"Many creatures, from mice and pigeons to lions, and dolphins, know a member of ‘our group’ from a stranger. All these creatures have been well studied in the past few decades, and the pattern is clear: fights within the group are limited and tend not to get out of hand, but fights between groups end in death. This in-group restraint, as the psychiatrist and author Jonathan Shay as said, gave rise to the widely believed myth that animals don’t kill their own kind. In fact many animals — lions, gorillas, chimpanzees, hyenas — are happy to kill their own. The victim just can’t be a member of their little band.
Animals, though, don’t make decisions about who is ‘in’ and who is ‘out.’ A dog guards her puppies because they are kin, and members of her human family because they are friends. But no dog quits her humans because they have converted to Catholicism or put a peace sign on their lawn. People can and will make that sort of change, because people, unlike animals, make choices based on signs — crosses, uniforms, peace signs, oaths, and other indicators of a particular human kind. Animals have kin and animals have friends, but only human beings trust symbols to tell us who is kin and who is friend."
— David Berreby, Us and Them: Understanding Your Tribal Mind (via savingpaper, desnoise)