Gibraltar's monkey's eat mud 'to soothe upset stomachs from tourist junk food'

Scientists have observed monkeys deliberately eating mud in Gibraltar, in what they believe is a reaction to consuming foods like ice cream and chocolate bars that upset their stomachs.

A population of around 230 Barbary macaques inhabiting the Rock of Gibraltar attract hundreds of thousands of visitors annually, who frequently flout rules banning them from feeding or touching the monkeys.

Scientists observing the monkeys found that geophagy – the deliberate consumption of soil – was highest when the macaques ate high levels of food from tourists.

They think the behaviour is likely to have been learned socially, as different troops of monkeys have preferences for certain types of soil.

A monkey eats crisps in Gibraltar. / Credit: Martin Nicourt/Gibraltar Macaques Project

“We think they are eating the soil to protect themselves against the adverse effects junk food can trigger,” said Dr Sylvain Lemoine, Assistant Professor in Biological Anthropology at the University of Cambridge.

Soil ingestion can help to rebalance the gut and relieve symptoms from upset stomachs, he explained.

Scientists believe the dirt also provides bacteria and minerals absent from junk food.

In a study published in Scientific Reports, the scientists found that the monkeys ate mud more in areas where there were more tourists and that geophagy was more common in the summer, when tourist numbers peaked.

Geophagy is common in monkeys, apes, lemurs and other non-human primates, but why it happens is still being debated. The scientists suspect that the Gibralta macaques ate soil to protect their stomachs from hard to digest human food.

The macaques are unable to digest milk after weaning, so they might experience the symptoms of an upset stomach after eating ice cream or dairy-rich junk foods, said Lemoine.

Gibraltarian authorities are responsible for feeding the macaques a diet of fruit, vegetables and seeds. The foods they might pick up from tourists, such as ice cream or crisps, are often high in calories, sugar, salt, fat and dairy and low in fibre.

Lemoine told ITV News that tourists should follow the rules and not feed the monkeys. He added they should also be careful with carrying food in the bottom of prams or when taking snacks out of their pockets, as the monkeys might snatch them.