Cerrone, Mirko. “Keepers As Social Companions: Tactile Communication and Social Enrichment for Captive Apes”. Sign Systems Studies 47, no. 3/4 (December 31, 2019): 453–479. Accessed June 17, 2021. https://ojs.utlib.ee/index.php/sss/article/view/SSS.2019.47.3-4.06.