Smell is an interesting point As travisjungroth points out, the most obvious mechanism is feed-back where they eat something, it works, and the body is design

Smell is an interesting point As travisjungroth points out, the most obvious mechanism is feed-back where they eat something, it works, and the body is designed to do more of the same. But if there is a feed-forward mechanism, i.e. where genetic information tells you "eat this thing now, even if you normally hate it*; then the most plausible thing would a pretty low level mechanism that matches chemicals detected by smell.