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Monkeys Keep Their Food Clean, Sort Of

By Elizabeth Preston
Nov 11, 2015 8:10 PMNov 20, 2019 4:13 AM


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We all have our standards. For humans, it's the five-second rule. For macaques, it's "think twice before eating food off a pile of poop." The monkeys have several ways of keeping their food (sort of) clean. And the most fastidious macaques, it seems, are rewarded with fewer parasites. On the Japanese island of Koshima, scientists have been studying Japanese macaques (Macaca fuscata) for nearly seven decades. The tiny, forested island is overrun with the monkeys, which live there naturally and sometimes move between the island and the nearby mainland. Back in the 1950s, researchers started feeding the island macaques treats of sweet potatoes and wheat, so they could study the animals more easily. In recent decades, researchers have cut back on the snacks as much as possible without hurting the population. Now they feed the macaques two or three times a week, at a dedicated sandy beach. Although the Koshima macaques run free, they're used to seeing humans around. Andrew MacIntosh of Kyoto University's Primate Research Institute calls the animals "extremely amenable" to experiments. He and graduate student Cécile Sarabian took advantage of this to learn about the monkeys' hygiene. Japanese macaques sometimes seem to clean their food before eating it—but is that really what they're doing? And does this habit keep them healthier? MacIntosh and Sarabian tested the monkeys' food hygiene in three ways. One experiment involved dirty sweet potatoes. In the 1950s, a young female monkey on the island started a trend by rinsing her sweet potatoes off before she ate them. The behavior famously spread through the other monkeys on the island. "This became one of the first examples of culture in nonhuman animals," MacIntosh says. For the new experiment, the researchers left sweet potato chunks at the snack site that they'd either rinsed clean, or wetted and then rolled in sand. In another experiment, the researchers left either grains of wheat or peanuts at the snack site. They placed some of these snacks directly on top of a fresh lump of monkey feces. In other cases, the snack sat on a plastic piece of poop, which looked convincing but didn't smell. As a control, they used a square of brownish plastic.

The real poop is on the left. Finally, the researchers watched macaques foraging for fallen acorns. They recorded how often each macaque rubbed an acorn between its hands or against its body before eating it. In all the experiments, monkeys cared about the cleanliness of their food—somewhat. They were least likely to eat a grain of wheat when it was sitting on a piece of poop. A little more than a third of them accepted this snack. They were somewhat more confident eating off the fake poop; more than half took these wheat grains. And all the monkeys took the wheat grains from the plain plastic control. When it came to sweet potato chunks, macaques were more likely to rinse off the potatoes that were covered in sand. This makes the researchers "fairly confident" that the macaques are trying to clean their potatoes, MacIntosh says. (Another hypothesis had been that the monkeys were salting their food.) With the acorns, as well as with the sweet potatoes and the wheat, the researchers saw a range of behaviors. Some macaques were fastidious. They were more likely to rinse their sandy potatoes, rub their acorns, and avoid poop-wheat. Others were less particular. Females tended to be more hygienic than males. When the researchers collected feces from their subjects and checked it for parasite eggs, they found that more fastidious macaques had fewer parasites. Counting eggs isn't a perfect way to measure parasite infection, MacIntosh stresses. But they saw a pretty dramatic difference among monkeys. The least hygienic animals had at least 7 times more parasites eggs in their feces than the most hygienic ones. Hygienic habits seem to help macaques stay healthier. However, any time the Koshima Island monkeys saw a peanut, they threw caution out the window. Every peanut got eaten—even if it was garnishing a heap of feces. MacIntosh points out that the sand at this site is "literally covered in feces" to begin with. When the macaques collect their snacks, they have to "get pretty intimate with the ground," he says. So they can't be too picky. But MacIntosh was still surprised to see the difference between how macaques treated wheat grains and peanuts. The monkeys may be weighing the riskiness of a food against its nutritional value. Maybe a low-calorie food is more tempting if it's clean, while calorie-dense peanuts are worth a little risk. "Classic behavioral economics," MacIntosh says. On the other hand, he adds, "They simply love peanuts."

Images: top by Alpsdake (via Wikimedia Commons); bottom, Sarabian & Macintosh (2015).

Sarabian C, & MacIntosh AJ (2015). Hygienic tendencies correlate with low geohelminth infection in free-ranging macaques. Biology letters, 11 (11) PMID: 26538539

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