At the tourist-packed Rock of Gibraltar, one of the most common sights is of monkeys begging for food and sometimes stealing sweet and salty snacks from unsuspecting visitors. Scientists have now documented unusual behavior among these macaques that may help them avoid stomach pain from all this junk food.
The researchers said the monkeys have been observed eating dirt more frequently, a behavior they said may help the macaques avoid stomach upset when consuming human snacks. They found that eating dirt was more common in groups of monkeys that consumed more of the tourists’ food, including chocolate, chips and ice cream, products high in sugar, fat and dairy, and low in fibre.
“We propose the idea that human food, not being adapted to their natural diet, causes stomach upset and potentially microbiome alterations, the negative effects of which are buffered by soil components,” said Sylvain Lemoine, a biological anthropologist at the University of Cambridge in England and lead author of the study published Wednesday in the journal Scientific Reports.
Compared to something that occurs in humans, eating dirt “probably acts as an antacid,” Lemoine said, adding that more research is needed to understand its effects on gut bacteria.
The researchers tracked Barbary macaques that lived in Gibraltar, a British territory on the southern tip of Spain, between August 2022 and April 2024. The macaques (around 230 animals in eight groups) make up the only free-ranging population of monkeys in Europe.
The monkeys live in close contact with the hordes of tourists who visit the place. Tourists often feed the monkeys, or steal their snacks, even though the animals also receive fruits, vegetables and seeds at designated feeding platforms managed by local authorities.
Barbary macaques, originally from North Africa, are believed to have arrived in Gibraltar during medieval Arab rule. They later became a symbol of British control after legend says they helped alert troops to a surprise attack in the 18th century.
Their population subsequently declined during World War II, prompting British leader Winston Churchill to order ape reinforcements from Morocco and Algeria, animals from which most of today’s macaques are believed to descend.
The deliberate consumption of soil, chalk or clay is called geophagy. It is seen in many animal species, including primates such as chimpanzees, lemurs, and other macaques.
“We don’t know the exact action of soil within the intestine, but soils, particularly those rich in clay, are known to alleviate the pH (acidity) of the intestine, absorb toxins, plaster the stomach and modify the composition of the microbiome,” Lemoine said. Reuters. “I wouldn’t say soil helps digest junk food, but it probably helps them feel better during difficult digestion,” Lemoine added.
The researchers documented 46 cases of geophagy in 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 a group of monkeys without access to human food did not eat dirt at all, they said.
The study suggests that behavior can be learned socially. Different groups of monkeys prefer specific types of soil, and most feeding occurs in the presence of other macaques, giving younger individuals the opportunity to observe and copy.
The findings show how primates can adapt to changing environments in similar ways to humans and learn these behaviors from each other, Lemoine added.
Lemoine noted that the findings could influence tourists’ behavior, which could help discourage illegal feeding. However, there are concerns that it could have the opposite effect if visitors hope to trigger unusual behavior.
“There is no systematic association between immediate consumption of junk food and subsequent consumption of dirt. It does happen in some cases, but generally they do not eat dirt immediately after eating some human food,” Lemoine said.
