Responses of gibbons (Hylobates lar) to their mirror images

Nine gibbons (Hylobates lar) and one gibbon-siamang crossbreed were observed during exposure to their mirror images over two 30 minute baseline condition periods (mirror off) and four 30 minute treatment condition periods (mirror on). Greater frequencies and durations of gazing at the mirror and mirror-contingent behaviors were noted during the reflective conditions than for nonreflective conditions. Despite two subjects' apparent use of the mirror to view body parts otherwise not visible to them, no subjects passed a modified mark test. One subject exhibited limited aggressive displays toward the mirror, but no subjects exhibited abnormal behaviors in the presence of the mirror. The results expand on and confirm similar findings by Lethmate and Dücker [Zeitschrift fur Tierpsychologie 33:248-269, 1973].