Gibraltar's Junk-Food Monkeys Found Self-Medicating With Soil

When roughly 230 Barbary macaques living on the British territory of Gibraltar started munching on tourist offerings—ice cream, chocolate, crisps—something unexpected happened. The monkeys didn’t just get fat. They got sick. And then they did something remarkable: they figured out their own fix.

A new study from researchers at Oxford, Cambridge, Paris-Sorbonne, and Gibraltar’s environment department has documented that these primates are deliberately eating soil as a form of self-medication to counteract the effects of their junk food binges. The behavior, called geophagy, occurs at exceptionally high rates during summer months when tourist numbers peak—and it’s virtually absent in a separate group of Gibraltar monkeys that have no contact with visitors. That’s the smoking gun right there.

The culprits are obvious.“Crisps, chocolate bars and ice cream”are high in sugar, salt, and dairy that macaques can’t digest, according to Sylvain Lemoine, assistant professor in biological anthropology at Cambridge who co-authored the study. The hypothesis is elegant: soil contains micro-fungi and micro-organisms that rebalance the gut microbiome disrupted by this processed food onslaught. It’s not just scavenging; it’s survival strategy. As Bethany Maxwell, technical officer at Gibraltar Botanic Gardens, explained, while primates eat soil for detoxification and nutrient supplementation,“this study has shown that not only are they doing it for those reasons, but also as a result of eating too much junk food, which is something that is quite novel.”

Signs around Gibraltar warn visitors of a £4,000 fine for feeding the macaques, but the rules are nearly impossible to enforce when thousands of daily visitors encounter animals weighing up to 15 kilograms that brazenly steal snacks right out of tourists’hands. One Danish visitor even acknowledged the problem: feeding them“is not a great idea, because you can hurt them, because you give them anything.”

Here’s what makes this story matter beyond the cute-animal angle: it’s a living laboratory of how human behavior cascades through ecosystems. These monkeys aren’t evolving—they’re adapting in real time to damage we’re directly causing. And they’re literally eating dirt to survive the consequences. That’s not cute. That’s a warning sign about what happens when we treat wildlife as mascots instead of respecting their boundaries.