Monkeys are not as selfless as many think, according to a study from Yale’s Comparative Cognition Lab.

In an experiment aimed at figuring out monkeys’ willingness to share, researchers placed computer screens in front of their capuchin monkey study participants and had them complete a touchscreen task that led to food being delivered to other monkeys. Surprisingly, results showed that the capuchin monkeys, thought to be prosocial and cooperative from previous research, did not end up delivering food to a partner. The study was published in the journal Behavioral Sciences in April.

“First, we wanted to test the consistency of capuchins’ prosocial preferences, which few primate studies had done,” director of the lab and study co-author Laurie Santos said. “But second, my student Lindsey [Dreyton GRD ’17] hoped to develop a touch screen prosociality task that we could use to test other questions, like whether monkeys showed reciprocity and tracked others’ intentions during prosocial behaviors.”

This experiment is based on a common test, in which monkeys have the option of delivering food to a partner or not. Either way, the monkey still receives a food reward for himself or herself. Results have generally shown that monkeys are prosocial, meaning they are willing to give to others even when it does not benefit them.

However, across different species and situations, there have been some mixed results, with some monkeys responding prosocially in some situations but not in others. Specifically, Dreyton and Santos’s experiment showed that the capuchin monkeys did not significantly prefer the prosocial action of giving food over the selfish action of not sharing food.

“This study shows that many of the phenomenon we aim to study may depend in large part on the paradigms we develop to probe these phenomena,” Co-Director of the Duke Canine Cognition Center Evan MacLean said. “Animals doing something in a particular context doesn’t necessarily allow us to make generalized comments about the nature of the species.”

The puzzling results of this study show that primates’ prosocial preferences are not always consistent from one situation to the next, Santos said. In fact, there is significant variation among individual monkeys of the same species, a phenomenon that merits further research, she added.

Based on the touchscreen aspect of the study, in which capuchins used a computer to receive or give food, psychology and neurobiology professor Steve Chang noted that animal cognition can also depend on the animal’s understanding of the task at hand.

According to Chang, the study’s results show the scientific community that, in order to come to a broader understanding of primate behavior, the animals have to be studied in varying contexts, performing different tasks.

It is possible that there may be more individual variability in prosociality between individual monkeys of the same species than previously thought, Santos said. She added that she is also interested in looking at whether the contexts that influence humans’ prosocial behavior affect primate behavior, too.

“Ultimately, the best science draws on convergent lines of evidence that allow us to look at a question from multiple angles,” MacLean stated. Answering those questions, he added, will require further research.

Capuchin monkeys generally live in groups of 10 to 40 members.