These Monkeys Learned To Eat Dirt Because Tourists Keep Feeding Them Junk Food

On the rocky slopes of Gibraltar, a troop of Barbary macaques lingers near a popular tourist overlook. An adult female pauses mid-stride, crouches beside a patch of rust-red earth poking through the concrete road, and begins pinching off small fragments with surprising precision before popping them into her mouth. She chews, swallows, and moves on. It looks bizarre. But according to a new study, the dirt may help her body cope with the chocolate bars, ice cream cones, and potato chips that visitors keep slipping her despite rules against it.

Published in Scientific Reports, this is the first study to formally document and analyze deliberate dirt-eating in Gibraltar’s population of roughly 230 wild monkeys. And the behavior isn’t some quirky one-off. It happens at rates among the highest reported in any primate species, and the data strongly link it to human tourists. When more visitors show up and more junk food gets consumed, more monkeys eat dirt. Researchers describe the behavior as a locally maintained tradition that appears to have been shaped by human contact.

How Researchers Tracked Dirt-Eating Monkeys in Gibraltar

Between August 2022 and April 2024, across five field seasons spanning 98 observation days and more than 612 hours of watching, researchers recorded 46 separate dirt-eating events performed by 44 individual monkeys. They combined detailed tracking of individual animals, broader group surveys, and video recordings (30 of the 46 events were captured on film) to confirm that what they were seeing was genuinely intentional soil consumption.

Close inspection of each site turned up no seeds, insects, or eggs that might explain the behavior as disguised foraging. Video analysis backed this up: the monkeys used precise, goal-directed hand movements to pick up tiny fragments of earth, which looked nothing like their usual ground-feeding postures of sweeping and digging. On average, each dirt-eating session lasted about 21 seconds and involved roughly four to five separate intakes, with some monkeys consuming pieces ranging from a few millimeters to several centimeters across.

About 83 percent of events involved red soil, a type of calcium-rich clay common across Mediterranean landscapes. Smaller numbers involved yellow soil, black earth, and even tar scraped from holes in asphalt roads. Most dirt-eating happened among groups living on the upper and central portions of the Rock, where both red soil outcrops and tourist traffic are most concentrated.

Why Tourists’ Junk Food May Be Driving Monkeys to Eat Dirt

To figure out what was driving the behavior, the research team tested two main ideas. One proposes that dirt-eating helps buffer the digestive system against harmful or irritating substances in food. Another suggests animals eat dirt to obtain minerals missing from their regular diet. Both explanations have support in other species, and they aren’t mutually exclusive.

Results pointed most strongly toward the digestive protection idea. Statistical analysis found that monkeys were significantly more likely to eat dirt when they had consumed greater amounts of tourist-provided food: biscuits, dairy ice cream, bread, crisps, and chocolate. In three specific cases captured during detailed individual tracking, dirt-eating directly followed consumption of such items, occurring as little as six or seven minutes after a monkey ate bread or ice cream.

Tourist numbers themselves drove how much junk food the monkeys ate. When more visitors crowded around, monkeys ate more human food. This effect was strongest in summer, when tourist numbers peak. Gibraltar’s Upper Rock Nature Reserve logged more than 859,000 visitors in 2024 alone, and dirt-eating was also more common in summer than winter.

Tourist-provided foods tend to be high in sugar, salt, fat, and dairy but low in fiber, essentially the opposite of what a wild primate’s digestive system typically handles. Because primates generally lose the ability to digest dairy sugars after infancy, dairy products in particular can cause stomach distress. One possibility the researchers put forward is that these foods disrupt the community of helpful bacteria living in the monkeys’ guts, producing discomfort that the animals then try to relieve by eating soil. Certain types of clay are known to absorb irritating substances, change stomach acidity, and even influence gut bacteria, properties that could explain why the monkeys seem to seek it out after eating human food.

A mineral-supplement explanation couldn’t be entirely ruled out. Gibraltar’s macaques eat very few insects compared to wild populations in North Africa, where bugs are a regular part of the diet. Some groups in Gibraltar also receive limited variety in their official food provisions, mostly seeds and nuts without much fresh fruit or vegetables. Soil-eating could partly make up for missing nutrients. But the lack of any clear link between dirt-eating and pregnancy or nursing, periods when mineral demands spike, weakened this explanation. Females across all reproductive states ate dirt at similar rates.

A Social Habit That May Spread From Monkey to Monkey

One of the study’s more intriguing findings is that dirt-eating in Gibraltar appears to be socially learned, likely passed from animal to animal through observation, much like cultural traditions in humans. Several lines of evidence support this idea.

About 89 percent of dirt-eating events happened with other monkeys nearby, within about 20 meters. While most individual sessions involved a single animal at a particular soil patch, nearly a third involved multiple monkeys feeding at the same spot or taking turns. Researchers captured video of a juvenile watching an adult female eat red soil, the kind of observational setting that could allow the behavior to spread through a group, though the team notes that confirming social transmission would require longer-term tracking.

Different groups also showed distinct preferences for specific soil types. To test this, the team ran 124 experiments in which individual monkeys were presented with trays containing all four locally available soil types at the same time. Red soil was the most popular first choice across the board. But one group called Apes Den, which lives lower on the Rock’s western slope where red soil is scarce, showed a distinctive preference for eating tar instead. That group accounted for 70 percent of all tar ingestion during the experiments, a statistically significant difference from other groups.

And then there was the control group of sorts. Middle Hill, the only group with no tourist contact whatsoever, was not observed eating dirt during the study period, though the team did log fewer hours with this group than with the tourist-exposed ones. Researchers who studied Middle Hill years earlier, when it did have regular contact with military personnel and tourists, reported that the monkeys did eat dirt back then. The behavior, it seems, may fade when the conditions that trigger it, namely access to human food, disappear.

How Gibraltar Compares to Other Barbary Macaque Populations

To put their findings in broader context, the research team surveyed scientists working with Barbary macaques at 26 other sites worldwide, including wild populations in Morocco and Algeria and groups kept in semi-captive settings in Germany, France, and England. Dirt-eating was reported at 9 of those 26 sites, but it was typically described as occasional, seasonal, or limited to younger animals. Gibraltar stood out for the sheer frequency of the behavior, its occurrence across all age and sex classes, and its apparent connection to tourist-driven diets.

At the population level, the Gibraltar macaques averaged more than 12 dirt-eating events per week, rates comparable to the highest reported in any primate species. Only a population of provisioned hybrid macaques in Hong Kong showed higher reported rates, at roughly 34 events per week.

“Foods brought by tourists and eaten by Gibraltar’s macaques are extremely rich in calories, sugar, salt and dairy,” explains lead author Dr. Sylvain Lemoine, a biological anthropologist from the University of Cambridge’s Department of Archaeology. “This is completely unlike the foods typically consumed by the species, such as herbs, leaves, seeds and the occasional insect.”

Despite rules prohibiting tourists from feeding or touching the macaques, such interactions remain common. Each candy bar or ice cream cone handed over contributes to a cycle of dietary disruption and compensatory behavior that reshapes how an entire population of primates lives. Gibraltar’s macaques haven’t simply adapted to the tourist economy. They’ve been changed by it.

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