When a lion takes over a pride, job #1 is killing any kid who's not his. This is not uncommon in the animal kingdom. One way, or another, the goal is to create an environment where your have a reproductive advantage.

So the base is genetics, but also you have to remember that humans and chimps understand timelines. So if you see a strange redhead by the watering hole, you slink back home and get the lads, gonna need to make an example out of him. After all, you only speak your tribes dialect, can't talk to him, and if even you could, what would you say?

¯\_(ツ)_/¯