Monkeys find unique way to avoid tourist junk food

Gibraltar’s Barbary macaques are increasingly eating soil in behaviour researchers believe may help counter stomach problems caused by tourist snacks, according to a study published on Wednesday.

Scientists observed that soil consumption was more common among monkey groups that had greater access to human food such as chocolate, crisps and ice cream. The researchers said these processed foods, which are high in sugar, fat and dairy and low in fibre, may be disrupting the animals’ digestion and gut microbiome.

The study tracked around 230 macaques across eight groups in Gibraltar between August 2022 and April 2024. Researchers recorded 46 instances of geophagy, or deliberate soil-eating, and found that the behaviour was most frequent in areas with heavy tourist traffic and during summer, when visitor numbers are highest.

One monkey group with no access to human food showed no soil-eating at all, the researchers said. They added that clay-rich soil may help reduce acidity, absorb toxins, protect the stomach and alter gut bacteria, though the exact effect remains unclear.

The study also suggested the behaviour may be socially learned, with younger macaques likely copying others in their group. Researchers said different groups appeared to prefer different types of soil, with most soil-eating taking place in the presence of other monkeys.

Barbary macaques are the only free-ranging monkey population in Europe. The animals live in close contact with tourists on the Rock of Gibraltar, where they are often fed by visitors or steal snacks despite also receiving fruits, vegetables and seeds at managed feeding platforms.