Troops of monkeys living on the Rock of Gibraltar have learned to eat soil, according to researchers. Scientists believe the monkeys eat soil to settle their stomachs after consuming junk food from tourists, according to research. Researchers observed intentional mud eating, known as geophagy, in groups of Barbary macaques in Gibraltar.
A direct correlation exists between tourist contact and soil consumption patterns, according to observations. Monkeys with the most contact with tourists ate the most soil, and consumption peaked in the holiday season, researchers found. The only macaques not seen eating soil belonged to a group isolated from visitors and tourists, according to observations. Macaques living around the top of the rock were more than twice as likely to eat junk food than others and consumed the most soil, researchers reported. When visitor numbers fell in winter, monkeys were 40% less likely to eat tourist food and more than 30% less likely to eat soil, according to research.
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. We think there's a protective effect of the soil.
The scale of junk food consumption is significant, with observations between summer 2022 and spring 2024 finding that nearly a fifth of all food consumed by the macaques was junk food from tourists. Monkeys are fed junk food by locals as well as visiting tourists, including items like salted peanuts, chocolate bars, crisps, dried pasta, bread, Coca-Cola, orange juice, M&M's, and ice-cream, according to Dr Sylvain Lemoine, a primate behavioral ecologist at the University of Cambridge. In three instances, macaques ate soil shortly after being fed ice-cream, biscuits, or bread, researchers observed.
About 230 macaques live on Gibraltar in eight distinct groups, according to authorities. Local authorities provide the macaques with daily helpings of fruit, vegetables, and seeds to supplement their natural diet.
There's a lot of ice-cream. They love Magnums and Cornettos. What they don't like very much is sorbet.
Research methodology involved detailed observation, with researchers recording 44 monkeys eating dirt on 46 occasions. Most monkeys search out terra rossa, a red clay found across Gibraltar, but the Ape's Den troop favors tar-clogged soil from potholes in asphalt roads, according to observations.
Key unknowns remain about this behavior. Researchers have not determined what specific health effects the junk food has on the monkeys' digestive systems beyond microbiome disruption, nor whether the soil-eating behavior has any long-term negative consequences. It is also unclear how effective the soil is at mitigating the negative effects of junk food consumption. According to Dr Lemoine, researchers think that eating junk food disrupts the microbiome and that soil components might help recompose it and alleviate negative effects. It remains unknown what measures, if any, are being taken to reduce tourist feeding of junk food to the monkeys, and whether the soil-eating behavior is observed in other monkey populations exposed to similar human food sources.