Gibraltar’s monkeys eat mud ‘to avoid upset stomachs from tourist junk food’

Troops of monkeys living on the Rock of Gibraltar have learned to eat soil in what scientists believe is an effort to settle their stomachs after all the junk food they receive – and sometimes steal – from crowds of tourists.

Researchers spotted the intentional mud eating, known as geophagy, while observing groups of Barbary macaques in the territory. Monkeys that had the most contact with tourists ate the most soil and consumption peaked in the holiday season, they found.

About 230 macaques live on Gibraltar in eight distinct groups, and while local authorities provide them with daily helpings of fruit, vegetables and seeds, tourists routinely feed them snacks ranging from bags of chips and chocolate bars to M&M’s and ice-creams.

The observations don’t prove why the monkeys eat soil, but scientists suspect it has a protective effect on the digestive system. The only macaques on the rock that were not seen eating soil belonged to a group that is isolated from visitors and tourists.

A macaque with an ice-cream. Monkeys on Gibraltar that had the most contact with tourists ate the most soil, scientists found. Photograph: Martin Nicourt/Gibraltar Macaques Project/PA

Dr Sylvain Lemoine, a primate behavioural ecologist at the University of Cambridge, said the monkeys may be eating the soil to rebalance their gut microbiomes, the populations of microbes that live in the digestive tract, which become disturbed by the fatty, salty and sugary snacks the monkeys binge on.

“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,” Lemoine said. “We think there’s a protective effect of the soil.”

Observations between summer 2022 and spring 2024 found that nearly a fifth of all food consumed by the macaques was junk food from tourists. Macaques that lived around the top of the rock, which is particularly popular with tourists, were more than twice as likely to eat junk food than others. They also consumed the most soil.

Lemoine said the monkeys were fed junk food by locals as well as visiting tourists, who have offered salted peanuts, chocolate bars, crisps, dried pasta, bread, Coca-Cola, orange juice, M&M’s, ice-cream and more. “There’s a lot of ice-cream. They love Magnums and Cornettos. What they don’t like very much is sorbet.”