From Sarah Hrdy's paper "The Past, Present, and Future of the Human Family":
When male mice encounter a strange pup, they either ignore it or eat it. When suffiently “primed,” however (that is, presented with pup after pup until the males become sensitive to pup signals), males finally quit cannibalizing and caretake: licking pups, gathering them in nests, hovering over them to warm them with their bodies. Primed males do just about everything mothers do, short of lactating. The hormonal basis of such maternal-seeming behavior in males—including humans—is only beginning to be studied.
We all know about the thin line between love and hate, but who knew the line between parental love and cannibalism was even thinner?