Population sex ratios under differing local climates in a reptile with environmental sex determination

Populations that experience different local climates, such as those along a latitudinal gradient, must match life history traits to local environmental conditions. In species with temperature-dependent sex determination, such as many reptiles, population sex ratio is strongly influenced by local climate, yet local climate differs substantially among populations in geographically-widespread species. We studied the painted turtle at three sites across the species’ geographic range to gain a mechanistic understanding of how sex ratios are produced under different local climates. We combined data on maternal nest-site choice, nest incubation temperature, and the resultant offspring sex ratio of populations across a climatic gradient, to demonstrate how geographic variation in behavior and physiology translates into sex ratios among populations of a widely-distributed species. We found that populations across the species’ geographic range match incubation conditions with local climatic conditions through population-specific adjustment of maternal nest-site choice. Incubation temperatures during the thermosensitive period were cooler and clutches were more male-biased in the south, with populations farther north having warmer incubation temperatures and more female-biased sex ratios, yet adult sex ratios were not strongly biased in any population. Most components of maternal nest-site choice varied latitudinally among populations, suggesting that the species may have a considerable repertoire for responding to climate change through adjustment of nest-site choice.