Reptilian Eggs and Temperature
The sex of reptilian offspring, such as crocodiles, alligators, and turtles, is determined by the temperature the eggs reach during the middle third of their incubation period.
Although the sex of humans and most other vertebrates is determined by the chromosomes passed on from their parents, alligators and crocodiles are amongst the group of animals that have what is called temperature-dependent sex determination. This means that the sex of the baby contained in the egg isn't determined until several weeks after the egg has been laid.
Among American alligators, for instance, eggs that exceed a temperature of 34 degrees Celsius during this crucial development stage become male alligators, while eggs that remain cooler than 30 degrees Celsius become female alligators. Eggs that incubate at temperatures in between those two extremes tend to give a mix of male and female alligators. Rules about temperature and sex determination vary among these animals according to species.
Temperature-dependent sex determination is also found in some other animals, including the Australian brush-turkey.
Check out some other interesting facts about eggs from this collection of Science NetLinks resources developed for the 2009 White House Easter Egg Roll. (No, no reptile eggs were rolled.)