2026-05-01 09:06:01
Junk food-addicted monkeys are feasting on soil to counteract the stomach upsets they suffer by gorging on greasy grub grabbed from tourists.
In a behavioural shift scientists say reflects the growing impact of human diets on wildlife, Barbary macaques on the Rock of Gibraltar have developed the habit of deliberately ingesting dirt to soothe digestive problems caused by foods high in sugar, salt and dairy.
Researchers from the University of Cambridge made the discovery and say it is the result of the macaques – the only wild monkey population in Europe – increasingly coming into contact with tourists, leading to a shift away from their natural diet of herbs, seeds and insects towards snacks such as crisps, chocolate and ice cream.
Their study on the phenomenon, published in Scientific Reports, observed 230 macaques across eight groups living on Gibraltar, with heightened soil-eating behaviour recorded during peak tourist seasons between summer 2022 and spring 2024.
Dr Sylvain Lemoine, a biological anthropologist at Cambridge’s Department of Archaeology, said: “This could alleviate gastrointestinal symptoms from nausea to diarrhoea.”
He added: “Soil may also provide friendly bacteria that helps with the gut microbiome.”
The findings suggest the macaques are using soil as a form of self-medication by using it to line their digestive tract to reduce irritation caused by rich foods.
According to the researchers, junk food is “extremely rich in calories, sugar, salt and dairy”, and while appealing to the monkeys, it has “negative digestive effects” including nausea and diarrhoea.
Despite this, the study notes the food remains “as delicious for them” as it is for humans.
Dr Lemoine added: “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.”
The research recorded 46 separate soil-eating “events” across 44 animals during 98 days of observation, with behaviour most common among macaques living closest to tourist-heavy areas.
Scientists believe the habit has spread socially within groups, with different troops showing preferences for particular types of soil.
The macaques’ traditional diet – consisting largely of leaves, seeds and occasional insects – has been replaced in part by processed foods “completely unlike” what the species evolved to consume.
Dr Lemoine also said: “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.”
Experts say the soil being consumed by the monkeys acts as a protective “barrier” in the gut, limiting the absorption of harmful compounds while also introducing minerals and beneficial microbes missing from processed snacks.
The behaviour, they suggest, may represent an adaptive response to an increasingly human-influenced environment rather than a natural dietary choice.
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