Yerkes, R. M. 1925. Almost Human. Jonathan Cape, London, UK.

Almost all of the primates like companionship, but there is good reason to believe that the gorilla cannot live normally without it. He becomes sulky, despondent, and - like chimpanzees or orang-utans captured when adult - he may refuse to eat, and thus end his life. Primates seem as eager as humans for the company of their kind and as dependent upon it for contentment and happiness. Companionship and play are not a whit less important to their health, comfort, and contentment than such factors of physical environment as temperature, moisture, food, and drink. No primate, if it can reasonably be avoided should be kept in isolation. Undoubtedly, kindness to captive primates demands ample provision for amusement and entertainment as well as for exercise. The greatest possibility of improvement in our provision for captive primates lies in the invention and installation of apparatus which can be used for play or work.