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

Abstract

Although several primates respond negatively to inequity, it is unknown whether this results from homology or convergent processes. Behaviours shared within a taxonomic group are often assumed to be homologous, yet this distinction is important for a better understanding of the function of the behaviour. Previous hypotheses have linked cooperation and inequity responses. Supporting this, all species in which inequity responses have been documented are cooperative. In this study, we tested this hypothesis by investigating the response to inequity in squirrel monkeys, which share a phylogenetic family with capuchin monkeys, but do not cooperate extensively. Subjects exchanged tokens to receive food rewards in conditions in which the level of effort required and reward received varied. Squirrel monkeys did not respond negatively to inequity. However, the monkeys were sensitive to the variation present in the task; male subjects showed a contrast effect and, as in previous studies, subjects were more sensitive to differences in reward in the context of a task than when rewards were given for free. Taken with other results, these results support the hypothesis that a negative response to inequity evolved convergently in primates, probably as a mechanism for evaluating outcomes relative to one's partners in cooperative species.

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

“…When effort and no-effort conditions involving the same outcomes were randomized, chimpanzees responded negatively to conditions with a task, but not to a no-effort "gift" condition (Brosnan et al, 2010b). Moreover, as with Neiworth et al's findings, even in species that do not respond to inequity, there is typically a greater frequency of refusals in exchange conditions than in the "gift" conditions without a task (squirrel monkeys: Talbot et al, 2011;marmosets;Freeman et al, in review;orangutans, Brosnan et al, 2011a). …”

Justice- and fairness-related behaviors in nonhuman primates

“…Macaques, too, both responded negatively to inequity (39,40) and showed cooperation in the form of coalitions and alliances (41). However, in squirrel monkeys, a species confamilial (and sympatric) with capuchins but lacking extensive social cooperation, males responded more strongly to the Contrast condition than either the Inequity or Equity conditions, indicating that their reactions were more strongly influenced by their previous offers than by rewards received by another monkey (27). Again considering orangutans, one way in which they differ from chimpanzees and bonobos is their relative lack of social cooperation.…”

“…The experimenter rewards subjects with the same rewards (Equity condition) or gives one a more preferred reward (Inequity condition). To rule out the possibility of contrast effects (25,26), in which subjects compare their outcomes to those present in the environment without attending to the social component, pairs are given a Contrast condition, in which both partners are shown the preferred reward but, after completing the task, receive the less-preferred one (27,28). These responses are referred to as Inequity and Equity rather than Inequality and Equality because, despite the fact that the outcomes are identical (or not), we cannot know whether they are perceived identically by subjects because of differences in rank, hunger level, etc.…”

The importance of risk tolerance and knowledge when considering the evolution of inequity responses across the primates

“…In particular, work with ten primate species spread across the great apes, Old World monkeys, and New World monkeys demonstrates that inequity is most common in species that show high levels of cooperation with non-kin, non-pair social group members (Brosnan 2011). That is, primates that routinely form coalitions and alliances for support, rank acquisition or territory defense, cooperatively hunt, or otherwise socially cooperate, including chimpanzees (Brosnan, Schiff et al 2005;Brosnan, Talbot et al 2010), capuchin monkeys (Brosnan and de Waal 2003;van Wolkenten, Brosnan et al 2007;Fletcher 2008;Takimoto, Kuroshima et al 2009), macaques (McCleam andHarlow 1954;Beran, Rumbaugh et al 2007), and, possibly, bonobos (Bräuer, Call et al 2009), respond to inequity while other, often closely related (Talbot, Freeman et al 2011) and equally largebrained species (Meyer, Polidora et al 1961;Bräuer, Call et al 2009) do not. Moreover, these results do not map on to other distributions, such as those based on brain size, performance on cognitive tasks, or extent of sociality (Brosnan 2011).…”