In mammals and birds, sex is determined by genotype at fertilization, but reptiles determine the sex of an individual by interaction with the environment, typically temperature. The Charnov–Bull model speculates that environmental sex determination will be favoured by selection if it could be shown that different temperature regimes maximized reproductive fitness for each sex. This has not been confirmed, partly because of the difficulty of setting up the 'control' experiment. However, hormone treatments have been used to overcome this difficulty, and a short-lived species of lizard shows that the Charnov–Bull model is correct.
