Several primate species form expectations based on others’ outcomes, responding negatively when their outcomes differ from their partners’. The function and evolutionary pathway of this behavior are unknown, in part because all of the species which have been tested thus far share traits related to a gregarious lifestyle, intelligence, and cooperativeness. Our goal was to test whether inequity is a homology among primates or a convergence by comparing one species known to show social comparisons, the chimpanzee, to another great ape which differs on several of these life history characteristics. Using a protocol identical to one used previously with chimpanzees, we tested whether orangutans, an intelligent but predominantly solitary species with few opportunities to cooperate, responded similarly. To allow for a strong comparison with chimpanzees (and other species), we used socially housed adults of both sexes, tested with members of their social group. Orangutans did not respond negatively to inequity, supporting previous findings and indicating that inequity responses in apes are likely a convergence based on either sociality or cooperative tendency. These results in such closely related species highlight the need for additional comparative studies to understand better the function and evolution of social behaviors.

Faces provide humans with information on the age, sex, individual identity, and emotional state of other individuals. Face discrimination was likely advantageous in the evolution of many group living species; however, little is known about how a species' sociality relates to their face discrimination skills. This may be particularly interesting in the context of discriminating familiar versus unfamiliar faces, as species that do not spend as much time in social groups may not differentiate between these two categories as compared to species that spend more time in groups. Previous studies in humans and chimpanzees, both group living species, have found differences in performance based on familiarity such that performance decreases across changes in viewpoint when discriminating unfamiliar, but not familiar faces. In this study, we tested a less gregarious species, orangutans, to determine if face discrimination skills differed from these other primates. Using a matching-to-sample paradigm, we found that two of the three orangutans performed significantly above chance when discriminating novel photographs of familiar individuals but not novel photographs of unfamiliar individuals. Thus, while additional data are needed on species that are even less gregarious than orangutans, our results indicate that, at least within the primates, more and less gregarious species show the same bias towards better discrimination of familiar faces. Further examination is needed to understand social organization and other social factors that may be important in the evolution of face-processing skills.

In summer 1998 a ♂ infant Western lowland gorilla Gorilla gorilla gorilla at Zoo Atlanta was hand‐reared and integrated into a social group. Because the biological mother would not accept the infant, he was introduced to a non‐lactating surrogate mother. Gorilla infants are usually ≥ 6 months old at the time of introduction but this infant was successfully introduced to the surrogate at only 11 weeks of age. Behavioural observations made during the first year of development indicate that this infant's behaviour is similar to that of mother‐reared gorillas and he shows no signs of abnormal or stereotyped behaviour. The choice of surrogate mother, the training of both the surrogate and infant for bottle feeding, and the dedication of the nursery‐care staff who worked long hours to provide the infant with a stimulating nursery environment, all contributed to the success of the introduction.

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