Spike: WAIT, how did you know I was a boy??!!

Me: Uuhhhhhh, temperature of the egg, Spike??!! C’mon, I thought you knew this already from Fluttershy’s Reptilian class!!

“Temperature-dependent sex determination” is a type of environmental sex determination in which the temperatures experienced during embryonic development determine the sex of the offspring. It is most prevalent and common among amniote vertebrates that are classified under the reptile class. TSD differs from the chromosomal sex-determination systems common among vertebrates. It is a type of environmental sex determination ; in other ESD systems, some factors such as population determine the sex of organisms . TSD was also thought to occur in some megapodes, such as the Australian brushturkey; however, their offspring sex ratios appear to result from temperature-dependent embryo mortality rather than from TSD.

The eggs are affected by the temperature at which they are incubated during the middle one-third of embryonic development. This critical period of incubation is known as the thermosensitive period. The specific time of sex-commitment is known due to several authors resolving histological chronology of sex differentiation in the gonads of turtles with TSD.

The thermosensitive, or temperature-sensitive, period is the point during development at which sex determination occurs in temperature-dependent sex determination systems, such as those present in alligators and turtles. Occurring during the middle third of incubation, the TSP lasts for 7–15 days, species-dependent. The temperature must be maintained during this time for a certain sex to be determined; often the differences between temperatures required to produce each sex are very slight and during the TSP, sex is susceptible to change with temperature. After this period, however, sex cannot be reversed