Like humans, chimpanzees are highly social animals, relying on group cooperation in order to survive. Furthermore, as the Internet so often reminds us, their ability to socialize and make friends knows no species, with the likes of this bulldog, this lynx and Michael Jackson all having succumbed to the famous chimp charm. Exactly what drives chimpanzees’ socializing tendencies, however, remains something of a mystery, although a new study suggests that trust may play a significant role in defining their interactions. As such, chimpanzee social bonds appear to closely resemble those of humans, and could represent the evolutionary roots of human friendships.

Detailing their research in the journal Current Biology, a team of researchers from the Max Planck Institute for Evolutionary Anthropology devised an experiment to test whether or not chimpanzees trusted their friends more than non-friends. To do so, they first spent time observing a group of chimps in order to identify pairs who appeared to have a close bond.

The animals were then presented with a modified version of a classic human trust game, whereby they were given the option to pull one of two ropes. One rope resulted in the chimpanzee receiving a small amount of fruit, while another chimpanzee on the other side of a screen received nothing. By pulling the other rope, however, the chimpanzee on the other side would gain access to a large amount of fruit, and would then have the opportunity to return the favor by pulling the same rope.

As such, the game essentially presented participants with a choice: By pulling the “no trust” rope, they guaranteed themselves a small reward. However, by pulling the “trust” rope, they placed their faith in the other chimp to deliver a larger reward, yet at the same time risked receiving no reward at all.

Results showed that the animals were “significantly more likely to voluntarily place resources at the disposal of a partner, and thus to choose a risky but potentially high-payoff option, when they interacted with a friend as compared to a non-friend.” Consequently, the researchers conclude that “chimpanzees, like humans, evolved robust forms of trust toward their close social partners.”

Interestingly, they also found that all chimps were equally likely to reciprocate when the “trust“ rope was pulled, with more than two-thirds upholding their end of the bargain by returning the favor – regardless of whether or not they had a relationship with the other chimp. In other words, even though participants were much more likely to show trust in their friends, these friends were no more likely to prove trustworthy than non-friends.

However, the study authors note that chimps were less concerned with a lack of reciprocation from friends than from non-friends, indicating that, like humans, they are willing to “tolerate inequalities” between themselves and their close companions. Because of this, the researchers hypothesize that interactions between unfamiliar chimpanzees are probably driven by strategic reciprocity – which explains why so many non-friend chimps returned the favors they received – while relations between closely acquainted chimps are more likely to be dependent upon “emotional trust.”