Orangutans (Pongo pygmaeus) Do Not Form Expectations Based on Their Partner’s Outcomes

Abstract

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.

Squirrel monkeys' response to inequitable outcomes indicates a behavioural convergence within the primates

“…First, it is possible that this trait is ancestral among primates, but was lost in squirrel monkeys. However, other studies find no evidence of inequity in orangutans [14,26,27], and little evidence in tamarins [24]. A second possibility is that the response is an emergent property of advanced cognition, seen primarily in species such as capuchins and chimpanzees, with high brain-to-body ratios [28].…”

To Each According to his Need? Variability in the Responses to Inequity in Non-Human Primates

“…Presumably this comparison requires some basic cognitive abilities (e.g., those necessary to compare one's outcome with another conspecific's), but the data indicate that within the primates, neither absolute brain size (Deaner et al, 2007) nor brain-to-body ratio (Jerison, 1973;Martin, 1984), correlate with inequity responses. Within the great apes, which have comparatively similar brain sizes, orangutans do not respond to inequity Brosnan et al, 2011a), thus a large brain is not sufficient to respond to inequity. Additionally, while both capuchins and macaques show negative responses to inequity, capuchins are highly encephalized, but macaques are not (Martin, 1984).…”

Kea show no evidence of inequity aversion

“…They also did not react to manipulations in their expectation of food type, as seen in crows and ravens [24]. Instead kea reacted like a wide range of other species, including bonobos [8], squirrel monkeys [19], orangutans [13,21] owl monkeys [20], marmosets [20], cleaner fish [35] and New Caledonian crows [25], in showing no significant reaction to inequitable situations.…”

“…Partners always exchanged with the experimenter before the actor and birds knew which tray the experimenter would take a reward from to give to them after they had exchanged the token, as in past studies, e.g. [4,10,11,21,23]. …”