"choice" men, for their part, can gain from this arrangement if they can detect fertile women.... or they can lose, unless they guard the "old lady" from the "wolves". my quotes.
the cost-benefit matrix is pretty simple - all women benefit from one caregiver and one or more hunks, same person or different. some men benefit from caring for one female, some from sleeping with many and caring for one or none, and some take care of others' kids!
Darwin's are examples of theories so clearly articulated as to be self-evident to the receptive listener (though he provided heaps of evidence in support thereof). Here too i see sense. The bar is the microcosm, & experiment bears it out. My fave - fertile fems prefer smell of symmetric males.