There are thousands of species of animals that have developed the ability to change their sex. It's not so much a matter of what chromosomes they carry, because only mammals have X or Y chromosomes. Other species have sex chromosomes, but they are so different they are labeled in their own way. These animals are classified as male or female by whether they produce sperm or eggs. And that can change over time, often due to environmental conditions.
For example, there is a snail that changes from male to female depending on where it is in the mating chain, and since they are snails, that takes a long time. Some amphibians change sex when there is a big imbalance between males and females in the population. Many species of turtle will change sex when the temperature of the water changes. One fish can change from male to female in a hurry if confronted by a male rival it doesn't want to fight. And the clownfish has a sex change scheme that would give a whole new meaning to the movie Finding Nemo if Disney had wanted to be completely accurate. Read up on seven species of animals that change their sex at Mental Floss.
(Image credit: Nick Hobgood)
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