Monkeys in a tourism hotspot have learned that swallowing dirt can quell the upset stomachs caused by overconsumption of sweet and salty snacks fed to them by holidaymakers, a new University of Cambridge-led study suggests.
Troops of macaques living on Gibraltar – the only free-ranging monkey population in Europe – have been scientifically observed for the first time regularly engaging in “geophagy”: the practice of intentionally ingesting soil.
Researchers monitoring monkey groups across the Rock of Gibraltar have tracked instances of geophagy, and found that animals in frequent contact with tourists eat far more dirt, and that dirt-eating rates are higher during peak holiday season.
The scientists believe that the chocolate, crisps and ice cream offered by or stolen from tourists – a substantial part of some Gibraltar macaques’ diets – are disrupting “gut microbiome composition” in the animals and leading to changes in their culture.
Eating soil may help rebalance monkey stomachs by providing bacteria and minerals absent from junk food, say researchers, and it is likely to help line the gut and soothe or prevent irritation caused by too much sugar and fat.
Scientists think this behaviour is transmitted socially, as different troops have preferences for certain types of soil, and say it is an example of an emerging animal culture and “tradition” created by living in a human-dominated environment.
“Foods brought by tourists and eaten by Gibraltar’s macaques are extremely rich in calories, sugar, salt and dairy,” said Dr Sylvain Lemoine, a biological anthropologist from Cambridge’s Department of Archaeology, who led the study, published in the journal Scientific Reports . “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,” said Lemoine, who founded the Gibraltar Macaques Project.
“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 behaviour in macaques is both a functional and cultural one, like nutcracking in chimps, except it is driven entirely by proximity to humans.”
Gibraltar’s macaque population averaged 12 soil-eating “events” a week, at the high end of recorded geophagy frequency in primates, and comparable to chimpanzees 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. However, researchers found no increase in macaque soil-eating during pregnancy or lactation, suggesting it is not driven by a need to supplement.
“We think the macaques started eating soil to buffer their digestive system against the high energy, low fibre nature of these snacks and junk foods, which have been shown to cause gastric upsets in some primates,” said Lemoine.
“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 diarrhoea. 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,” said Lemoine.
A few instances of observed geophagy came shortly after researchers had watched the same macaque consume bread and ice cream.**
The scientists recorded not just when and where these animals ingested soil, but also types of dirt and social context, to try and understand whether geophagy has become part of the macaque “cultural repertoire”.
Some 30% of geophagy occurred in groups, with multiple animals eating dirt together from the same outcrop, and 89% of all incidents took place in the presence of other macaques who were often watching, suggesting this behaviour is “socially learned”.
Additionally, the monkeys clearly favoured 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 scientists say these firm 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.
Gibraltar’s macaques number around 230 animals across eight stable groups that inhabit different areas of the Rock. The monkeys are a major tourist draw, plastered across t-shirts and postcards.
Local authorities support the macaques through daily provisioning of fruit, vegetables and water at designated feeding stations. Visitors are forbidden to feed the monkeys, yet many do, and the animals seek out and often pilfer snacks brought by tourists.
In fact, across the whole population and observation time, 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 two and a half times more likely to consume tourist junk food than animals in other areas. The three monkey groups that live atop the Rock accounted for over two-thirds (72%) of all dirt-eating incidents.
Those lower down on Gibraltar’s slopes showed far fewer geophagy cases. One group had no contact with tourists or access to human food, and this was the only group in which researchers observed no soil consumption whatsoever.
The macaques as a whole population were about 40% less likely to eat tourist food in winter than in summer, and geophagy also fell by about 31% in the winter.
Barbary macaques, native to north Africa, are believed to have arrived in Gibraltar during medieval Moorish rule, with Berber soldiers keeping them as pets.
Following the British capture of Gibraltar in the 18 th century, legend has it the monkeys were disturbed by a surprise attack, alerting British forces, who repelled the French and Spaniards and adopted the animals as a symbol of British rule of the Rock.
Gibraltar’s dwindling macaque population became a concern for British morale during World War II, prompting Winston Churchill to order monkey replenishments from Morocco and Algeria. Previous research suggests that most of the Rock’s present-day macaques may be descended from those imports.
“Gibraltar’s macaques are deeply entwined with human history, offering a striking example of a human-primate interface,” added Lemoine. “The range of human interaction across Gibraltar’s macaque groups create a natural experiment for understanding how anthropogenic landscapes affect primate behaviour and culture.”
Notes:
*Geophagy in the Barbary macaques of north Africa is very rare. The only macaque species with higher recorded rates of geophagy than those on Gibraltar are the semi-feral macaques that live in Hong Kong’s Kam Shan Country Park, which access a huge amount of human food from visitors, and average over 33 geophagy events a week. Ring-tailed lemurs average 16 events per week. East African chimpanzees average 14 events per week.
** Three instances of geophagy were observed directly after an animal consumed tourist food: ice cream (7 minutes earlier); biscuits (48 minutes earlier); bread (6 minutes earlier).
***During a total of 98 observation days between summer 2022 and spring 2024 the scientists recorded 46 dirt-eating “events” in 44 different animals.
Scientific Reports
Observational study
Animals
Geophagy in Gibraltar Barbary macaques is a primate tradition anthropogenically induced
22-Apr-2026