For humans and other mammals, sex is neatly determined by the X and Y chromosomes. If you have a Y you are male, and without it you are female. Reptiles however, use a variety of strategies, and the mammalian X/Y system is just one of them.

In some species, the female is the one with different chromosomes, in this case Z and W, and the male has two Zs. And some reptiles ignore sex chromosomes altogether. For them, an individual’s sex is determined by the temperature that their eggs were incubated at.

Scientists had long believed that these strategies were mutually exclusive with each species choosing one of the other.

But Alexander Quinn and colleagues form the University of Canberra have found that