FIGURE 1. Theoretical predictions of the Charnov–Bull model for the adaptive significance of TSD based on the TSD pattern in Jacky dragons (A. muricatus).

In this species, females are produced at thermal extremes and males are produced at intermediate temperatures. The solid line represents fitness of sons and the dashed line represents fitness of daughters. The Charnov–Bull model predicts that TSD will enhance individual fitness if the fitness of sons is greatest for individuals that hatch from eggs incubated at temperatures that naturally produce males, and fitness of daughters is greatest for individuals from eggs incubated at temperatures that naturally produce females. These conditions might be satisfied if: a, male-producing temperatures are optimal for sons, and female-producing temperatures are optimal for daughters; b, female fitness is unaffected by incubation temperature, but fitness is optimized by intermediate incubation temperatures for males; or c, male fitness is unaffected by incubation temperature, but female fitness is optimized by cool and warm incubation temperatures. d, Many other scenarios should not favour TSD. For example, when incubation temperature affects fitness, but does so in similar directions for males versus females, TSD is not favoured by selection.