Monkeys ‘self-medicate' to help them digest tourists' junk food

Gibraltar's monkeys "self-medicate" to help them digest tourists' junk food, reveals new research.

Troops of macaques living on the Mediterranean holiday hotspot have learned that swallowing soil can quell upset stomachs caused by munching chips, chocolate and ice cream.

The only free-ranging monkey population in Europe have been scientifically observed for the first time regularly engaging in "geophagy" - intentionally ingesting soil.

University of Cambridge researchers monitoring the roughly 230 monkeys on the Rock of Gibraltar tracked instances of geophagy.

The team also found that animals in frequent contact with tourists eat far more dirt, and that dirt-eating rates are highest during peak holiday season.

The researchers believe that sweet and salty snacks offered by or stolen from tourists - a major part of some Gibraltar macaques' diets - are disrupting their gut microbiome composition.

They say eating soil may help "rebalance" monkey stomachs by providing bacteria and minerals absent in junk food.

The team also suspect soil-eating is likely to help line the gut and soothe or prevent irritation caused by too much sugar and fat.

They believe the behavior is transmitted socially, as different troops have preferences for certain types of soil.

The team say it is an example of an emerging animal culture and "tradition" created by living in a human-dominated environment.

Study leader Sylvain Lemoine, a biological anthropologist at Cambridge, said: "Foods brought by tourists and eaten by Gibraltar's macaques are extremely rich in calories, sugar, salt and dairy.

"This is completely unlike the foods typically consumed by the species, such as herbs, leaves, seeds and the occasional insect.

"Humans evolved to seek out and store energy-dense fats and sugars to survive periods of scarcity, leading us to crave high-calorie junk food.

"Availability of human junk food could trigger this same evolutionary mechanism in macaques.

"Soil-eating may allow them to keep consuming food that has negative digestive effects, but is as delicious for them as it is for us.

"The emergence of this behavior in macaques is both a functional and cultural one, like nutcracking in chimps, except it is driven entirely by proximity to humans."

The findings, published in the journal Scientific Reports, showed that Gibraltar's macaque population averaged 12 soil-eating "events" a week.

Lemoine says that is at the "high end" of recorded geophagy frequency in primates, and comparable to chimps and lemurs, which eat dirt for minerals and to mitigate against plant toxins.

Geophagy is common in some human cultures, often associated with nutrient intake during pregnancy.

But the Cambridge team found no increase in macaque soil-eating during pregnancy or lactation, suggesting it is not driven by a need to supplement.

Lemoine said: "We think the macaques started eating soil to buffer their digestive system against the high energy, low fiber nature of these snacks and junk foods, which have been shown to cause gastric upsets in some primates.

"Our findings are more supportive of this protection hypothesis.

"The consumed soil acts as a barrier in the digestive tract, limiting absorption of harmful compounds.

"This could alleviate gastrointestinal symptoms from nausea to diarrhea.

"Soil may also provide friendly bacteria that helps with the gut microbiome.

"Non-human primates become lactose intolerant after weaning, so dairy is known to cause digestive issues in monkeys, and ice cream is hugely popular with Gibraltar's tourists and consequently its macaques."

Three instances of geophagy were observed directly after a macaque consumed tourist food: ice cream, biscuits and bread.

The researchers recorded not just when and where the monkeys ingested soil, but also types of dirt and social context, to try and understand whether geophagy has become part of the macaque "cultural repertoire."

The findings showed that some 30% of geophagy occurred in groups, with several macaques eating dirt together from the same outcrop.

And 89% of all incidents took place in the presence of other macaques who were often watching, which the research team say suggests the behavior is "socially learned."

The findings showed that the monkeys clearly favored certain types of soil, with most macaques seeking out the "terra rossa" or red clay earth found across Gibraltar, which made up 83% of all geophagy incidents.

However, one troop, the Ape's Den group, which roam the Rock's lower western slopes, had a taste for tar-clogged soil collected from potholes in asphalt roads - which made up 70% of this group's geophagy, despite the prevalence of terra rossa.

The researchers say the preferences - which they also tested experimentally by presenting some animals with a range of soil samples laid out on trays - suggest "local traditions" of dirt-eating have emerged within different monkey groups.

The authorities on Gibraltar support the macaques through daily provisioning of fruit, vegetables and water at designated feeding stations.

Tourists are forbidden to feed the monkeys, yet many do, while the macaques seek out and often steal snacks brought by tourists.

Across the whole population and observation time between summer 2022 and spring 2024, almost a fifth (18.8%) of all food consumed by macaques was junk food from tourists.

Macaques in areas with lots of people, primarily around the top of Gibraltar Rock, were 2.5 times more likely to eat tourist junk food than animals in other areas.

The monkey groups that live atop the Rock accounted for almost three-quarters of all dirt-eating incidents (72%).

Barbary macaques, native to north Africa, are believed to have arrived on Gibraltar during Moorish rule, in the Middle Ages.

After Britain captured Gibraltar in the 18th century, legend has it the monkeys were disturbed by a surprise attack, alerting British troops, who repelled French and Spanish forces and adopted the animals as a symbol of British rule.

Lemoine added: "Gibraltar's macaques are deeply entwined with human history, offering a striking example of a human-primate interface.

"The range of human interaction across Gibraltar's macaque groups create a natural experiment for understanding how anthropogenic landscapes affect primate behavior and culture."

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This story was originally published April 22, 2026 at 7:12 AM.