Why do hyenas laugh?

It's not a laugh like a human laugh, those vocalisations are far reaching ways of hyenas to communicate with their clan. Hyenas are social animals which live in huge groups (called clans) that may be spread out over a large area. They depend on numbers to scavange effectively from other predators, such as a pride of lions, so they "laugh" to signal that they have found food or some other reason for other hyenas of the clan to join them.