 Even Darwin recognized, however, that nature presented some cases that did not fit this picture. In particular, seemingly altruistic behaviors in the animal kingdom posed a puzzle (Wilson 2015). Take social mammals like ground squirrels that whistle alarm calls to notify other members of the group of a nearby threat. On the original Darwinian formulation of natural selection, alarm calling should not arise because it is not beneficial to the ground squirrels that display the behavior. A ground squirrel that is more prone to notifying its counterparts of a looming threat is presumably also more likely to get killed by that threat, and so the proneness to warn shouldn’t get propagated in the population if all the altruistic ground squirrels are dying by very virtue of their altruistic behavior (entry:  biological altruism).