Researchers have observed Barbary macaques on the Rock of Gibraltar eating soil, a behaviour known as geophagy, which they believe helps the monkeys counteract the effects of junk food consumed from tourists. The study, published in Scientific Reports, found that monkeys with the most contact with tourists ate the most soil, with consumption peaking during the holiday season.
About 230 macaques live on Gibraltar in eight distinct groups. Although local authorities provide them with fruit, vegetables and seeds, tourists routinely feed them snacks such as chips, chocolate bars, M&M's and ice-creams. Observations between summer 2022 and spring 2024 revealed that nearly a fifth of all food consumed by the macaques was junk food from tourists.
Dr Sylvain Lemoine, a primate behavioural ecologist at the University of Cambridge, said the monkeys may eat soil to rebalance their gut microbiomes, which become disturbed by fatty, salty and sugary snacks. “We think that eating this junk food disrupts the composition of the microbiome and we know that bacteria and minerals in soil can help recompose the microbiome and alleviate the negative effects,” he said.
The researchers recorded 44 monkeys eating dirt on 46 occasions. In three instances, the macaques ate soil shortly after being fed ice-cream, biscuits or bread. When visitor numbers fell in winter, the monkeys were 40% less likely to eat tourist food and more than 30% less likely to eat soil. The only macaques not seen eating soil belonged to a group isolated from tourists.
Tourists are told not to feed the monkeys, but the rule is not well enforced. Dr Lemoine expressed concern about pollutants in the soil, as much of it is found near busy roads. “We want to analyse the soil. We’re very interested in seeing the levels of pollutants,” he said.
