The tortoise and the air: climate shapes sex-ratio reaction norm variation in turtles

For some species, gonadal sex determination relies on environmental cues and thus ought to match local conditions, which can be accomplished through changes in maternal behavior and/or the sex-ratio reaction norm. Here, we investigate the evolution of the latter at broad spatial and phylogenetic scales for turtle species with temperature-dependent sex determination (TSD). We find evidence that pivotal temperatures (Tpiv, where a 1:1 offspring sex ratio is produced) and average temperatures during incubation increase together across the phylogeny, while on contemporary timescales, Tpiv increases with incubation temperature variability. Limited evidence also indicates that the breadth of the logistic-shaped reaction norm decreases with temperature variability, contradicting expectations and suggesting that this reaction norm feature may relate to intrinsic biological factors rather than to environmental conditions alone. In sum, these findings indicate that adaptive sex-ratio reaction norm variation within turtle species with TSD is driven by climate variability, while adjustments to the maternal behavior of nest-site choice may primarily compensate for differences in climate averages.