Gibraltar’s monkeys eat mud ‘to avoid upset stomachs from tourist junk food’ | Gibraltar

Troops of monkeys living on the Rock of Gibraltar have learned to eat dirt, which scientists say is an effort to calm their stomachs after all the junk food they get — and sometimes steal — from crowds of tourists.

Researchers spotted intentional slime consumption, known as geophagy, while observing groups of Barbary macaques in the territory. The monkeys who had the most contact with tourists ate the most soil, and their consumption peaked during the holiday season, they found.

Around 230 macaques live in Gibraltar, divided into eight distinct groups, and while local authorities provide them with fruit, vegetables and seeds daily, tourists regularly feed them snacks ranging from bags of chips and chocolate bars to M&M’s and ice cream.

Observations don’t prove why monkeys eat dirt, but scientists suspect it has a protective effect on the digestive system. The only macaques on the rock that were not seen eating dirt belonged to a group isolated from visitors and tourists.

Dr Sylvain Lemoine, a primate behavioral ecologist at the University of Cambridge, said monkeys might eat soil to rebalance their gut microbiome, the populations of microbes that live in the digestive tract, which are disrupted by the fatty, salty and sugary snacks the monkeys gorge on.

“We think that eating this junk food disrupts the composition of the microbiome and we know that bacteria and minerals in the soil can help recompose the microbiome and mitigate the negative effects,” Lemoine said. “We think the soil has a protective effect.”

Observations between summer 2022 and spring 2024 revealed that almost a fifth of all food eaten by macaques was junk food from tourists. The macaques who lived on top of the rock, particularly popular with tourists, were twice as likely to eat junk food as others. They were also the ones who consumed the most land.

Lemoine said the monkeys were fed junk food by locals as well as visiting tourists, who offered them salted peanuts, chocolate bars, chips, dried pasta, bread, Coca-Cola, orange juice, M&M’s, ice cream and more. “There’s a lot of ice cream. They love Magnums and Cornettos. What they don’t really like is sorbet.”

In total, the researchers recorded 44 monkeys eating dirt on 46 occasions. In three cases, the macaques ate dirt shortly after eating ice cream, cookies or bread. When visitor numbers declined in winter, monkeys were 40% less likely to eat tourist food and more than 30% less likely to eat dirt.

In Scientific Reports, the researchers describe how monkeys appear to learn this habit from others, with macaques favoring different types of soil depending on their troop. Most monkeys seek out the terra rossa, or red clay, found in Gibraltar, but the Ape’s Den troop, which occupies the lower western slopes, prefers soil clogged with tar from the potholes of asphalt roads.

Humans around the world eat dirt, especially pregnant women in parts of Africa, Asia and South America, where it is eaten to relieve nausea or to provide essential minerals. But the researchers found no increase in soil consumption in pregnant or lactating monkeys, suggesting that this behavior is not driven by the need to supplement their diet.

Instead, Lemoine said the macaques appeared to eat the ground to “protect their digestive systems” from high-energy, low-fiber snacks and junk foods that are known to cause stomach upset in some primates.

Tourists are asked not to touch or feed the Gibraltar monkeys, but the rule is not well enforced. If junk food can be harmful to macaques, so can the soil, as much of it is found near busy roads on the rock. “There are a lot of vehicles that pass by every day, and most of them are not electric yet,” Lemoine said. “We want to test the soil. We’re very interested in pollutant levels.”

Dr. Paula Pebsworth, a primatologist at the University of Texas at San Antonio, said geophagy serves several purposes related to detoxification and mineral supplementation. In his own work on chacma baboons in South Africa, the monkeys consumed significant amounts of soil, likely in response to plant toxins.

“The idea that soil consumption could help monkeys cope with tourism supply is also plausible and has been documented at [Japan’s] Arashiyama Monkey Park. However, while geophagy can serve as a coping mechanism, a more effective management approach is to reduce or eliminate the human food supply,” she said.