It found that groups of mammals with relatively bigger brains tended to live in stable social groups. The brains of more solitary mammals such as cats, deer and rhino, grew much more slowly during the same period.

Previous research had suggested that the growth rate of the brain relative to body size followed a general trend across all animal groups.

The study published in Proceedings of the National Academy of Sciences overturns this view, showing there is a wide variation in patterns of brain growth across different groups of mammals and not all of them have larger brains – suggesting social animals had to think more.