"The fact that some kinds of animals rely on environmental forces to determine sex shows that specific sex genes are not even necessary. In many reptile species, for example, the incubation temperature of their eggs dictates whether their offspring will be male or female. Temperature changes appear to trigger the release of hormones that sway the developing embryo's sex one way or another.
A good example of this phenomenon occurred in 1999. About a quarter of a million endangered Olive Ridley sea turtles showed up at the world's largest known nesting site, the Bhitarkarnika sanctuary on the east coast of India. The turtles usually arrive in January, but they held off until April, possibly because the females had been deterred by fishing trawlers.
The delay, along with unseasonably warm spring temperatures, meant that they laid their eggs in beach sands about 12 degrees warmer than normal. The effect was that the hatchlings were almost all female. Climatic variability over the years helps to balance the sex ratio in the populations of such long-lived species. "
'Men, From Stone Age to Clone Age: The Science of Being Male'