Environmental Sex Determination
For most turtles and alligators, the sex of an individual is determined by the incubation temperature of the eggs.
In many turtles, for instance, eggs that incubate at low temperatures become females, and the eggs that incubate at high temperatures become males (in other species, the pattern is reversed).
In some cases, sex can be determined by the social or physiological status of the individual. Small or poorly fed individuals are male and larger individuals are female, or vice-versa. This is especially common in sequential hermaphrodites.
In some cases, cytoplasmic parasites such as Wolbacchia sp. Can feminize their hosts, converting males into females...