In some species of fish and amphibians, females can change into males if males are in short supply and vice-versa.
When Do Changes Happen?
Changes only happen when the sex ratio in a certain area is off kilter. So how do fish and frogs go about fixing the mate shortage? African reed frogs carry both ovarian and testicular tissue, although only one type is active at a time.
Females can activate their testes and deactivate their ovaries.
Males And Females
As their male tissue becomes active, the females adopt male behaviors such as fighting and courting. When the testes are fully developed, the females come to look like males and are indistinguishable from other male frogs.
Like frogs, some fish change sex just once and live happily as the opposite sex, but certain female fish are able to change into males and then change back if a larger male comes along. And some fish species can flip-flop back and forth as many as 10 times.
Actually, the whole sex change process takes up a lot of energy. This might be why larger creatures can't change their sex just because there is a shortage of mates... well, not without some pretty radical surgery.