[_pigpen__]

“Animals don’t have socially defined gender.” Perhaps, but they can definitely have have socially defined sex. Clownfish live in hierarchical units. The dominant fish is an aggressive female. When she dies, the next dominant male will change sex and assume the role. If that isn’t social I don’t know what is. (And that’s one example there are at least a dozen species of fish that can change sex due to external factors.)

[pvaldes]

Those fishes are true hermaphrodytes in fact. Its reproductive organs have a part male and a part female. Both perfectly able to produce functional gametes but not at the same time. Not equivalent to trans people case, really.