A group of monkeys living on the Rock of Gibraltar have learned to eat dirt, and scientists think this is to settle their stomachs after receiving, and sometimes stealing, junk food from crowds of tourists.
Researchers discovered deliberate mud feeding, known as geophagy, while observing a group of Barbary macaques in the area. It was found that the monkeys that had the most contact with tourists ate the most dirt, with consumption peaking during the holiday season.
Gibraltar is home to around 230 macaques divided into eight different groups, and while local authorities feed them fruit, vegetables and seeds daily, tourists regularly feed the macaques snacks ranging from bags of chips and chocolate bars to M&M’s and ice cream.
The observations don’t prove why monkeys eat soil, but scientists suspect that soil may have a protective effect on their digestive systems. The only macaque not seen feeding on soil on the rocks belonged to a group isolated from visitors and tourists.
Dr Sylvain Lemoine, a primate behavioral ecologist at the University of Cambridge, said the monkeys may be eating soil to rebalance their gut microbiota (the population of microorganisms that live in their digestive tracts) through the fatty, salty and sugary snacks they binge.
“We believe that eating this junk food disrupts the composition of the microbiome, and we know that bacteria and minerals in the soil can help reconstitute the microbiome and mitigate the negative effects,” Lemoine said. “We think it has a soil protection effect.”
Observations from summer 2022 to spring 2024 revealed that nearly one-fifth of the food consumed by macaque monkeys was junk food from tourists. In particular, monkeys that lived near the top of rocks popular with tourists were more than twice as likely to eat junk food than other monkeys. They also consumed the most soil.
Lemoine said the monkeys are fed junk food by local residents and visiting tourists, including salted peanuts, chocolate bars, potato chips, dried pasta, bread, Coca-Cola, orange juice, M&M’s and ice cream. “We have a lot of ice cream. They love magnums and cornettos. What they don’t really like is sorbet.”
The researchers recorded 44 monkeys eating dirt on a total of 46 occasions. In three cases, the monkeys ate dirt immediately after being given ice cream, biscuits, and bread. When the number of visitors decreased in winter, the monkeys were 40% less likely to eat tourist food and more than 30% less likely to eat dirt.
Researchers explain in Scientific Reports how monkeys learn their habits from other monkeys, with monkeys prefering different types of soil depending on their group. Most monkeys seek out terra rossa (red clay), which is found throughout Gibraltar, but the monkey den colony that occupies the lower western slopes prefers the tar-filled dirt that forms in asphalt road holes.
Humans around the world, especially pregnant women in parts of Africa, Asia, and South America, consume soil to relieve nausea and to provide necessary minerals. However, the researchers found no increase in dirt-eating behavior among pregnant or lactating monkeys, suggesting that this behavior is not driven by a need to supplement their diet.
Instead, Lemoine said the macaques appear to be eating dirt to “buffer their digestive systems” against high-energy, low-fiber snacks and junk foods that are known to cause stomach upset in some primates.
Tourists in Gibraltar are told not to touch or feed the monkeys, but the rules are not fully enforced. Junk food can be harmful to macaque monkeys, but soil can be as well, since much of it is found on rocks near busy roads. “There are a lot of vehicles passing by every day, and most of them are not yet electrified,” Lemoine said. “We want to analyze the soil. We’re very interested in seeing the levels of contaminants.”
Dr. Paula Pebsworth, a primatologist at the University of Texas at San Antonio, said geophagy serves multiple purposes related to detoxification and mineral replenishment. In her own research on Chacma baboons in South Africa, the monkeys consumed significant amounts of soil, perhaps in response to toxins in the plants.
“The idea that soil ingestion may help monkeys cope with tourist food is also plausible and has been documented at the following sites: [Japan’s] Arashiyama Monkey Park. “However, while geophagy can act as a coping mechanism, a more effective management approach is to reduce or eliminate human food provision,” she said.
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