Monkeys in Gibraltar have developed a striking survival tactic: eating soil to soothe stomach troubles caused by their junk food diet. This unusual behavior helps protect their digestive system from irritation triggered by the calorie-dense, sugary, salty, and dairy-laden snacks they obtain from tourists.
The Role of Soil in Soothing Macaque Stomachs
Researchers from Cambridge University reveal that the soil lines the monkeys’ digestive tracts, acting as a protective barrier that reduces the absorption of harmful compounds from unhealthy foods. Beyond this, the dirt supplies essential bacteria and minerals missing from the human treats monkeys consume, such as chocolate bars, crisps, and ice cream.
These foreign snacks wreak havoc on the macaques’ digestion, causing symptoms ranging from nausea to diarrhea. Yet, the monkeys find these foods as irresistible as humans do, which drives their persistent consumption despite the negative effects.
Social Learning and Seasonal Behavior
The soil-eating habit spikes during peak tourist seasons when monkeys encounter more visitors offering or leaving behind junk food. Scientists observed 46 instances of dirt-eating across 44 different monkeys during 98 days of study from summer 2022 to spring 2024. The behavior varies between troops, suggesting it is socially learned, with some groups preferring specific soil types.
Expert Insights on Macaque Adaptation
Dr. Sylvain Lemoine, a biological anthropologist at Cambridge, explains that the soil acts as a buffer against gastrointestinal distress, easing symptoms such as nausea and diarrhea. He also notes that soil may enrich the gut microbiome by introducing beneficial bacteria.
“After weaning, non-human primates become lactose intolerant, so dairy products like ice cream—popular with Gibraltar’s tourists and its macaques—can cause significant digestive trouble,” Dr. Lemoine said.
Junk Food vs. Natural Diet
The macaques’ diet traditionally consists of herbs, leaves, seeds, and occasional insects—foods vastly different from the human junk food they now frequently consume. This dietary shift is driven entirely by their close contact with humans.
Dr. Lemoine highlights an evolutionary perspective: “Humans evolved to crave and store energy-dense fats and sugars to survive scarcity. The availability of human junk food triggers similar cravings in macaques, exploiting the same survival mechanisms.”
Population and Study Details
Gibraltar hosts approximately 230 macaques divided into eight stable troops across the Rock. The dirt-eating behavior, documented extensively in the new study published in Scientific Reports, reflects the species’ remarkable adaptability in response to human influence.
