Troops of Barbary macaques (Macaca sylvanus) 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 to settle their stomachs from all the junk food they consume.
Scientists believe the dirt helps the monkeys line the gut to stop irritation from foods which are "extremely rich in calories, sugar, salt and dairy".
The soil also provides bacteria and minerals missing from junk food. The snacks have negative digestive effects for the macaques and can cause symptoms from nausea to diarrhoea, but the food is "as delicious for them" as it is for humans, according to the Cambridge University study.
Observations between summer 2022 and spring 2024 found that nearly a fifth of all food consumed by the macaques was junk food from tourists. Macaques that lived on the Rock of Gibraltar, which is particularly popular with tourists, were more than twice as likely to eat junk food than others. They also consumed the most soil.
Dr Sylvain Lemoine (left), a biological anthropologist from Cambridge's Department of Archaeology, said the monkeys were fed junk food by locals as well as visiting tourists, who have offered salted peanuts, chocolate bars, crisps, dried pasta, bread, Coca-Cola, orange juice, M&M’s, ice-cream and more. “There’s a lot of ice-cream. They love Magnums and Cornettos. What they don’t like very much is sorbet”.
In total, the researchers recorded 44 monkeys eating dirt on 46 occasions. In three instances, the macaques ate soil shortly after being fed ice-cream, biscuits or bread. When visitor numbers fell in the winter, the monkeys were 40% less likely to eat tourist food and more than 30% less likely to eat soil.
Writing in Scientific Reports, Vol. 16, Article number: 13139 (2026), "Geophagy in Gibraltar Barbary macaques is a primate tradition anthropogenically induced", the researchers describe how the monkeys appear to learn the habit from others, with macaques favouring different types of soil depending on their troop. Most monkeys search out the terra rossa, or red clay, found across Gibraltar, but the Ape’s Den troop, which occupies the lower western slopes, favours tar-clogged soil from potholes in asphalt roads.
Humans around the world eat soil, particularly pregnant women in parts of Africa, Asia and South America, where it is consumed to help with nausea or to provide critical minerals. But the researchers saw no rise in soil-eating among pregnant or lactating monkeys, suggesting the behaviour is not driven by a need to supplement their diets.
And, Lemoine added that the macaques seemed to eat the soil to “buffer their digestive system” against high-energy, low-fibre snacks and junk foods that are known to cause stomach upsets in some primates.
Tourists are told not to touch or feed the monkeys on Gibraltar, but the rule is not well enforced. While the junk food may be harmful to the macaques, so might the soil, as much of it is found close to busy roads on the rock. “There are a lot of vehicles passing every day, and most are not electric yet”, Lemoine said. “We want to analyse the soil. We’re very interested in seeing the levels of pollutants”.
The junk diet was "completely unlike" foods normally eaten by the species, such as herbs, leaves, seeds and the occasional insect – with the behaviour being "driven entirely by proximity to humans".
Lemoine posited: "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".
