High on ice cream and crisps, a Barbary macaque in Gibraltar swallows soil to soothe its stomach—a form of self-medication that researchers have documented for the first time in this remarkable species. The roughly 230 primates living in this British exclave have become dependent on junk food snatched from tourists' hands and abandoned in bins, and their bodies are paying the price. But in a striking example of animal adaptation, the monkeys have learned to fight back against digestive disruption by consuming earth, revealing an elegant survival strategy born from human interference.
The Barbary macaques have become the star attraction of Gibraltar, a British territory of just 30,000 people perched on the southern Spanish border. For decades, visitors have arrived with one goal: to see Europe's only wild monkeys. The experience has been transformative for the macaques' diet. Rather than subsisting on their natural foods—fruit, vegetables, and seeds—the animals now brazenly snatch ice cream, chocolate bars, and crisps from unsuspecting tourists, and scavenge from overflowing bins. Though Gibraltar's environment authorities have posted signs threatening fines of up to £4,000 for feeding the macaques, enforcement proves difficult amid the crowds and the animals' remarkable independence.
A new study, conducted between August 2022 and April 2024 by researchers from Oxford, Cambridge, and Paris-Sorbonne universities working with Gibraltar's environment department, has uncovered the hidden toll of this dietary shift. The scientists observed that the macaques engage in geophagy—the deliberate consumption of soil—at exceptionally high rates compared to other macaque populations worldwide. Crucially, this behavior spiked during summer months when tourist numbers peak, and was entirely absent in a control group of Gibraltar macaques with no contact to visitors. The pattern points unmistakably to cause and effect.
Sylvain Lemoine, an assistant professor in biological anthropology at Cambridge and co-author of the study, explains the mechanism. The tourist food is "high in sugar, high in salt, high in dairy, which the macaques can't digest." When consumed in quantity, these foods disrupt the animals' gut microbiome. The soil the macaques eat, Lemoine hypothesizes, contains micro-fungi and micro-organisms that help rebalance their digestive systems. Bethany Maxwell, technical officer at Gibraltar Botanic Gardens, notes that while primates have long been known to 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."
The finding reveals an unexpected resilience in the macaques—they are not passive victims of tourist culture, but active problem-solvers adapting to circumstances beyond their control. Yet the deeper message is sobering. These primates, weighing up to 15 kilograms and descended from North African ancestors, have evolved in response to human disruption. Their self-medication is a band-aid solution to a wound that shouldn't exist. As Gibraltar's tourism continues to draw visitors seeking encounters with these charismatic animals, the study underscores an uncomfortable truth: our presence, however well-intentioned, has fundamentally altered their world. The macaques' solution is ingenious, but prevention would be infinitely better.