Gibraltar’s famous colony of Barbary macaques, a top draw for international visitors to this British overseas territory on southern Spain’s border, have developed an unexpected adaptive behavior: they deliberately eat soil to counteract gastrointestinal distress caused by consuming large amounts of human junk food, according to groundbreaking new research published by an international team of biologists.
Originating from North Africa, the population of roughly 230 macaques holds a unique status as the only free-ranging colony of wild monkeys in all of Europe, per data from the Gibraltar Ornithological and Natural History Society. For millions of tourists who flock to the territory’s iconic Rock of Gibraltar each year, seeing these charismatic primates is often the highlight of their trip. “We came here specifically for the monkeys, because this is the only place in Europe you can see wild populations of them,” 29-year-old Danish visitor Elish told Agence France-Presse. But the tourist attention comes with a hidden cost: despite repeated warnings, many visitors either feed the macaques directly or leave food waste accessible, and the animals regularly raid snacks from unaware guests.
Local authorities have long enforced a ban on feeding the macaques, with posted warning signs across the territory and fines for violations reaching as high as £4,000 ($5,350). But enforcement remains a major challenge: thousands of tourists visit the Rock daily, and the macaques, which can grow up to 15 kilograms, are bold, independent, and skilled at snatching ice cream, cakes, crisps, chocolate and other processed treats from unguarded bags, picnic baskets and public waste bins. Over time, this steady access to unhealthy human food has drastically altered the macaques’ natural diet, which originally consists of wild fruits, leafy vegetables, seeds and native vegetation.
The new study, conducted by researchers from the Universities of Oxford, Cambridge, Paris-Sorbonne and Gibraltar’s local environment department between August 2022 and April 2024, documents this soil-eating behavior — formally called geophagy — for the first time in this Gibraltar macaque population. The research team found that geophagy occurs at far higher rates among this colony than it does among other macaque populations around the world, and that the behavior spikes in summer, when tourist numbers to Gibraltar reach their annual peak. Critically, the behavior was not observed at all in a separate group of Gibraltar macaques that have no regular contact with tourists and do not access human junk food.
“That is a strong argument for the direct association between soil-eating and the consumption of human food,” explained Sylvain Lemoine, co-author of the study and assistant professor of biological anthropology at the University of Cambridge. Lemoine noted that the processed junk food the macaques consume is extremely high in sugar, salt and dairy — ingredients that the primates’ digestive systems are not evolved to process properly, leading to frequent stomach discomfort and disrupted gut microbiomes.
The research team classifies the behavior as an early form of self-medication. They hypothesize that the soil the macaques consume carries beneficial microfungi and natural microorganisms that help rebalance the disordered gut microbiome after a junk food binge, in addition to absorbing toxins to reduce gastrointestinal distress. Bethany Maxwell, technical officer at the Gibraltar Botanic Gardens, pointed out that while primate geophagy is already well-documented in scientific literature, the link to excess junk food consumption from human tourism is an entirely new finding. “We already know primates eat soil mostly to detoxify or to supplement missing nutrients, but this study shows that this behavior is also being driven here by eating too much unhealthy human food — that’s something quite novel,” Maxwell said.