Ontogenetic shifts in sex ratios support multiple adaptive explanations for temperature-dependent sex determination

Abstract Temperature-dependent sex determination (TSD) is found in all reptile groups, but its adaptive significance is a mystery. This is because most TSD reptiles feature extreme longevity, so assessing lifetime fitness is incredibly difficult. Here we combine classical sex ratio theory with a dataset of stage-specific sex ratios from a diversity of wild reptile populations to infer patterns of sex-specific mortality (fitness) across ontogeny. We disentangle whether there is a single or multiple adaptive explanations for TSD, provide insight into what those explanations are, and align the adaptive explanations with major reptile clades. We find that TSD evolved in turtles and crocodilians so that males develop in high-quality patches, supporting an extension of the Trivers-Willard hypothesis where the fitness of males under sexual selection is more sensitive to body condition than female fitness. We further find that stage-specific patterns of mortality are different in squamates, where TSD may have evolved to match females with high-quality patches that facilitate early maturation and high fecundity. We suggest if TSD evolved in long-lived reptiles because incubation temperature is a pervasive agent that influences stress, then it may be embryonic stress in general, not just thermal stress, that is the fundamental driver of TSD evolution.