According to John
No greater love is there than this, than to lay down one’s life for one’s friends.
According to Ewen Callaway
On average, warfare caused 14 per cent of the total deaths in ancient and more recent hunter-gatherers populations.
The cost of losing an armed conflict as a group is high enough to balance out the individual risks of warfare, especially if a population is relatively inbred, Bowles’ model concludes. Since evolution acts on genes, it makes more sense to make more sacrifices for a related neighbour than an unrelated one.
Since Bowles had no way of knowing how inbred Pleistocene populations were, he compared contemporary hunter-gatherers such as African pygmies and native Siberians. Individuals in these populations were closely related enough to justify going to war, he found.
On the other hand, do Diana monkeys go to war? Yet according to Deborah Blum
In one lab study of Diana monkeys, for instance, the animals had to put tokens into a slot to receive their food. When an elderly female couldn’t manage hers, a neighbouring male inserted the tokens for her. In a different kind of experiment, rats refused to push a lever for food when they realised their action meant another animal got an electric shock.