" The attitude toward members of one's own species should of course not be equated with that toward other species. Lack of concern for other species is to be expected, given the virtual absence of attachment. Animals often seem to regard those who belong to another kind as merely ambulant objects. Sue Boinsky reports that when an angry capuchin male in the wild ran out of ammunition while hurling things at her, he simply turned around, grabbed an unsuspecting squirrel monkey who sat nearby, and threw it at her as if it were just another branch. The capuchin, who would never have acted in this way with a member of his own species, clearly could not care less about the shrieking little monkeys with whom he shared the forest. Cruelty to other animals is something that we humans may have begun worrying about; it is a concern without precedent in nature. Hunters judge the hunted by caloric rather than emotional value, and even if other species are not perceived as food, usually nothing is to be gained by investing care in them. "

― Frans de Waal , Good Natured: The Origins of Right and Wrong in Humans and Other Animals