Humans aren't the only mammals to make friends -- elephants, horses, whales and plenty more form strong social bonds that aren't connected to reproduction -- but it's only the smartest that do. Why is that?
In fact, mammals form alliances to help other, to ensure safety -- essentially to make life easier -- and it requires intelligence to realise that's a smart thing to do. But, as this video explains, whether you're a human or a horse, it takes a lot of brainpower to support a complex social group -- and that puts a natural limit on just how many friends we can have. [New Scientist]