From Lucy Kelly:

Re: Is humor tied to male aggression? (Dec. 21): with the man on the unicycle, he may have gotten very different responses if he had been female.

Also, animals have humour, although I have seen it more in omni and carnivores than in herbivores. Eg adult female chimps laughing at a baby chimp who falls whilst monkeying about.

I think humour is simply a message to the self that all is well. Some situations message the self to get tense, and there may be a physiological response to a non-threatening situation, which people would enjoy.