Geographic variation in nesting behavior and thermally-induced offspring phenotypes in a widespread reptile
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Taxa with large geographic ranges experience considerable variation in climate. To the extent these organisms have environmentally sensitive traits, they provide an excellent opportunity to evaluate how different populations have responded to dissimilar local climates. These eco-evolutionary processes are of particular interest in organisms with traits that are intrinsically connected to temperature, such as embryonic development in oviparous taxa. Maternal effects, particularly oviposition-site choice, greatly affect offspring phenotypes and survival, and thus have the potential to mitigate the effects of divergent climatic conditions across a species range.
In our first study, we delineated natural nesting areas in each of six populations of painted turtles (Chrysemys picta) between the Mississippi River and the Pacific Ocean, and quantified spatial and temporal variation in nest microhabitat characteristics, such as thermal environment and canopy cover. To quantify the microhabitats available for females to choose, we also identified sites within the nesting area of each location that were representative of available shade cover. Females nested non-randomly, generally selecting nest sites with less canopy cover and therefore a warmer nesting environment when compared to the area available for nesting in a given location across the wide geographic range. Natural nest microhabitats differed among locations, but did not follow a predictable pattern of varying with latitude or historic mean air temperature during development. The distributions of our thermal data suggest that females are choosing nest sites to buffer against developmental minimum temperatures rather than hot conditions. Thus, nest-site choice may be unlikely to compensate for the novel stressor of rapidly increasing ambient temperatures in these populations.
Lastly, we examined potentially adaptive spatial patterns of phenotypic variation in a widespread vertebrate, we quantified fitness-related embryonic and hatchling traits of the painted turtle (Chrysemys picta) from seven locations across its geographic range (in Idaho, Minnesota, Oregon, Illinois, Nebraska, Kansas, and New Mexico). We incubated eggs from these sites under constant conditions across a range of environmentally relevant temperatures. Thermal reaction norms for developmental rate, hatchling mass, carapace width, and survival to hatching varied among localities. We further found evidence to support local adaptation in some of these traits in common-garden conditions. However, latitude and the 30-year mean July air temperature (i.e., during the middle of development) for each location were not strong predictors of these geographic differences. These findings suggest that common proxies, such as latitude, used to encapsulate geographic patterns in phenotypic variation may not be reliable predictors. Thus, complex interactions between abiotic and biotic factors likely drive among-population phenotypic variation. Understanding spatial variation in key traits provides important perspective on current adaptation to climatic conditions and future phenotypic responses to projected climate change.
Overall, these studies have contributed to the existing body of literature leverage an integrative approach that uses lab and field studies to assist in understanding how these organisms accommodate different thermal conditions during development. Elucidating how organisms with temperature-sensitive traits are persisting in these vastly different environments is key to predicting how they may respond to rapidly changing thermal conditions predicted under climate change models.