F. Zaragoza, M. Ibáñez, B. Mas, S. Laiglesia, and B. Anzola, “Influence of environmental enrichment in captive chimpanzees (Pan troglodytes spp.) and gorillas (gorilla gorilla gorilla): behavior and faecal cortisol levels”, Rev. Cient. FCV-LUZ, vol. 21, no. 5, 1.