Bonobos prefer bad guys

The quality of being helpful is a valued trait among humans, so much so that by the age of three months, infants have been shown to exhibit a strong preference for individuals whom they observe helping others. The same, however, cannot be said of bonobos, the great apes that are one of humans’ two closest relatives. 

Although bonobos (Pan paniscus) are demonstrably able to tell the difference between those who help and those who hinder, a study published this week in the journal Current Biology has found they show the opposite bias, consistently favouring hinderers over helpers.

The report says this early-emerging preference for helpful individuals could account for many uniquely human forms of co-operation. 

“It was striking and unexpected and suggested that these sorts of motivations may be really central to humans’ unusually co-operative nature,” says Christopher Krupenye of Duke University, US, who produced the new study along with lead author Brian Hare.

They wanted to find out whether the motivation to prefer helpers might be unique to humans. Because of the friendly reputation of the bonobos, it made sense to start by looking at them. 

“Bonobos exhibit a high level of social intelligence, tracking others’ social interactions and evaluating novel social partners based on these observations,” Krupenye says.  “However, what motivates social preferences may be fundamentally different in bonobos and humans.” 

The report says bonobos are generally regarded as the most socially tolerant ape, and as such provide a “powerful phylogenetic test of whether this trait is derived in humans”. 

Further, it says they exhibit “a suite of characteristics associated with greater sensitivity to others, and their sociality is hypothesised to have evolved due to selection against male aggression”. 

The reality, however, turned out to be more complicated.

“We were surprised, in that many have characterised bonobos as being the most co-operative, ‘hippie’ ape,” Krupenye says. 

“Our experiments show that the issue is much more nuanced. Bonobos are highly socially tolerant in food settings and help and co-operate with food in ways that we don’t see in chimpanzees. However, dominance still plays an important role in their lives.”

In four experiments, the researchers found that bonobos did not exhibit the human preference for helpers. Instead, they reliably favoured a hinderer that obstructed another’s goal. 

They also found that bonobos preferred a dominant individual over a subordinate, from which they suggest a preference for hinderers may reflect attraction to dominant individuals, while in humans a preference for helpers over hinderers supports the hypothesis that positive social preferences “played a central role in the evolution of human development and co-operation”.

“In primate society, it pays to be dominant and have dominant allies,” they write.

The researchers conducted their studies at Lola ya Bonobo, a sanctuary for orphaned bonobos in the Democratic Republic of Congo. Bonobos are said to be the most endangered great ape, with perhaps as few as 5000 remaining in the wild.

Twenty-four apes participated in the primary experiment, 12 each of males and females, completing both a test and a control condition. Each participant was shown two different animated videos on an iPad.

The videos depict a pair of two-dimensional shapes interacting. They were modelled closely after processes used in an important 2007 study titled Social evaluation by preverbal infants, by Yale University psychologists J. Kiley Hamlin, Karen Wynn and Paul Bloom, to test the social evaluation capacities of human infants. 

Each shape has two eyes and moves in ways that clearly denote specific goals. The shapes have eyes because previous experiments have shown that apes, especially bonobos, are sensitive to eye contact and direction. 

In the test, each video begins with a circle, representing a climber, trying but failing three times to ascend a steep hill. On the third attempt, the climber encounters another figure. In one of the videos, this second shape, a blue triangle, enters the scene as a helper and pushes the climber up to the top of the hill before returning down the hill and exiting. 

In the second video, a different shape, a red square, enters the scene from above, as a hinderer, and pushes the climber back down the hill before returning to the top of the hill. 

The test and control sessions each involved four trials in which the bonobos watched the helper and hinderer animations, as well as a series of control sequences, and then were allowed to choose between paper cutouts of the red squares or blue triangles placed on top of small pieces of apple. Reaching for one shape or the other was used as a measure of preference.

The bonobos exhibited a significant bias for selecting the hinderer. Only two individuals chose the helper on a majority of trials whereas 11 favoured the hinderer. 

This finding suggests that bonobos can discriminate between prosocial and antisocial agents, but, unlike humans, that they do not show a preference for prosocial ones. 

Separate analysis of adult and sub-adult subjects revealed that only adults showed a significant preference. Adults were defined as aged nine or older.

Because the youngest testable bonobos were already four years old, the researchers say they cannot be certain about the preferences of younger infants. 

“However,” they write, “we found no evidence that bonobos discriminate helpers from hinderers, or at least that they exhibit strong social preferences based on third-party interactions, until adulthood, in contrast to humans’ early emerging prosocial preference.”

In additional experiments, bonobos were given a choice to interact with unfamiliar humans they’d observed either helping or hindering. In every case, bonobos showed an ability to differentiate helpers and hinderers. They showed a preference for hinderers every time. 

Krupenye says he and Hare intend to continue to explore social preferences and social evaluation in bonobos, trying to understand what types of social information they track and what motivates their preferences. They also plan to conduct similar studies in chimpanzees.