Hediger, H. 1964. Wild Animals in Captivity. Dover Publications, New York, NY.

Recommendations for the species-appropriate housing of captive animals. The quality of the space at the disposal of the animal is of the greatest importance for its welfare. ... We must put ourselves into the animal's position as far as possible and foresee its needs. ... Through training the impoverished surroundings of the captive animal become enriched by new interests.