Researchers from Cambridge University spotted intentional mud eating, known as geophagy, while observing groups of Barbary macaques in Gibraltar. The study, conducted between summer 2022 and spring 2024, found that nearly a fifth of all food consumed by the macaques was junk food from tourists, including chips, chocolate bars, M&M's and ice-creams. Dr Sylvain Lemoine, a primate behavioural ecologist at the university, said the monkeys may be eating soil to rebalance their gut microbiomes. "We think that eating this junk food disrupts the composition of the microbiome and we know that bacteria and minerals in soil can help recompose the microbiome and alleviate the negative effects," he said. "We think there's a protective effect of the soil."
About 230 macaques live on Gibraltar in eight distinct groups. The only macaques not seen eating soil belonged to a group isolated from visitors. Macaques living around the top of the rock, a popular tourist spot, were more than twice as likely to eat junk food than others and also consumed the most soil. In total, researchers recorded 44 monkeys eating dirt on 46 occasions. When visitor numbers fell in winter, the monkeys were 40% less likely to eat tourist food and more than 30% less likely to eat soil.
The observations do not prove why the monkeys eat soil, but scientists suspect it has a protective effect on the digestive system. In three instances, macaques ate soil shortly after being fed ice-cream, biscuits or bread. Dr Lemoine noted the monkeys' preference for certain treats: "There's a lot of ice-cream. They love Magnums and Cornettos. What they don't like very much is sorbet." 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 favours tar-clogged soil from potholes in asphalt roads. Local authorities provide the macaques with daily helpings of fruit, vegetables and seeds, but tourists and locals also feed them junk food, according to Dr Lemoine. The behaviour mirrors geophagy in humans, particularly among pregnant women in parts of Africa, though the exact mechanism by which soil consumption protects the digestive system remains unknown. The long-term health impacts of the macaques' junk food diet and geophagy are also unclear.