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 behavior 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 behavior 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 fiber.
"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.
The researchers tracked Barbary macaques living in Gibraltar, a British territory at the southern tip of Spain, between August 2022 and April 2024. The macaques – around 230 animals across eight groups – comprise the only free-ranging monkey population in Europe.
The monkeys live in close contact with the hordes of tourists who visit the site.
Tourists often feed the monkeys - or have their snacks stolen - despite the animals also receiving fruits, vegetables and seeds at designated feeding platforms managed by local authorities.
Barbary macaques, native to North Africa, are thought to have arrived in Gibraltar during medieval Moorish rule. They later became a symbol of British control after legend has it they helped alert troops to an 18th-century surprise attack.
Their population later dwindled during World War Two, prompting British leader Winston Churchill to order simian reinforcements from Morocco and Algeria - animals from which most of today's macaques are believed to have descended.
The deliberate consumption of soil, chalk or clay is called geophagy. It is seen across many animal species, including primates such as chimpanzees, lemurs and other macaques.
"We don't know the exact action of soil within the gut, but soils, particularly those rich in clay, are known to alleviate gut pH (acidity), adsorb toxins, plaster the stomach and modify microbiome composition," Lemoine told Reuters. "I would not say that soil helps digest junk food, but that likely helps them feel better during a rough digestion," Lemoine added.
Researchers documented 46 instances of geophagy across the Gibraltar monkey population.
The behavior was especially common in areas with heavy tourist traffic and peaked in summer, when visitor numbers are highest, while one group of monkeys with no access to human food showed no soil-eating at all, they said.
The study suggests the behavior may be socially learned. Different groups of the monkeys favor specific types of soil, and most soil-eating occurs in the presence of other macaques, giving younger individuals a chance to observe and copy.
The findings show how primates can adapt to changing environments in ways similar to humans and learn these behaviors from one another, Lemoine added.
Lemoine noted the findings could influence tourist behavior, potentially helping discourage illegal feeding. However, there are concerns it could have the opposite effect if visitors expect to be able to trigger unusual behavior.
"There is no systematic association between immediate junk food consumption and subsequent soil-eating. It happens that way in some cases, but generally they don't immediately eat the soil after having some human food," Lemoine said.
