"The bold individuals approached these ‘predators' quickly and aggressively, while the shy individuals stayed back," she said.
Earlier research found that humans also tend to select buddies who have similar levels of friendliness and assertiveness. Traits like neuroticism, which refers to the general tendency to respond with stress and anxiety to situations, do not lead to bonding, so a more neurotic human or chimp is not likely to connect with a similarly neurotic individual.
There could be a genetic component to friendship, given that primates appear to be born more or less social and bold, with those qualities reinforced -- or not -- via environment, upbringing and experiences.
Anthropologist Joan Silk of the University of California at Los Angeles has also studied social interactions among primates. She said the new paper presents "very interesting results."
"In our own work on baboons, we have found that certain pairs of females form social bonds that are characterized by high levels of friendly interactions and well-balanced grooming relationships; high ratios of friendly to aggressive interactions; and higher levels of support in agonistic conflicts," Silk told Discovery News.
"Pairs that form the strongest relationships also have the most enduring relationships over time," she continued, adding that baboons with the closest, longest-lasting friendly interactions display lowered stress levels.
Since all of this happens among baboons, a basic form of bonding might therefore go beyond the common ancestor of chimps and humans, possibly to as far back as 36 million years ago when the earliest socially living primates emerged.
The researchers are also quick to add that friendships are not unique to primates.
"Durable friendships have, for example, been described for elephants, horses and dolphins," Massen said, "and also in birds, like the common raven."