The latest episode of the Explanimator by New Scientist features an animated explanation of Dunbar’s Number, a concept that correlates brain size with the ability to make and maintain friendships.
You have to know how your actions will affect others in the group and that takes a big brain. It also takes a lot of time to maintain those relationships. In the wild, that time is usually spent grooming. Grooming releases endorphins which help an individual feel relaxed and trusting of the company they’re in. In nature the more time two individuals spend grooming
each other, the stronger their bond tends to be. But there’s only so much time in
the day and that’s why brain size put a limit on how many friends and individual can have. This is known as Dunbar’s number and it’s a way to predict how big a natural group of animals will be. …so friendship is great but it takes a lot of time and intelligence which helps explain why in nature of these good friends really are hard to find