 TSD may be advantageous and selected for in turtles, as embryo energy efficiency and hatchling size are optimized for each sex at single-sex incubation temperatures and are indicative of first-year survivorship.This suggests that natural selection would favor TSD, as TSD may enhance the fitness of offspring.An alternative hypothesis of adaptive significance was proposed by Bulmer and Bull in 1982 and supported by the work of Pen et al.