On the tourist-packed ⁠Rock of Gibraltar, one of the most common sights is monkeys ⁠begging for food – and sometimes stealing sweet and salty snacks from unsuspecting visitors.

Scientists now have documented an unusual behaviour among these macaques that may help them ward off a bellyache from all this junk food.

Researchers said the monkeys have been observed eating soil more frequently, a behaviour they said may help the macaques avoid stomach upset from consuming human snacks.

They found that soil-eating was more common in groups ‌of monkeys that consumed more food from tourists, including chocolate, crisps and ice cream – items high in sugar, fat and dairy, and low in fibre.

“We propose the idea that human food, being not adapted to their natural diet, triggers upset stomachs, and potentially microbiome disruption, of which negative effects are buffered by the soil components,” said Sylvain Lemoine, a biological anthropologist at the University of Cambridge in England and lead author of the study published on Wednesday in the journal Scientific Reports.

If compared to something in humans, soil-eating “likely acts as antacids”, Lemoine said, adding that more research is needed to understand its effects on gut ⁠bacteria.