In crocodilian species—the most studied of which is the American alligator— both low and high temperatures result in females and intermediate temperatures select for males. 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.
Warner and Shine used hormonal manipulations to produce males and females across a range of temperatures in a species with TSD.
Valenzuela, Nicole. In these reptiles, the temperature of the eggs during a certain period of development is the deciding factor in determining sex, and small changes in temperature can cause dramatic changes in the sex ratio Bull National Center for Biotechnology InformationU.
Journal of Heredity. Printer-friendly version PDF version. The rich tapestry of mountains, forests, rivers, lakes and coastlines provides habitats for both northern and southern species of animals. An alternative hypothesis of adaptive significance was proposed by Bulmer and Bull in  and supported by the work of Pen et al.
At certain levels of PCBs, the feminizing effects can override the influence of temperature, negating the effects of TSD in an organism.
While sex hormones have been observed to be influenced by temperature, thus potentially altering sexual phenotypes, specific genes in the gonadal differentiation pathway display temperature influenced expression.
However, in the bipotential gonads of those turtles raised at male-promoting temperatures, Sox9 expression was retained in the medullary sex cords destined to become Sertoli cells Spotila et al. Other work centers on a theoretical model the Charnov — Bull model ,   predicted that selection should favour TSD over chromosome -based systems when "the developmental environment differentially influences male versus female fitness";  this theoretical model was empirically validated thirty years later  but the generality of this hypothesis in reptiles is questioned.
Hatchlings from single-sex producing temperatures also had higher first-year survivorship than the hatchlings from the temperature that produces both sexes.