But what’s true for mammals isn’t always true for the rest of the animal kingdom. The sex of baby birds is also determined by two different sex chromosomes, but in birds, it’s the male that has two of the same-shaped sex chromosomes and the female that has two different sex chromosomes. A lot of reptiles, including crocodiles and some turtles don’t even have sex chromosomes. The sex of these animals is determined by the temperature of the eggs before they hatch. And in some species of fish, female adults can turn into males if there aren’t enough males in the population, while in others males turn into females.