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. 2021 Nov 10;288(1962):20211937.
doi: 10.1098/rspb.2021.1937. Epub 2021 Nov 3.

Chimpanzees and bonobos use social leverage in an ultimatum game

Alejandro Sánchez-Amaro  1   2 , Federico Rossano  2
Affiliations

Chimpanzees and bonobos use social leverage in an ultimatum game

Alejandro Sánchez-Amaro et al. Proc Biol Sci. .

Abstract

The ultimatum game (UG) is widely used to investigate our sense of fairness, a key characteristic that differentiates us from our closest living relatives, bonobos and chimpanzees. Previous studies found that, in general, great apes behave as rational maximizers in the UG. Proposers tend to choose self-maximizing offers, while responders accept most non-zero offers. These studies do not rule out the possibility that apes can behave prosocially to improve the returns for themselves and others. However, this has never been well studied. In this study, we offer chimpanzee and bonobo proposers the possibility of taking into account the leverage of responders over the offers they receive. This leverage takes the form of access to alternatives for responders. We find that proposers tend to propose fairer offers when responders have the option to access alternatives. Furthermore, we find that both species use their leverage to reject unequal offers. Our results suggest that great apes mostly act as rational maximizers in an UG, yet access to alternatives can lead them to change their strategies such as not choosing the self-maximizing offer as proposers and not accepting every offer higher than zero as responders.

Keywords: bonobo; chimpanzee; leverage; prosociality; ultimatum game.

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Figures

Figure 1.
Figure 1.
Representation of offers and alternatives for both conditions. P stands for proposer while R stands for responder. Responder ideal choice refers to the response that allows individuals to maximize their gain. (Online version in colour.)
Figure 2.
Figure 2.
Proportion of trials where proposers maximized their choices as a function of the type of session (control, test session with no leverage and with leverage). Differences between conditions are significant. The box plot represents the median and q1 and q3 quartiles. The dotted lines in purple represented the fitted values of the model and the predicted 95% CI. (Online version in colour.)
Figure 3.
Figure 3.
Proportion of trials in which the responders accessed the alternative as a function of the ‘proposers' decisions. Differences between species and between previous proposers decisions are significant. The box plot represents the median and q1 and q3 quartiles. The dotted lines in purple represented the fitted values of the model and the predicted 95% CI. (Online version in colour.)
See this image and copyright information in PMC

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