There have been two major reviews of research into animal play, and both have said it is extremely difficult to define. Play is almost universal in very young mammals, and also in some groups of birds such as parrots and ravens. Therefore, it must have a function, but what is that function? It is definitely not immediate survival. In fact, the young animals of many species may risk their lives playing. Yet, since it survives in so many species, it must have survival value. That, as Fagan says, is a paradox.