental outcomes have long-term implications at the population scale, since sex-specific recruitment translates to operational sex ratios on a time lag of years, or even decades (Chaloupka & Musick, 1997; Schwanz, Janzen, & Proulx, 2010). Many organisms with TSD are subject to considerable spatial and temporal variation in developmental temperatures (Refsnider & Janzen, 2016), and several studies suggest that populations have responded in potentially adaptive directions to local climates (Doody et al, 2006; Ewert, Lang, & Nelson, 2005; Morjan, 2003; Pen et al, 2010)
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