At what ratio does male expendability stop being true, and female expendability take over?

For example, let's say we have a tribe of 99,999 women, and 2 men. Women are obviously more expendable in this case.

But, this is not a good scenario to be in. Although those 2 men are getting laid out the wazoo, they essentially have only 2 capable workers/soldiers/providers. The women will not only be less capable at these jobs, but will be less willing to perform them.

Basically, I'm saying that at some point, when women sufficiently outnumber men, male and female expendability become equal. It won't get to the situation I just described, because the society will likely maintain an equilibrium around this point.

I was wondering if anyone knew of any studies (perhaps in other animals) regarding this. It sounds like an economics problem, where you weigh the females' reproductive potentials against the males' potentials to provide and fight off enemies