 Although it is typically reported that chimpanzees consistently “pass” the mirror-mark test, a survey of the scientific literature by Shumaker &amp; Swartz (2002) indicates that of 163 chimpanzees tested, only 73 showed mark-touching behavior (although there was considerable variation in the age and mirror experience among these animals). Shumaker &amp; Swartz also report mark-touching behavior in 5 of 6 tested orang utans and 6 of 23 gorillas. They suggest that the lower incidence of mark touching by gorillas may be due to avoidance of socially-significant direct eye contact.