On a tour of the Galápagos Islands, we had the opportunity to visit a field of Galápagos giant turtles, some who may have been the grandchildren or great-grandchildren of the same turtles Charles Darwin saw when he visited the islands in the 1820s (they can live to be more than 100 years old). Our guide told the group that, unlike humans and other mammals, male and female Galápagos turtles are not genetically different. For these turtles, as well as for other reptiles including alligators and crocodiles, sex is not determined by differences in genes, but by differences in the temperature at which the eggs are incubated. We could, theoretically, have genetically identical twin turtles, one a male and one a female. The guide told us the mnemonic he uses to remember the relationship between incubation temperature and sex for Galápagos giant turtles: “Hot chicks and cool dudes.”

Peter Gray & David Bjorklund, Child and Adolescent Development: An Integrated Approach, Belmont, California, 2011, p. 85