Are capuchin monkeys (Cebus apella) inequity averse?

Area of Science:

• Primate behavior
• Comparative psychology
• Animal cognition

Background:

• Capuchin monkeys exhibit food rejection when observing unequal outcomes.
• Previous studies suggested inequity aversion as the primary driver.
• Alternative explanations, such as frustration or mere presence of desirable items, were underexplored.

Purpose of the Study:

• To investigate the underlying mechanisms of food rejection in capuchin monkeys.
• To differentiate between inequity aversion and frustration-based rejection.
• To test the hypothesis that the mere presence of a more preferred food influences behavior.

Main Methods:

• A controlled experimental paradigm was designed using capuchin monkeys.
• Subjects were presented with a less preferred food (LPF) and a more preferred food (PF).
• Experimental conditions included PF presence (visible, hidden, accumulated) and partner interaction.

Main Results:

• Food rejection (refusals to initiate, take, or eat) increased when the PF was visible but inaccessible (hidden or accumulated).
• Rejection rates returned to baseline levels when the partner consumed the PF.
• Capuchins showed higher acceptance of LPF when the partner consumed the PF.

Conclusions:

• Capuchin monkeys' food rejection is likely driven by frustration from perceiving but not obtaining a more preferred food.
• The mere presence of a more desirable food, rather than inequity, is a significant factor.
• Observing a partner successfully consume the preferred food mitigates frustration and increases acceptance of the less preferred food.