On January 17th, 2016 we learned about

Primate sharing, sociability and trust tested with treats

As my two-year-old reminds me on a regular basis, sharing is hard. It requires that you can somehow trust that the other party will reciprocate to some degree, returning the borrowed item or at least expressing gratitude about what was given to them. These concerns aren’t limited to two-year-olds of course, as plenty of adults, as well as various other primates, show that tension over sharing is hard wired into our brains.

Since even a human can’t always speak rationally about sharing or giving up resources, tests had to be designed where monkeys and chimps could demonstrate their attitudes about giving food to their peers. Two test subjects were separated from each other, with a special table between them. The table had two snack options, one less attractive, while the other was bigger and tastier. To eat the snacks, one subject could pull one rope and retrieve the lesser snack only for themselves, or they could use the other rope to give the better treat to their counterpart. The most beneficial outcome is the latter option, as there should be enough of the tastier food for both animals if the passive recipient chooses to share with the rope-puller.

If I can’t have it, no one can

Not every test left everyone happy, of course. Most primates in the study were prone punish their partners if they somehow stole food out of turn. Brown capuchin monkeys (Cebus apella) took this interest in vengeance a bit further, and would often collapse the test table if they thought their partner simply got more food then them. Before this, this kind of action out of spite was thought to be unique to humans. Aside from realizing that we’re not the only animal who can act like a jerk, it informs researchers about how deeply ingrained the concept of social expectations might be in the primate brain. So I guess my two-year-old is off the hook a bit.

From picking nits to breaking bread

On the flip side, chimpanzees demonstrated how our social relationships shape our comfort with sharing resources. Chimps who were partnered with animals they had prior, positive relationships with (ie, friends) were much more likely to choose the mutually beneficial rope option. Previous experiences grooming and eating together largely predicted if a chimp would trust that their partner wouldn’t hog all the treats, and they were generally correct in these assumptions.

While research with other primates is great for finding evolutionary roots of behaviors in humans, recognizing the goodwill of others may go further back the family tree than that. When given a similar option to choose rewards for their peers, rats seem to remember who has been generous and who has been stingy as well. It’s possible that sharing is just a feature of social species, rather than a special skill that’s exclusive to primates.

Source: Chimps Reveal Defining Element of Friendship by Jennifer Viegas, Discovery News

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