Reports of endowment effects in nonhuman primates have received considerable attention in the comparative literature in recent years. However, little is known about the mechanisms underlying these effects. Continuing to explore endowment effects across different species of primate may reveal subtle differences in behavior that can help formulate specific hypotheses about the relevant mechanisms and the social and ecological factors that have shaped them. In this study, we use a paradigm that has previously been used to test chimpanzees (Pan troglodytes) and orangutans (Pongo spp.) to explore whether western lowland gorillas (Gorilla gorilla) exhibit comparable endowment effects. We find that gorillas exhibit endowment effects when in possession of food, but not nonfood, items, and that they show a statistically stronger effect than chimpanzees but not orangutans. These findings are consistent with the hypothesis that mechanisms for endowment effects in primates may be related to inhibitory control or risk aversion.

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Although individuals in some species refuse foods they normally accept if their partner receives a more preferred one, this is not true across all species. The cooperation hypothesis proposes that this species‐level variability evolved because inequity aversion is a mechanism to identify situations in which cooperation is not paying off, and that species regularly observed cooperating should be more likely to be averse to inequity. To rule out other potential explanations of inequity aversion, we need to test the converse as well: species rarely observed cooperating, especially those phylogenetically close to more cooperative species, should be less likely to be inequity averse. To this end, we tested eight zoo‐housed Western lowland gorillas (Gorilla gorilla gorilla) on a token exchange task in which subjects received either the same food reward or a less‐preferred reward for the same or more effort than their partner, recording both refusals to participate in the exchange and refusals to accept the reward. Supporting the cooperation hypothesis, even with procedural differences across sessions, gorillas were significantly more likely to refuse in all conditions in which they received a low‐value food reward after completing an exchange, regardless of what their partner received, suggesting that gorillas were not inequity averse, but instead would not work for a low‐value reward. Additionally, gorillas were more likely to refuse later in the session; while the pattern of refusals remained unchanged after accounting for this, this suggests that species should be tested on as many trials as is practical.

A goal of the comparative approach is to test a variety of species on the same task. Here, we examined whether the factors that helped capuchin monkeys improve their performance in a dichotomous choice task would generalize to three other primate species: orangutans, gorillas, and drill monkeys. In this task, subjects have access to two options, each resulting in an identical food, but one (the ephemeral option) is only available if it is chosen first, whereas the other one (the permanent option) is always available. Therefore, the food-maximizing solution is to choose the ephemeral option first, followed by the permanent option for an additional reward. On the original version (plate task), the options were discriminated by the color and pattern of the plates holding the food, while on two subsequent versions we used altered cues that we predicted would improve performance: (1) the color of the foods themselves (color task), which we hypothesized was relevant to primates, who choose foods rather than substrates on which foods are found when foraging, and (2) patterned cups covering the foods (cup task), which we hypothesized would help primates avoid the prepotent response associated with visible food. Like capuchins, all three species initially failed to solve the plate task. However, while orangutans improved their performance from the plate to the color task, they did not for the cup task, and only a few gorillas and no drills succeeded in either task. Unfortunately, our ability to interpret these data was obscured by differences in the subjects' level of experience with cognitive testing and practical constraints that precluded the use of completely identical procedures across species. Nonetheless, we consider what these results can tell us, and discuss the value of conducting studies across multiple sites despite unavoidable differences.

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