San Francisco Frontiers - Gibraltar monkeys eat soil in junk food detox: study

Believed to originate from North Africa, the roughly 230 primates are the main attraction in the British exclave of 30,000 people that borders southern Spain, according to the Gibraltar Ornithological and Natural History Society.

"We are here for the monkeys, to take a look at them, because it's the only place in Europe where the monkeys are," Danish visitor Elish, 29, told AFP.

But feeding the Barbary macaques is "not a great idea, because you can hurt them, because you give them anything", added the constructor designer.

Signs dotted around Gibraltar remind visitors of a ban on feeding the macaques, with infringements punishable by fines of up to £4,000 ($5,350).

But the rules are hard to enforce given the large numbers of daily visitors to "The Rock" and the independence of the animals, which can weigh up to 15 kilograms (33 pounds).

The marauding monkeys brazenly snatch ice cream, cakes and crisps from unwitting tourists in addition to gobbling abandoned leftovers from bins or food directly offered to them.

These unhealthy treats have contributed to modifying a diet usually made up of fruit, vegetables and seeds.

To soothe the resulting stomach aches, the macaques have developed a habit of "geophagy, the deliberate consumption of earth", a recent study has found.

- 'Self-medication' -

Experts from the universities of Oxford, Cambridge and Paris-Sorbonne as well as Gibraltar's environment department say their research, carried out between August 2022 and April 2024, reported this behaviour "for the first time" among the monkeys.

"Geophagy occurred at exceptionally high rates compared to other macaque species and locations, and it was more common in summer when tourist numbers peak," the study said.

In contrast, the behaviour was not observed in a group of Gibraltar monkeys that have no contact with visitors, "a strong argument for this association between soil-eating and human food", said Sylvain Lemoine, assistant professor in biological anthropology at Cambridge who co-authored the study.

Crisps, chocolate bars and ice cream are identified as the culprits of this tendency, which the study viewed as "representing an early form of self-medication".

The monkeys with the most frequent soil consumption munch on tourist junk food that is "high in sugar, high in salt, high in dairy, which the macaques can't digest," said Lemoine.

"We make the hypothesis that the soil can bring micro-fungi and micro-organisms that could rebalance the micro-biome that has been disrupted by the ingestion of junk food," he told AFP.

Bethany Maxwell, technical officer at Gibraltar Botanic Gardens, added: "We know that primates eat soil especially to detoxify or for nutrient supplementation."

"But 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."

K.Hernandez--SFF