Read more: 鈥� Instant Expert: Evolution of social networks鈥�
We tend to think of social networks as being distinctly human. In fact, they occur wherever animals live in 鈥渂onded鈥� groups 鈥� where individuals gather together because of their personal relationships rather than being forced to by environmental factors such as a food source or safe sleeping site. Bonded groups are found among all primates and a few other mammals including cetaceans. Here, as with humans, social relationships give rise to a nested hierarchical structure based on each relationship鈥檚 intimacy. Such networks have benefits, but they are also costly to maintain and are only an option for the smartest of species
Grooming and gossip
Monkeys and apes create and nurture social relationships by grooming each other. The physical action of being groomed is rather like massage and triggers the release of endorphins. This creates a light euphoria that seems to make it possible for animals that groom each other to build a relationship based on friendship and trust.
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The average time spent grooming by members of a species correlates with the size of their social group. Those, such as gibbons, which typically live with only three or four others, groom for 5 per cent of their day at most. Baboons, meanwhile, live in groups of 50 or more and can spend as much as 20 per cent of their time grooming. However, as group size and time spent grooming increases, this social effort is concentrated on fewer and fewer partners.
Although we use grooming in intimate relationships, the very intimacy of the activity makes it ineffective as a tool for bonding our large social groups. Instead, we have evolved alternative ways to create the same endorphin surge on a bigger scale. One of these is laughter, another is communal music-making. Language, too, plays an important role 鈥� not only can we speak to many people at the same time, we can also exchange information about the state of our networks in a way that other primates cannot. Gossip, I have argued, is a very human form of grooming.
Net benefits
Being part of a bonded society provides protection against predators, so there are benefits from the wider social network. Intimate relationships have their own rewards. In baboons, for example, the number of close grooming friends a female has directly influences the number of offspring she rears successfully. Such friends provide protection against more dominant animals in the group, increasing a female鈥檚 fertility. They also help support and protect her offspring, improving their chances of survival.
Friends also help combat the stress of living in a large bonded social group. Female baboons that have more friends have lower levels of stress hormones on a day-to-day basis than those with fewer friends. On the other hand, intimacy seems to be more important than an individual鈥檚 total number of friends when dealing with one-off stressful events, such as when a very aggressive male joins the group. Then, female baboons cope better when they reduce the number of social partners they have and concentrate on their closest friends.
A mind for friendship
Group living needn鈥檛 tax your intelligence too much. In a loose herd, cues such as body size or aggressiveness may be enough to judge whether you should challenge or steer clear of another individual. In bonded networks, however, you need to know each member鈥檚 personal characteristics and those of the friends and relations that might come to their aid. Keeping track of the ever-changing web of social relationships requires considerable mental computing power.
As a reflection of this, there is a correlation between the size of a species鈥� brain 鈥� in particular its neocortex 鈥� and the typical size of its social groups. In other words, brain size seems to place a limit on the number of relationships an individual can have. This link between group size and brain size is found in primates and perhaps a handful of other mammals that form bonded societies such as dolphins, dogs, horses and elephants. In all other mammals and birds, unusually large brains are found only in species that live in pair-bonded (monogamous) social groups.
As group size increases so too does the number of relationships that need servicing. Social effort is not spread evenly. Individuals put most effort into their closest relationships to ensure that these friends will help out when they need them. At the same time they maintain the coherence of the group. As a result, social networks resemble a nested hierarchy with two or three best friends linked into larger groupings of more casual friends, and weaker relationships bonding the entire group. This hierarchy typically has a scaling ratio of three 鈥� each layer of decreasing intimacy is three times larger than the one before it.
We don鈥檛 know exactly how monkeys and apes maintain this social balancing act. But one thing that seems important is being able to appreciate another individual鈥檚 perspective. In humans this has developed into theory of mind or mentalising 鈥� the understanding that another individual can hold ideas and beliefs that differ from your own. Brain scans of people doing perspective-taking tasks reveal that these cognitive processes use a lot of neural activity, particularly in the ventromedial prefrontal areas. In monkeys the size of these same brain regions have now been shown to correlate with social-group size and social rank.
