Tuesday, January 15, 2013

Milk of Human Kindness Also Found in Bonobos (New York Times)

From the New York Times, this is a cool article on one of our primate cousins, one the more social and generous of the primate species (perhaps we can learn a bit from them). The findings are interesting - bonobos will gladly share food with strangers, but only if the stranger provides social interaction. That does not seem too far removed from the days when a traveler could a hot meal and a blanket in exchange for a song or two, or some stories.

The article is open access from the PLoS ONE system, link with abstract and introduction is below.

Milk of Human Kindness Also Found in Bonobos

Jingzhi Tan - Two bonobos shared food and affection at the Lola ya Bonobo sanctuary.

Published: January 7, 2013

Bonobos will happily share their food with a stranger, and even give up their own meal — but only if the stranger offers them social interaction, evolutionary anthropologists at Duke University report in the journal PLoS One. The researchers, Jingzhi Tan and Brian Hare, say their findings may shed light on the origins of altruism in humans.

Along with chimpanzees, bonobos are among the closest primates to humans. Chimpanzees, however, do not display similar behavior toward strangers.

“If you only studied chimps you would think that humans evolved this trait of sharing with strangers later,” Mr. Tan said. “But now, given that bonobos do this, one scenario is that the common ancestor of chimps, humans and bonobos had this trait.”

The subjects were all orphaned bonobos at the Lola ya Bonobo sanctuary in the Democratic Republic of Congo. In one phase of the study, bonobos were given a pile of food, then given the opportunity to release a stranger or a group mate (or both) from other rooms.

The bonobos chose to release strangers and share their food. Not only that, but the just-released bonobo would then release the third.

“This was shocking to us because chimpanzees are so xenophobic,” Mr. Tan said. “They won’t approach a stranger unless they outnumber them.”

The apes did have a limit — they would not share their own food when no social interaction was involved.

They were, however, willing to help a stranger get food even without social interaction. Mr. Tan compared this to certain human acts of kindness.

“It’s like when you donate money and you don’t tell people,” he said, “so there’s no way for you to get any benefit.”

A version of this article appeared in print on January 8, 2013, on page D3 of the New York edition with the headline: Milk of Human Kindness Also Found in Bonobos.

Bonobo parents with infant

Here is the abstract and introduction from the whole article. The whole text is available by following the link below.

Bonobos Share with Strangers

Jingzhi Tan1,* and Brian Hare1,2


Humans are thought to possess a unique proclivity to share with others – including strangers. This puzzling phenomenon has led many to suggest that sharing with strangers originates from human-unique language, social norms, warfare and/or cooperative breeding. However, bonobos, our closest living relative, are highly tolerant and, in the wild, are capable of having affiliative interactions with strangers. In four experiments, we therefore examined whether bonobos will voluntarily donate food to strangers. We show that bonobos will forego their own food for the benefit of interacting with a stranger. Their prosociality is in part driven by unselfish motivation, because bonobos will even help strangers acquire out-of-reach food when no desirable social interaction is possible. However, this prosociality has its limitations because bonobos will not donate food in their possession when a social interaction is not possible. These results indicate that other-regarding preferences toward strangers are not uniquely human. Moreover, language, social norms, warfare and cooperative breeding are unnecessary for the evolution of xenophilic sharing. Instead, we propose that prosociality toward strangers initially evolves due to selection for social tolerance, allowing the expansion of individual social networks. Human social norms and language may subsequently extend this ape-like social preference to the most costly contexts.


One of the most puzzling human behaviors from an evolutionary perspective is our species' propensity to share with non-relatives and even strangers [1], [2]. Across numerous cultures and early in development, humans engage in spontaneous helping and costly sharing with strangers [3], [4]. Some have suggested this human form of sharing is inconsistent with the predictions of kinship theory and reciprocal altruism (see [1], but see [5]) while others have proposed our species has evolved unique motivation and cognition for sharing [6][9].

Nonhuman primates are known to help and voluntarily share food with other groupmates (e.g.[10][16]). This prosociality, or voluntary behavior that benefits others [17][21], can be driven by selfish or other-regarding motivations [17], [22]. Therefore, while a primate can be prosocial even if pursuing selfish goals, they only demonstrate other-regarding forms of prosociality if their actions do not result in immediate selfish benefit (see SI for disambiguation of prosocial, other-regarding and altruistic behaviors). A number of experiments have now shown that a variety of primates will even help another individual obtain food when there is no immediate, tangible reward for their help (chimpanzees: [4], [23][27]; old world monkeys: [28]; new world monkeys: [29][31]). This type of prosociality suggests in some contexts primates also have other-regarding motivations (but see critique of this interpretation by [9]). However, there remains little evidence that nonhuman primates show any form of prosociality toward non-group members [7], [9], [13], [31], [32]. Primates typically compete against non-group members, resulting in agonistic intergroup relations [33]. This hostility goes to the extreme in chimpanzees that opportunistically kill neighbors [34], [35] and sometimes even immigrants[36][38]. Therefore, it is unlikely that most primates have tolerance levels that would allow for prosocial or other-regarding tendencies toward strangers. Moreover, designing such an experiment for most primate species would be extremely difficult given the high potential for stress, injury and aggression.

Bonobos are known for relatively high-levels of tolerance within and between groups when compared to chimpanzees [34], [39][43]. In the wild, bonobos have even been observed to have affiliative intergroup interactions. For example, females from neighboring communities have been seen traveling together for days, feeding in the same trees and even participating in socio-sexual behavior ([39], [40], also see [44]). In a preliminary experiment seven bonobos were given the opportunity to voluntarily share with another bonobo [12]. All three bonobos paired with a non-groupmate voluntarily shared their food while only one of the four bonobos paired with an in-group shared. No aggression of any form was ever observed. This suggests that with the relative tolerance of bonobos they can afford such prosociality with strangers. In turn, sharing with a stranger might aid them in extending their social network and in forming new “friendships” [5], [45]. However, it remains unclear whether the observed prosociality represents a preference to share with strangers over groupmates. In addition, it is unclear if the voluntary sharing observed only represents a selfish tactic to obtain a novel social interaction or whether bonobos will also share with strangers if there is no immediate, tangible reward. Therefore, we conducted four experiments with 15 wild-born bonobos that are orphans of the bushmeat trade living at Lola Ya Bonobo Sanctuary in Kinshasa, Democratic Republic of Congo [46]. We designed these experiments based on the relative costs and benefits of the prosocial behavior to the actor and this serial design allowed us to identify whether the prosocial motivation is selfish or other-regarding (Table 1). In experiment 1 and 2 we presented bonobos with a task in which they could choose whether to share food and physically interact with either a groupmate or stranger. In experiment 3 and 4 we presented bonobos with a second task in which they could either ignore or help another bonobo in obtaining out-of-reach food. In this second task helping allowed no immediate benefit to the actor (e.g. physical interactions) and the cost of helping was altered between experiment 3 and 4 (see Table 1).

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