Singing birds, playing cats, and babbling babies: why do they do It?

Rarely, animals do what they do because they are aware of the function of the behaviour or its outcome. Instead, they will very often perform behaviour out of context, spontaneously, as play. The impression (strengthened by introspection in the human species) is that they do it because they get some kind of internal reward. Nevertheless, such seemingly meaningless behaviour may have an ultimate function to adjust behavioural programs to the body, to practice, to perfect the execution of the behaviour. If the proximate reason for doing what the animal does may be to attain a pleasurable state, the ultimate, evolutionary reason may still be that increased practice will give some gain in fitness. If one presupposes internal rewarding and punishing systems as intervening factors, it becomes much simpler to explain why birds sing, kittens play or babies babble without any outer reward and out of any functional context, more than needed from a strictly functional view, spontaneously and just for the fun of it.