In The Red Queen, Matt Ridley draws attention to the work of Kim Hill. Firstly you might want to bone-up on sexual selection. Natural selection is indeed powerful, but that is only half the story - or rather the impact of sexual selection is omitted in preference for the 'survival of the fittest' sound-bite we've ended up with.
Hill worked with the Ache people of Paraguay and found that meat was used a form of payment,
- Ache men would donate any spare meat they had to women they wanted to have sex with. They were not doing so in the hope of helping to feed children they had already fathered but as a direct payment for an affair. It was not easy to discover. Hill found that he was gradually forced to drop questions about adultery from his studies because the Ache, under missionary influence, became increasingly squeamish about discussing the subject.
As I said before, this is a great book!