MOST animals have acquaintances but only a few species are capable of true friendship. This select group of mammals includes the higher primates, members of the horse family, elephants, cetaceans and camelids. It is no coincidence that all of these animals live in stable, bonded social groups. Group living has its benefits, but it can also be stressful and you cannot simply leave when the going gets tough – which is where friendship comes in. Friends form defensive coalitions that keep everyone else just far enough away, without driving them off completely.

Friendship gives social groups a very different structure from the amorphous herds of deer or antelope. From the point of view of each animal within it, a bonded society is made up of layers, like an onion, with your best friends at the core and successive layers filled with individuals with whom you are decreasingly intimate. Whatever the species, the core tends to consist of some five intimates, with the next layer taking the group to around 15, and the widest circle encompassing a total of around 50 friends. Each layer provides different benefits. So while intimates offer personal protection and help, you may rely on a larger friendship group for food, and the entire society for defence against predators.