Gender refers to social and cultural roles of behavior between and within sexes. It's a human thing in the way that your initial respondent described morality. We can see the basics of it in nature, but if we want to really study it we have to turn to humans. If we use morality as a case study, we might study how animals might behave in terms of sharing and fairness. But it would be totally useless in studying ethical theories of behavior because those are largely cultural. In America, freedom and autonomy are important concepts while in Japan obedience and greater good mentalities are more highly prized. In a similar fashion, we can look at nature to see whether being gay is entirely cultural or whether there is a biological basis. Finding gay animals lends credence to the idea that there's a biological component. However, for people like transvestites there's no way to study that in nature because that's an entirely cultural concept. Culture and society exist in only the most rudimentary forms in nature apart from what we do ourselves, so in order to study cultural phenomena we have to study humans, not animals.