I developed a concept called 'typical intensity'. You know when a telephone rings, it goes brrp-brrp, brrp-brrp, brrp-brrp, like that. Now, it could be a casual call, it could be an urgent call – the telephone can't tell the difference. The brrp-brrp goes on at a fixed intensity. The only way that you can make it feel more urgent is to let it ring longer, but that's not very... So telephone's not very good at telling you how intense a particular call is or how urgent a particular call is because it has a fixed signal. Now animal signals aren't fixed like that. They're variable, and the stronger the intensity, the stronger the movement. But if that happens... if that goes too far, then there's nothing strange about the ritual. And for the ritual to become strange, it has to develop a typical intensity, that is, that the bird may bob its head slightly faster, but it won't be as fast as it might be if it was just an action like, say, pecking: if there... if there's a hard nut, the harder the nut, the harder the peck.

It isn't like that with animal display. What happens is that the animal develops a typical intensity so the head will be bobbing like this at a particular intensity. It won't be absolutely fixed because it might get a little more excited. But it won't... let me give you an example. Human clapping – you see, because I, of course, later was going to apply all this to humans – human clapping – if you clap like this, it's a very different signal to if you clap like this. And when people applaud, if you measure the speed of clapping, you'll find that there is a typical intensity, that when people are applauding, they tend to clap at this sort of speed. If they're really excited, they might try and clap even faster, but it's very difficult. So they have a...normal applause goes at a certain speed. And if you do it very slowly, it gives a different signal altogether. So visual signals or sound signals have about them this typical intensity which marks them out and makes them into special rituals. And this was the kind of thing I was studying at Oxford.