Western lowland gorillas (Gorilla gorilla gorilla) do not show an aversion to inequity in a token exchange task

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.

Behavioral and physiological response to inequity in bonobos (Pan paniscus)

“…Dubreuil et al, 2006; Hopper et al, 2014; Neiworth et al, 2009) and a missing contrast control condition (cfr. Brosnan et al, 2010; Hopper et al, 2014; Sosnowski et al, 2021), we cannot completely differentiate social IA from the social disappointment hypothesis and the reward expectation hypothesis. We did rule out the expectancy violation hypothesis as explanation for the trial refusals in our study by running additional analyses.…”

“…While looking at the effect of dominance relations on IA, several studies found that dominant individuals were more sensitive to inequity than subdominants (Bräuer et al, 2006; Brosnan et al, 2010; Dale et al, 2020; Essler et al, 2017; Oberliessen et al, 2016), others have found no such rank effects (Brosnan & de Waal, 2003; Massen et al, 2012; Range et al, 2012; Sosnowski et al, 2021; Van Wolkenten et al, 2007). Again, the socioecology of the species could explain these differing results.…”

“…Blake et al, 2015; Rochat et al, 2009), but its occurrence is influenced by various factors like culture (Henrich et al, 2001), personality (Colquitt et al, 2006; Wiesenfeld et al, 2007) and relationship quality between the individuals that cooperate (Clark & Grote, 2003). To better understand the evolutionary origins of IA, its occurrence has been studied in a variety of animal species across different taxa, including birds, mammals and mostly nonhuman primates (reviewed in Brosnan & de Waal, 2014; McGetrick & Range, 2018; Sosnowski et al, 2021; Talbot et al, 2016). In the first study of IA in nonhuman animals, capuchin monkeys ( Sapajus apella ) had to exchange tokens for a food reward (Brosnan & de Waal, 2003), which has now become a standard method to study IA in animals.…”

Are Nonhuman Animals Averse to Inequity? A Meta-Analysis [Pre-Print]

“…PubMed: inequity aversion Duplicates were removed using EndNote 68 , after which the entries were imported into Covidence 69 for screening. Two relevant papers were published after the search was conducted and screened when their authors notified us of their existence 47,53 . The first author undertook the preliminary title and abstract screening; the first and fourth authors both conducted full text review of papers identified as potentially eligible.…”

“…The importance of this distinction has been acknowledged by much of the literature on IA in nonhuman animals, and most studies investigating IA in animals included conditions designed specifically to rule out the disappointment hypothesis. Typical examples of these are conditions in which the experimenter hands a high quality reward to an empty cage 27,[46][47][48] , and conditions in which the experimenter holds up a high quality reward prior to exchanging but then hands both subject and partner a low quality reward [49][50][51][52][53][54][55] . Some studies successfully demonstrated an effect of social comparison above and beyond disappointment 27,37,52,56,57 , while others did not 37,46,47 .…”