“Friendship," wrote CS Lewis, “is born at that moment when one person says to another: ‘What! You, too? Thought I was the only one.’” He wasn't the only one. Plato wrote "similarity begets friendship" in his 360 BCE play Phaedrus. And Aristotle had the same idea when he wrote, "some define it as a matter of similarity; they say that we love those who are like ourselves."
Friendships blossoming on the basis of similar ideas, outlooks or tastes may seem intuitive, but that intuition is deceiving. Most friendships develop between people who are not family members or sexual partners, so friendship can't be explained on the basis of genetic or reproductive interests. Instead, evolutionary biologists have typically relied on a tit-for-tat process known as reciprocal altruism to explain friendship: you scratch my back, and I'll scratch yours.
The problem, however, is that social psychologists have discovered that people do not maintain mental ledgers of favours given and received. Primatologist Joan Silk described the riddle of friendship neatly: "reciprocity and equity are important among friends, but tit-for-tat reciprocity is antithetical to the formation and maintenance of close friendship. If these seemingly contradictory claims are correct, then friendship presents a puzzle for evolutionary analysis."
As with any evolutionary puzzle, it makes sense to look towards the animal kingdom for clues. Recently, a group of French shark scientists looked at whether aggregations among sharks could be explained in social terms – that is, if they were friendships – or whether sharks occupied the same space at the same time simply because of overlapping home ranges or mutual food sources.
The researchers focused their attention on 133 blacktip reef sharks, Carcharhinus melanopterus, a species known to patrol coral reefs in the Indian and Pacific Oceans. They learned that certain sharks preferred the company of certain others, and that those friendships persisted over time. And other pairs of sharks went out of their way to avoid each other, even if their territories overlapped. Geographic or territorial proximity is not enough to explain the riddle of their friendship.
Perhaps bigger-brained mammals like dolphins can help. Dolphin societies, like those of non-human primates such as chimpanzees, are characterized by two levels of social hierarchy: groups of two or three males, called "first-order alliances" work together to guard females from other males; and larger groupings comprised of several first-order alliances, appropriately referred to as "second-order alliances," cooperate to steal females from other groups. Typically, the individuals within first- and second-order alliances are related, so this sort of cooperation can be explained by shared genes, or kin selection.
But an Australian research team working in Shark Bay, Australia discovered, a third level of social hierarchy: coalitions among second-order alliances, which may be called "third-order alliances," or more imposingly as "second-order super-alliances," involving dolphins that were unrelated.
As with human friendships, the interactions among second-order super-alliances could not be explained by reciprocity. For example, a group of dolphins the researchers called PD prevailed over another group called KS in two separate occurrences. A social exchange-based theory of dolphin friendship would predict that PD and KS never cooperate. But that's exactly what the researchers saw when a third coalition, WC, attacked KS. To their amazement, PD swam into the fray to help their one-time nemesis defeat WC.
It turns out that dolphin friendships are not based solely on prior interactions, they are politically motivated. Cooperative decisions are based on who else is around at any given moment. Even if the PD and KS groups had historically been enemies, their mutual interest in defeating the WC group turned them into friends, at least temporarily. This seems to require an extraordinary amount of cognitive resources.