We humans have a strong sense of justice. Various experiments suggest that some animals, including monkeys, wolves and crows, may be similar. But apparently their sense of justice differs significantly from ours, as a recent study shows. Accordingly, monkeys experience a mixture of social disappointment and competition for food rather than injustice in the human sense.
If we feel unfairly treated, for example when distributing snacks, then we usually show it clearly. We complain or do without the dry pretzel stick if our counterpart got a piece of cream cake instead. Similar behavior has often been observed in animals in experiments. If a conspecific receives a better reward for the same performance, the experimental animal often foregoes its own reward “in protest”. But does that really mean that animals can feel unfairly treated compared to their peers?
Unsatisfactory leverage tricks
To find out, researchers led by Rowan Titchener from the German Primate Center in Göttingen conducted various behavioral experiments with cynomolgus monkeys. The basic principle: the animals had to pull a lever and were rewarded with inferior feed. High-quality rewards were also visible, but they were out of reach for the monkeys. The experimental setup could be varied in two places. The inferior food either came from a vending machine or was given to the monkeys by a human trainer. In addition, the animals were either alone or within sight of a partner, who was rewarded with high-quality food for the lever trick.
Titchener’s team observed how the monkeys reacted to the situation — whether they refused the inferior food when they saw their peers getting better rewards. In this way, the researchers were able to test three common hypotheses that are supposed to explain the animal’s apparent sense of justice in previous experiments. The first is that animals refuse the inferior food because they feel unfairly treated in a human sense and protest. The second assumes that the animals expected better food and therefore reject the worse. The third hypothesis is based on social disappointment. The animals feel “cheated” by their trainer because they are used to better snacks from him.
Different sense of justice than ours
The result: when the inferior reward came from the machine, the cynomolgus monkeys almost never refused it. However, when a trainer handed them the food, they spurned it more than 20 percent of the time. From this, Titchener and her colleagues conclude that the monkeys were disappointed in their trainer because he made a conscious decision to give them an inferior reward. “The monkeys have no social expectations of a machine and are therefore not disappointed,” explains Titchener. According to the researchers, the behavior of the monkeys can best be explained by the third hypothesis, that of social disappointment.
According to the scientists, a certain degree of competition for food also plays a role. Because as soon as a conspecific was present, the monkeys grabbed the reward faster on average, even if it was objectively not the tastiest snack. The results of the study confirm that monkeys also have a sense of justice, but this apparently differs from the typical human sense of justice.
Source: Deutsches Primatenzentrum GmbH – Leibniz Institute for Primate Research; Specialist article: Royal Society Open Science, doi: 10.1098/rsos.221225