Wild macaque washing sand from cucumber slices on Koram Island, Thailand. Image credit: Amanda Tan (CC BY 4.0)
While picnics at the beach sound fun, sand is notorious for sticking to food surfaces and nobody likes the feeling of grit on their teeth. Therefore, it is hardly surprising that monkeys living near beaches tend to clean sand from their food. While some briefly brush food with their hands, others wash food items in the ocean with care.
Although washing food in water might seem like the best way to eliminate sand, the most dominant monkey in a social group almost never does it. This difference in cleaning behavior raises the possibility that these monkeys make a shrewd calculation in their minds: is eating more quickly worth the risk of tooth damage from sand?
To explore this idea, Rosien et al. studied wild monkeys living on Koram Island in Thailand, which are known to wash their food in water. Measuring the properties of the sand that stuck to the food showed that 78% of it is made up of quartz – a mineral that is known to damage teeth. The researchers then studied how much time the monkeys spent washing or brushing slices of cucumber with different amounts of sand on them. This revealed that monkeys spent more time cleaning food with more sand on it, confirming the idea that they are averse to sand on their food and intentionally remove it.
The experiments also showed that monkeys tend to spend more time cleaning sand from their food than Rosien et al. had predicted to be necessary, indicating they prioritize this careful cleaning over efficient energy intake. However, this was not the case for the most dominant monkeys in the social group. They favored the quicker but less effective method of brushing food with their hands, suggesting that their main priority is immediate feeding, despite the long-term risk of tooth damage from sand.
The findings will be of interest to evolutionary biologists focused on the tradeoffs between foraging behavior and other vital needs, such as growth, reproduction, and ageing. The experiments also suggest that tooth wear can vary among individuals as a result of different cleaning behaviors, rather than just food types, which will be relevant for future studies by paleoanthropologists.
