If you have seen a child just eat an entire ice-cream, and she begs you to buy her one, what will your reaction be?
Here we ask if monkeys understand the physical needs of others. In order to provide help or share food, it would be useful for them to know what others want or need. Such understanding is probably simpler than an understanding of the other's state of knowledge, known as Theory-of-Mind (ToM). Sensitivity to wants and needs appears earlier than ToM in the development of children. While there is some evidence for ToM in the great apes, such as chimpanzees, monkeys are rarely tested on intersubjectivity (empathy with and understanding of others) since the common assumption is that they are behavior-readers rather than mind-readers, i.e. that they merely react to another's behavior. They may not make any inferences about the internal states of others.
The present study, published in Behaviour, questions this assumption, however, since it shows that brown capuchin monkeys (a large-brained South-American primate) are less willing to share with another if they know that the other has just eaten. This condition was compared with one in which they saw the other not eat or in which the other's interactions with food were obscured from view. The specific withholding reaction of the monkeys suggests that they understood that a monkey who has just eaten does not need any more food. Alternative hypotheses were considered, but the most important one -- that they merely reacted to the behavior or food motivation of the other -- could be excluded thanks to a control condition in which the other had eaten out of view. Under this condition, they readily shared with them. The study provides the first tentative evidence for "empathetic perspective-taking" in primates.