Food Cleaning by Japanese Macaques: Innate, Innovative or Cultural?

Japanese macaques (Macaca fuscata) display a number of cultural behaviours including food washing, stone handling and certain grooming techniques. These are deemed cultural behaviours because it is presumed that they are socially learned and, importantly, that social learning is essential for their emergence. Recently, however, research has revealed that culturally naïve primates can re-innovate presumed cultural behaviours. These behaviours are said to fall within that species' "zone of latent solutions" (ZLS). A notable cultural behaviour of Japanese macaques is food washing, first reported by Japanese researchers studying wild Japanese macaques in the 1950s. To test whether culturally naïve Japanese macaques would spontaneously wash food and also, therefore, whether food-washing behaviour is within their ZLS, we presented 12 zoo-housed macaques with sweet potato covered in sand near a pool in their exhibit. Over 11 days we recorded the macaques' behaviour. While 11 of the 12 macaques ate the potato pieces, none washed them. However, 4 macaques cleaned their food, brushing off the sand using their hand or rubbing the potato against their body or another food item, using three distinct techniques. We found no change over time in the rate at which monkeys cleaned or consumed potato, but there was a significant positive correlation between the number of potato pieces a monkey ate and the number of cleaning behaviours performed. We conclude that, minimally, food-cleaning behaviour is within macaques' ZLS.