A study by the University of Cambridge reveals that monkeys in Gibraltar began eating dirt to alleviate digestive problems caused by consuming processed foods provided by tourists
A new study released this Wednesday indicates that Gibraltar monkeys learned to eat dirt to alleviate gastrointestinal discomfort caused by excessive consumption of sweets and savory foods offered by tourists.
Groups of these primates – the only free-ranging population of monkeys in Europe – were scientifically observed for the first time to regularly resort to “geophagy”, the intentional ingestion of earth, says a statement about the work from the University of Cambridge, in the United Kingdom.
Researchers monitoring groups of monkeys on the Rock of Gibraltar in the south of the Iberian Peninsula, also known as Barbary macaques, recorded cases of geophagy and discovered that “animals in frequent contact with tourists eat much more soil”, and “earth ingestion rates are higher during peak season”.
“During a total of 98 days of observation, between the summer of 2022 and the spring of 2024, scientists recorded 46 ‘events’ of soil ingestion in 44 different animals”, states the statement, adding that “three cases of geophagia were observed immediately after an animal had consumed tourist food: ice cream (7 minutes before), cookies (48 minutes before), bread (6 minutes before)”.
According to scientists, chocolate, snacks and ice cream offered or stolen to tourists – a substantial part of the diet of some Gibraltar monkeys – alter the animals’ “intestinal microbiome composition” and have led to changes in their behavior.
“Eating dirt may help rebalance monkeys’ stomachs by providing bacteria and minerals missing from processed foods,” and “is likely to help coat the gut and soothe or prevent irritation caused by excess sugar and fat,” they say.
“The foods brought by tourists and consumed by the Gibraltar monkeys are extremely rich in calories, sugar, salt and dairy products”, says Sylvain Lemoine, who led the study, published in the journal Scientific Reports, cited in the statement.
“This is completely different from the foods typically consumed by the species, such as herbs, leaves, seeds and, occasionally, insects”, adds the primatologist from the Department of Archeology at Cambridge.
The scientists recorded not only when and where the animals ingested soil, but also the types of soil and the social context, concluding that this is a socially transmitted behavior, given that different groups have preferences for certain types of soil.
According to the study, around 30% of geophagy cases occurred in groups, with several animals eating dirt together from the same rocky outcrop, and 89% of all incidents occurred in the presence of other monkeys who often watched them.
The monkeys clearly preferred certain types of soil, with most seeking out the “terra rossa” or red clay found in Gibraltar, which accounted for 83% of all geophagy episodes.
Lemoine, founder of the Monkeys of Gibraltar Project, says researchers believe “the monkeys started eating soil to protect their digestive systems from the high energy and low fiber content” of junk food.
“Consumed soil acts as a barrier in the digestive tract, limiting the absorption of harmful compounds. This can alleviate gastrointestinal symptoms, from nausea to diarrhea. Soil can also provide beneficial bacteria that help the intestinal microbiota,” he explains.
An average of 12 soil ingestion “events” were observed per week, considered a high value among the geophagy frequencies already recorded in primates, comparable to that of chimpanzees and lemurs, which eat soil to obtain minerals and protect themselves from plant toxins.
There are around 230 monkeys in Gibraltar, distributed in eight stable groups that inhabit different areas of the Rock.
Local authorities provide fruit, vegetables and water daily at designated feeding points and visitors are prohibited from feeding them, but many do and the animals often seek out and steal food brought by tourists.
During the observation period, almost a fifth (18.8%) of all the food consumed by the monkeys was ‘junk food’ brought by tourists and the three groups of monkeys living at the top of the Rock, one of the places with a large concentration of people, were responsible for more than two thirds (72%) of all cases of dirt ingestion.
One group that had no contact with tourists or access to human food was the only one in which researchers did not observe any soil consumption.
“In winter, compared to summer, the total monkey population was around 40% less likely to consume food offered by tourists and geophagy also decreased by around 31%.”
In some human cultures, geophagy is common, often associated with nutrient intake during pregnancy, but in this case researchers did not find an increase in soil intake by the monkeys during pregnancy or lactation, suggesting it is not driven by a need for supplementation.
“The emergence of this behavior in monkeys is both functional and cultural (…), but it is driven entirely by proximity to humans”, points out Lemoine.
