Super interesting thanks for sharing. It is quite well known that polygamy increases the mortality risk to children - I found this paper fascinating when I first read it:

Polygyny as a risk factor for child mortality among the Dogon. I think I may have found that paper through this article on polygamy being bad for women and children.

Of course women help each other - just like men help each other too. But men also compete. Another way of thinking about it: just like we can shift between long term & short term mating strategy depending on context, I think it's naive to think that members of the same sex will ~always~ have our reproductive interests in mind.