As gender theorists like Judith Butler and Anne Fausto-Sterling have pointed out, sex and gender cannot be fully pulled apart. Facts about our sexed bodies influence the cultural rules surrounding gender… And facts about gender in turn shape our sexed bodies. (For instance, norms of what is attractive lead to different patterns of exercise, like weightlifting for men and running on the elliptical for women.) And these feedback on each other. (When men only weightlift, this creates further sex differences that reinforce our cultural norms.) But despite this intertwining, peacocks still do not have genders. And the reason is that peacocks do not have culture.
[G]enuine examples of animal culture are relatively rare, and do not involve sex-specific patterns of behavior and learning the way human gender does. We do not, for instance, see killer whale pods where females learn special behaviors only from females, and pass these on only to other females. Even if we did, this sort of proto-gender would be missing many of the key features of human gender systems. Most importantly, human gender systems involve healthy doses of normativity—not only do women behave a certain way by dint of their cultural training, but we agree as a society that they should behave that way. This element is absent from animal cultures.