Researchers have observed Barbary macaques on the Rock of Gibraltar eating soil, a behaviour believed to help them cope with stomach upsets caused by junk food given by tourists. The study, published in Scientific Reports, found that the monkeys most exposed to tourists consumed the most soil, with peak intake during the holiday season.
Approximately 230 macaques live on Gibraltar in eight distinct groups. Although local authorities provide daily fruit, vegetables and seeds, tourists routinely feed them high-fat, sugary snacks such as crisps, chocolate bars, ice-cream and M&M's. Nearly a fifth of the macaques' diet came from such junk food, according to observations between summer 2022 and spring 2024.
Dr Sylvain Lemoine, a primate behavioural ecologist at the University of Cambridge, suggested the soil may help rebalance the gut microbiome disrupted by the fatty, salty and sugary treats. The team noted that the only macaques not seen eating soil belonged to a group isolated from tourists. When visitor numbers fell in winter, the monkeys were 40% less likely to eat tourist food and 30% less likely to eat soil.
In total, researchers recorded 44 instances of soil consumption by 46 monkeys. The animals appeared to learn the habit from each other, with different troops favouring different soil types—most sought red clay, while the Ape's Den troop preferred tar-clogged soil from road potholes. Lemoine said the soil likely acts as a 'buffer' for the digestive system, as humans in some regions also eat soil for medicinal purposes. However, no increase was observed in pregnant or lactating monkeys, ruling out dietary supplementation.
Despite rules forbidding feeding, enforcement is lax. The researchers expressed concern about pollutants in the soil near busy roads. Dr Paula Pebsworth, a primatologist at the University of Texas at San Antonio, noted that geophagy has multiple functions, including detoxification, and similar behaviour has been seen in other primate populations exposed to tourist food.
