Solving a 50-year-old mystery, a US-China research collaboration has revealed why eggs incubated at 32ºC produce all female hatchlings, while eggs kept at 26ºC hatch as males. Researchers showed cooler incubation temperatures turn up a gene called Kdm6b in the turtle's immature sex organs. The gene acts as a biological "on" switch, activating other genes which allow testes to develop.