Monday, January 11, 2016

Find yourself being spiteful? Blame these primate relatives.

It looks like humans aren't the only ones who make an effort to punish or act negatively towards those we believe are undeserving. Leimgruber and colleagues (2015) recently published a new study on inequality and fairness in the journal Evolution and Human Behavior. They looked at whether capuchins (Cebus apella) would punish individuals who stole food or who benefited from an unequal distribution of food. The results and how they compare to other research might surprise you.

Capuchins are a New World Monkey that are closely related to us. Humans and capuchins last shared a common ancestor roughly 30 million years ago (Fragaszy et al., 2004). These social primates live in large groups with dominance hierarchies. To learn more about this species, including their behavior, click here.

C. apella Photo: John Mittermeier
As you can probably guess, capuchins were tested in captivity to determine how they responded when a reward was not equally given and when a resource was stolen from them. Six individuals were tested. All individuals were tested against a low-ranking member of their social unit. Monkeys were tested in different enclosures but with a shared table that spanned both enclosures. Multiple conditions were tests: one where the test individual had access to the food, one where the test individual briefly had access to the food and then a researcher moved it to the low-ranking individual, and one where the low-ranking individual could steal the food outright. When the low-ranking individual gains control over a food source (due to the researcher moving the food source towards the individual), capuchins will pull on a rope that collapses the table holding the food. They will also do this when the low-ranking individual is given the ability to steal the food.

Intentions don't seem to matter if you're a scorned capuchin. Capuchins punish the low-ranking individual even when that individual was not to blame for the unequal distribution of food. As Leimgruber stated when interviewed for, capuchin monkeys appear to have a sort of "If I can't have it, no one can"attitude.

Similar inequality studies have been conducted with chimpanzees, our closest living relatives. When we think of chimpanzees in comparison to capuchins, chimpanzees may seem more ruthless. They will kill one another and they've been observed going to "war" with other troops of chimpanzees. Yet, chimpanzees will only make an effort to punish if the other individual is responsible for the inequality (Riedl et al., 2012). Perhaps capuchins are the anomaly or maybe this spiteful behavior does have its evolutionary origins 30 million years ago, making chimpanzees the anomaly. Similar studies will need to be done in other primate species to better understand the evolutionary history of this trait in primates.

Of course, we can't actually blame capuchins for our spiteful actions (nor should we: let's not forget the power of human agency), but this study adds to our knowledge of human behavior while, perhaps, raising more questions.

Links of potential interest: article
Video of capuchin fairness test
Frans de Waal Ted Talk on Moral Behavior

Works cited:

Fragaszy, D. M., Visalberghi, E., & Fedigan, L. M. (2004). The complete capuchin: the biology of the genus Cebus. Cambridge University Press.

Leimgruber, K. L., Rosati, A. G., & Santos, L. R. (2015). Capuchin monkeys punish those who have more. Evolution and Human Behavior.

Riedl, K., Jensen, K., Call, J., & Tomasello, M. (2012). No third-party punishment in chimpanzees. Proceedings of the National Academy of Sciences, 109(37), 14824-14829.