Resources are always constrained and populations had to make the choice between devoting more resources to either agriculture or raising protein. Harris details the choice of many ancient populations opting to grow more crops and therefore eat less animal protein because doing so gave them the most calories per unit of production. Of all of the domesticated animals, pigs proved to be the most energetically taxing to raise; they deplete the most resources to raise. Harris argues that the ecological consequences are important in cultures making pig protein taboo, but allows religious importance to also be a significant factor for the ban in many populations.