Nancy has type 2 diabetes. Her treatment regime includes daily insulin injections and a low-sugar diet. Her urine is tested regularly to check for the presence of glucose – a sign of poorly-controlled diabetes. Nancy is also a black-capped capuchin monkey.

capuchin photo sigit
One of our capuchins. Photo by Sigit Rimba.

She had been well-controlled for years when we began to see glucose in her urine tests. From the keepers’ perspective, nothing else had changed. She still received her special diet while separated from the other monkeys, she was accepting the insulin injections without fuss, she seemed well.

We performed blood tests and a clinical exam. These confirmed that her diabetes was poorly controlled, but no other problems were found. We were about to adjust her insulin dosage, but then… the keepers saw the other capuchin monkeys passing her bits of banana and other sugary fruit through the mesh of the cage.

Had we had run up against the well-studied capuchin sense of fair play?

A research group in Atlanta has published several papers over the years testing social interactions between capuchin monkeys when they are given food. In a study published in 2000 that pretty much mimics the situation of Nancy and her neighbours, researchers separated two capuchins by a mesh screen and gave food to only one monkey. They found that the monkey with the food will often sit near his/her less fortunate neighbour and allow them to take some of the food. Sometimes food is actively handed through the mesh.

An image from de Waal's 2000 paper.
An image from de Waal’s 2000 paper.

Observational studies in wild capuchin groups also show extensive cooperation and sharing. It seems the social benefits of sharing outweigh the consequences of losing food, at least in capuchin society (similar studies in macaques have shown that in contrast, they’re selfish gits).

Capuchins that share food equally are better at cooperating to get food. In addition, if given the choice between a task that delivers food to themselves, and one that delivers food to themselves AND another monkey, capuchins will usually choose to feed the other monkey too. As you’d expect, this is influenced by how well they know the other animal.

Capuchins have also been shown to have some sense of fair play, as shown in this video from an experiment published in 2003. The experimenter asks two monkeys to do the same task, but then rewards them unequally. See what happens:

A capuchin monkey will happily perform a simple task for a cucumber reward – until s/he sees another monkey getting grapes for doing the same thing. Grapes are way better than cucumber, and the underpaid monkey reacts with frustration. But if you think logically, throwing the cucumber back at the researcher is daft, since then the monkey ends up with nothing.

This behaviour makes sense to us though, as we humans also angrily reject unequal pay for equal tasks. In similar situations, we’ll often make the same illogical choice to go without rather than accept a reward we think is unfair. The authors of the capuchin paper took this as evidence of an evolutionary, emotional basis for the concept of fairness.

Sadly for Nancy, a sugar-rich diet made up of friendly but illicit treats had led to her condition worsening. New, smaller-gage mesh was put between the cages to prevent food-sharing, and her glucose levels dropped to normal.

capuchin photo jamie
Cebus atella with kong puzzle toy. Photo by Jamie Robertson.

Homepage image by Amy Hughes. Thanks to Sigit, Jamie, Amy and others for offering capuchin photos! Sorry I didn’t have space for them all.

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