"The ways in which animals play had long been studied. Ornithologists had observed many types of bird play, such as when they hang upside down from tree branches and swing back and forth with apparent glee. Chimpanzees had been observed playing and chasing each other in ways that looked much like children playing tag. Even some insects had been observed to play. In 1929, August Forel, an influential ant researcher, wrote in his book The Social World of the Ants as Compared to Man: 'On fine, calm days when they are feeling no hunger or any other cause of anxiety, certain ants entertain themselves with sham fights, without doing each other any harm; but these games come to an end directly [if] they are scared. This is one of their most amusing habits.'"
- from How to Tame a Fox (and Build a Dog)