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]