FunNature & AnimalThis is how monkeys learned to wash food

This is how monkeys learned to wash food

On the Japanese island of Koshima , it was common for visitors to feed the macaques that live there. Handing food to such an animal does not inspire much confidence, so most people just throw it on the ground for the primates to pick up. And among the most common foods that they were given, was the sweet potato, a cheap product that the macaques appreciate a lot. It was September 1953 when Imo , a young macaca, tired of having her food full of sand, picked up her sweet potato from the ground and approached the stream. He held the sweet potato with one hand under the water and rubbed it with the other hand to clean it well before eating it.

The rest of the macaques were not very clear about what Imo was doing with their food. After all, they had been eating dirty sweet potatoes all their lives, and Imo was just a one and a half year old. But, perhaps by imitation, or perhaps because the young macaca communicated the benefits of her discovery, the rest of the herd soon acquired the habit . In five years, 17 monkeys in the herd already repeated the process, being much more common for other cubs to learn it. In fact, only two were adults when they got it. In 1962, nine years after the first sweet potato washing, all the monkeys in the herd performed the ritual , with the exception of a minority of adults, those older than 12 years. Once the technique was learned, the herd began to improve it, innovating: if instead of washing it in the stream, they did it in the sea, the taste of the sweet potato was more pleasant . Today that herd of macaques continues to wash their sweet potatoes in the sea.

This event is considered to be the first observed case of precultural behavior in a non-human primate . And he is not the only one, because not everyone threw sweet potatoes at them. Some of them threw grain, highly valued by the macaques, but the problem was still clear: it was always full of earth. The solution of washing the food in the sea was spread, applying it to those grains of wheat as well. Clean food and to the point of salt.

It was Imo herself who invented grain washing, in 1956, when she was four years old . As with the sweet potatoes, at first she was the only one to do it. But over time, the other members of the pack, especially the younger ones, began to imitate her. In 6 years, almost 20 monkeys in the herd repeated the ritual of washing wheat.

There were some monkeys that, instead of learning to wash the grain, acquired a new habit: stealing the grains that other macaques threw into the water before they could retrieve them . The first to display this stealing behavior was an adult macaque named Eba . And, as in the previous case, it was an innovation that others learned later; even some who had already learned to wash those grains.

The question, however, lies in a key question: is this type of behavior a true precultural innovation, or are they nothing more than latent solutions inherent to the species? Namely. Are macaques born with the instinctive tendency to wash their food, or is it really something they develop intellectually to solve a problem?

To answer these types of questions, it is best to test other macaques who have not had contact with the Koshima herd . This is what a group of researchers from the Lincoln Park Zoo in Chicago did in a study published in 2020 in the journal Folia Primatologica . They used macaques from the zoo that, having been in captivity, did not have the preculture associated with wild macaques. The researchers provided the primates with a pool filled with water, and gave them dirty sweet potatoes . It turned out that none of them washed the food in the water; however, some did clean it in other ways. They invented, in total, three ways to remove the earth; shaking it with your hand, rubbing it against your body, or against another piece of food .

With this they concluded that the act of cleaning food must be something innate , that it is part of the latent instinctive solutions of the species, but that, however, the specific cleaning technique is part of intellectual innovation .

Since those events, the Koshima pack has been learning and acquiring new skills. Among them, the handling of stones as a tool , or certain techniques to improve cleanliness.

Therefore, it is not that Imo , that young Japanese macaque, invented that thing about washing food; The act of cleaning food from dirt was instinctive for her, but she innovated, inventing the technique that would allow her to do it in the most appropriate way and maximizing profit . And he transmitted those teachings to the rest of his community, in a pre-cultural way.


Fiore, A. M., Cronin, K. A., et al. 2020. Food Cleaning by Japanese Macaques: Innate, Innovative or Cultural? Folia Primatologica; International Journal of Primatology, 91(4), 433-444. DOI: 10.1159/000506127
Hirata, S., Watanabe, K., et al. 2008. “Sweet-Potato Washing” Revisited. En T. Matsuzawa (Ed.), Primate Origins of Human Cognition and Behavior (pp. 487-508). Springer Japan. DOI: 10.1007/978-4-431-09423-4_24
Kawai, M. 1965. Newly-acquired pre-cultural behavior of the natural troop of Japanese monkeys on Koshima islet. Primates, 6(1), 1-30. DOI: 10.1007/BF01794457

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