Monkeys living on Gibraltar have developed a unique behavior: swallowing soil to alleviate stomach upsets caused by consuming sweet and salty snacks offered or stolen from tourists, according to new research.
Junk Food's Appeal and Consequences
Snacks such as chocolate bars, crisps, and ice cream are as appealing to the macaques as they are to humans, but they have negative digestive effects, a Cambridge University study suggests. Eating soil may allow the primates to continue indulging in these treats by helping to line the gut and prevent irritation from excessive sugar and fat.
Scientists believe the dirt also provides bacteria and minerals absent from junk food. Animals in frequent contact with tourists were observed to eat more dirt, with rates peaking during the holiday season. The behavior is likely learned socially, as different monkey troops show preferences for specific types of soil.
Protection Hypothesis
Dr. Sylvain Lemoine, a biological anthropologist from Cambridge’s Department of Archaeology and lead author of the study, explained: “We think the macaques started eating soil to buffer their digestive system against the high energy, low fibre nature of these snacks and junk foods, which have been shown to cause gastric upsets in some primates. Our findings are more supportive of this protection hypothesis.”
He elaborated that the consumed soil acts as a barrier in the digestive tract, limiting absorption of harmful compounds and potentially alleviating gastrointestinal symptoms from nausea to diarrhoea. Soil may also introduce beneficial bacteria to the gut microbiome.
Dietary Contrast
Dr. Lemoine noted that foods brought by tourists in Gibraltar are “extremely rich in calories, sugar, salt and dairy,” unlike the macaques’ natural diet of herbs, leaves, seeds, and occasional insects. He drew parallels to human evolution, where the drive to seek energy-dense foods leads to cravings for junk food. “Soil-eating may allow them to keep consuming food that has negative digestive effects, but is as delicious for them as it is for us. The emergence of this behaviour in macaques is both a functional and cultural one, like nutcracking in chimps, except it is driven entirely by proximity to humans.”
Observations and Data
Gibraltar’s macaque population averages an estimated 12 soil-eating “events” per week, comparable to the highest reported rates. Three instances were observed directly after the animals consumed tourist food: ice cream (seven minutes earlier), biscuits (48 minutes earlier), and bread (six minutes earlier). The macaques number around 230 animals across eight stable groups inhabiting different areas of the Rock.
During 98 observation days between summer 2022 and spring 2024, scientists recorded 46 dirt-eating “events” in 44 different animals. The study is published in the journal Scientific Reports.
