Food-washing monkeys

Monkeys washing their food have provided scientists with an opportunity to understand how animals weigh up the cost of a clean meal against their place in the social pecking order. Taking time out to rinse sandy food ultimately limits how much food you end up eating. But forego the wash in favour of a greater energy intake to preserve your place in the social hierarchy, and you end up with sand-damaged teeth that could shorten your life. It’s a price some “top dogs” are clearly willing to pay for the reproductive opportunities that it opens up. Nathaniel Dominy, who did the work, is at Dartmouth College…

Nathaniel - Shortly after the tsunami of December 2004, researchers were moving along the coast of Thailand and Burma on boats, doing biodiversity damage assessments. They were caught by surprise when they noticed monkeys on the beach using stone tools. This was in Thailand. Primarily, they were using the tools to harvest shellfish—oysters, mussels, that sort of thing. These primates eventually became quite famous, and tourists began to visit to observe them. As tourists often do, they wanted a closer look, so they started throwing fruit onto the beach. This began happening regularly, and the monkeys got used to it. They could hear the boats coming and began to expect food from the tourists. The problem was that the food landed in the sand and became very sandy.

The monkeys quickly figured out that they could take the food to the ocean, dunk it, and remove the sand. That was exciting, because it recalled a very famous moment in primatology where something similar happened in Japan. Those food-washing behaviours inspired us to conduct an experiment where we gave the monkeys food designed to trigger washing, to see what would happen.

Chris - What might be the reason why an animal wants to wash food? I know why I would—sand tastes awful, and the sensation on your teeth is horrible. Do you think the monkeys are thinking along the same lines?

Nathaniel - It stands to reason, right? It’s a relatable problem. Anyone who’s had a picnic at the beach knows that sand in your mouth is objectionable. Back in 1952, when monkeys were first observed washing sweet potatoes that researchers had given them, the assumption was that they didn’t like the feeling of sand in their mouths. That was thought to be the motivation behind washing their food. That idea has persisted ever since, but no one had actually tested it.

Chris - So how did you test it?

Nathaniel - We had three trays and gave the monkeys cucumber slices, which have a sticky surface that’s good at picking up sand. We offered three options: very sandy cucumber, non-sandy cucumber, and something in between. If the monkeys cared about sand, we expected them to spend more time cleaning the sandier pieces. And that’s exactly what we found. When the cucumber slice had a lot of sand, they spent significantly more time trying to remove it.

Chris - Thing is, whenever there’s an action, there’s a consequence. And in this case, there’s a delay before they get their reward. Depending on how sandy the food is and how far away the water is, there’s an energy cost too. So what are the impacts of that? Is there a trade-off? Do they weigh up how far they’ll have to go and how much sand they’ll tolerate? Or did they always wash the food regardless?

Nathaniel - You raise a great point. What we found is that some animals were making shrewd calculations. For some, it just wasn’t worth the time and energy to walk to the ocean. Because the cucumbers are in their hands, they have to rise onto their hind legs and walk bipedally—like a human. But their muscles and bones aren’t well-suited to walking upright, so it’s very energy-intensive. There’s also an opportunity cost: if their hands are full and they’re walking to wash the cucumber, they’re missing out on eating other slices. So some animals chose to tolerate a bit of sand so they could eat more quickly. Others were more averse to the sand and were willing to pay the price of washing properly.

Interestingly, these decisions varied depending on the monkeys’ social rank. Dominant animals, for instance, have higher energy needs. They need to fight off rivals, stay alert, and expend a lot of energy asserting dominance. They have to eat a lot and eat quickly to meet those energy demands. We found that it was these dominant animals who couldn’t be bothered to wash the food properly. Walking to the water just took too long. Instead, they would brush the cucumber a few times with their hands and eat it—sand and all. And we calculated that a lot of sand was going into their mouths. That was probably ruining their teeth.

Sand is highly abrasive. It can chip teeth and cause serious wear. So we think they were paying a long-term price by shortening their lives—just to eat faster in the short term.

Chris - How does this fit with our theories about how hierarchy affects behaviour? It sounds like this supports the idea that animals—and perhaps people—make trade-offs: choosing short-term gains, even if they come with long-term health costs, because they improve reproductive chances.

Nathaniel - That’s exactly right. There’s an idea in evolutionary biology called the "disposable soma hypothesis." It suggests that some tissues in the body are expendable—and teeth might be one of them. A famous study from the UK looked at red deer and found that male teeth wear out quickly. Their strategy seems to be: get energy now, find mates as quickly as possible, and don’t worry about long-term survival. The benefits of reproducing early outweigh the benefits of a longer life. We think these monkeys were doing the same—making a strategic decision. They knew the sand was bad for their teeth, but they were willing to pay that price for more mating opportunities in the short term. It’s a “live fast, die young” kind of strategy.