Chimpanzees are regarded as more intelligent than marmosets. Yet, like humans, it is marmosets that will often come to the aid of their fellow group members, even unprompted. This willingness to help is derived from cooperative breeding – and it explains the unique abilities of the human brain, as suggested by a study funded by the Swiss National Science Foundation involving comparative tests on monkeys and nursery children.

For several decades many characteristics originally classed as being specific to humans have been seen in a new light. This exclusive interpretation has given way to the view that our ability to plan and remember does not differentiate us from other great apes. In fact, the opposite is true. These cognitive abilities, along with our use of tools, link us to our closest biological relatives. And yet there is a substantial difference to which reference is frequently made when it comes to explaining the unique nature of humans’ cognitive and cultural skills.

Spontaneous assistance

This difference relates to behaviour. Chimpanzees only rarely come to the assistance of other chimpanzees without being prompted. Additionally, they do not spontaneously share their food with other chimpanzees. This “pro-social” disposition is demonstrated by children – and common marmosets – much more frequently, as reported in the journal Nature Communications by Judith Burkart and her colleagues from the Anthropological Institute and Museum at the University of Zurich.

The scientists used a standardised test carried out with fifteen different primate species (including humans) to compare the extent to which the members of a group cooperate with one another. To do this, the researchers laid out food on a board outside the monkeys’ cage.

Food placed out of reach

One side of the board had a lever that could be used to pull the board towards the cage. The food was located on the other side of the board and could be reached by a member of the group whenever another member operated the lever (but as soon as the lever was released, a coil spring caused the board to return to its starting position away from the cage). The board was too long to enable one individual member to operate the lever and reach the food at the same time. However, one individual could do the other members of the group a service by bringing the food into reach.

Whilst the investigated groups of macaques and chimpanzees almost never retrieved the food from the board, the marmosets and the nursery children (who were playing with the board in a see-through cabinet rather than from behind the bars of a cage) almost always managed to get to the food. Given that the willingness to help is not linked to brain size, this has nothing to do with intelligence. “Marmosets have small brains and are not particularly bright,” explains Burkart. They perform less well than chimpanzees in memory tests or in recognising causal relationships, but are at times superior to chimpanzees in terms of their social learning. The mother marmosets will for example demonstrate advantageous behaviour to their young.

Potential of human brain unfolded

A key factor in the level of willingness to help is the amount of care that a child has received from group members other than the mother. According to Burkart, the marmosets appear to be as willing to help with the raising of offspring, as they are willing to share their food or information. In the case of humans, it takes a proverbial village to raise a child. Consequently, the abilities of the human brain exceed the individualistic cognitive skills of a chimpanzee’s brain. “It is only through cooperation that the potential of the human brain has flourished,” sums up Burkart.