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: Western lowland gorillas do not show an aversion to inequity in a token exchange task

Western lowland gorillas (Gorilla gorilla gorilla) do not show an aversion to inequity in a token exchange task. Meghan J. Sosnowski, Lindsey A. Drayton, Laurent Prétôt, Jodi Carrigan, Tara S. Stoinski, Sarah F. Brosnan. American Journal of Primatology, September 3 2021. https://doi.org/10.1002/ajp.23326

Abstract: 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.

Research Highlights

We tested Western lowland gorillas, a species that is rarely observed cooperating in the wild, on a token exchange paradigm in which reward and effort differed between the subject and the conspecific partner.

Gorillas did not respond negatively to inequitable outcomes, but instead refused to participate in all conditions where they must put in effort to receive a low-value reward. We also found that gorillas were more likely to refuse to participate later in a session.

We discuss the implications for the cooperation hypothesis of the evolution of inequity aversion, as well as interesting future directions for research in responses to inequity.