Gibraltar’s monkeys may be eating soil to cope with junk food from tourists

Coverage Comparison

Scientists have documented that Barbary macaques on the Rock of Gibraltar, a British territory at the southern tip of Spain, are deliberately eating soil, a behavior that may help them cope with the stomach upsets caused by the junk food they receive or steal from tourists. The research, published in the journal Scientific Reports, was covered by The Guardian, the Jerusalem Post, and the South China Morning Post, with consistent reporting of the key findings.

The Guardian, in its report, emphasized the scientific discovery and its implications for understanding how animals adapt to human impact. The Jerusalem Post and South China Morning Post took a more straightforward scientific framing, focusing on the unusual behavior itself. All three sources attributed the research to its lead author, Sylvain Lemoine, a biological anthropologist at the University of Cambridge.

Key Claims

The study tracked Barbary macaques living in Gibraltar between August 2022 and April 2024. Researchers followed approximately 230 animals across eight distinct groups, making them the only free-ranging monkey population in Europe. The macaques live in close contact with tourists who often feed them, despite local authorities providing designated feeding platforms with fruits, vegetables, and seeds.

The researchers found that soil-eating behavior was more common among groups of monkeys that consumed more tourist-provided food, including chocolate, crisps, and ice cream. "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," Lemoine said. He added that soil-eating "likely acts as antacids" when compared to something in humans.

The Guardian added additional observations, reporting that researchers spotted 44 monkeys eating dirt on 46 occasions, with behavior peaking in summer when tourist numbers are highest. The Guardian also quoted Dr. Paula Pebsworth, a primatologist, who said geophagy—the intentional eating of soil—serves multiple purposes, including detoxification and mineral supplementation.

Perspectives

The study's authors suggest that soil-eating may help monkeys rebalance their gut microbiomes, which become disturbed by the fatty, salty, and sugary snacks. "We think that eating this junk food disrupts the composition of the microbiome and we know that bacteria and minerals in soil can help recompose the microbiome and alleviate the negative effects," Lemoine told The Guardian. This hypothesis is framed as a protective effect, but the researchers acknowledge that more research is needed to fully understand the impact on gut bacteria.

The Guardian also noted a human parallel: in some parts of the world, pregnant women eat soil to help with nausea or to obtain critical minerals, a practice that has been observed across cultures. This comparison suggests that geophagy is not unique to monkeys.

Some experts might argue that the most effective management approach would be to reduce or eliminate the provisioning of human foods to the monkeys, rather than relying on soil-eating as a coping mechanism. However, this view is not explicitly stated in the covered reports.

No sources disputed the findings, and the study appears to be based on solid observational data. The Jerusalem Post reported that the only macaque group not seen eating soil was isolated from tourists, which supports the link between tourist food and geophagy.

The study adds to a growing body of evidence about how wildlife adapts to human-altered environments, and it underscores the unintended consequences of feeding wild animals, even when done with good intentions.