Life histories evolve in response to constraints on the time available for growth and development and selection can acto on both the average expressed pehnotype and phentoypic plasticity. Nesting date and its plasticity in response to spring temperature may therefore be important components of fitness in oviparous ectotherms near their northern range limit, as reproducing early provides more time for embryos to complete development before winter and more plasticity could allow for a better match between the phenotype and the environment. We used data collected over several decades to compare air temperature and nest date plasticity in populations of painted turtles and snapping turtles from three distinct populations, two from a relatively warm environments (southeastern Michigan, and Iowa) near the southern extent of the last glacial maximum and a relatively cool environment (central Ontario) near the northern extent of post-glacial recolonization.
For painted turtles, population-level differences in reaction norm elevation for two phenological traits were consistent with adaptation to time constraints, but no differences in reaction norm slopes were observed. For snapping turtle populations, the difference in reaction norm elevation for a single phenological trait was in the opposite direction of what was expected under adaptation to time constraints, and no difference in reaction norm slope was observed. Finally, among-individual
variation in individual plasticity for nesting date was detected only in the northern population of snapping turtles, suggesting that reaction norms are less canalized
in this northern population. Overall, we observed evidence of phenological adaptation, and possibly maladaptation, to time constraints in long-lived reptiles. Where present, (mal)adaptation occurred by virtue of differences in reaction norm elevation, not reaction norm slope. Glacial history, generation time, and genetic constraint may all play an important role in the evolution of phenological timing and its plasticity in long-lived reptiles.