A macaque on the Rock of Gibraltar clutches a tube of crisps. A macaque on the Rock of Gibraltar clutches a tube of crisps.
Monkeys in a tourism hotspot have learned that swallowing dirt can quell the upset stomachs caused by overconsumption of sweet and salty snacks fed to them by holidaymakers, a University of Cambridge study suggests.
Troops of macaques living on Gibraltar – the only free-ranging monkey population in Europe – have been scientifically observed for the first time regularly engaging in “geophagy”: the practice of intentionally ingesting soil.
Researchers monitoring monkey groups across the Rock of Gibraltar have tracked instances of geophagy, and found that animals in frequent contact with tourists eat far more dirt, and that dirt-eating rates are higher during peak holiday season.
The scientists believe that the chocolate, crisps and ice cream offered by or stolen from tourists – a substantial part of some Gibraltar macaques’ diets – are disrupting “gut microbiome composition” in the animals and leading to changes in their culture.
Eating soil may help rebalance monkey stomachs by providing bacteria and minerals absent from junk food, say researchers, and it is likely to help line the gut and soothe or prevent irritation caused by too much sugar and fat.
A macaque chews on red clay soil gathered from an outcrop on the Rock of Gibraltar. A macaque chews on red clay soil gathered from an outcrop on the Rock of Gibraltar.
Scientists think this behaviour is transmitted socially, as different troops have preferences for certain types of soil, and say it is an example of an emerging animal culture and “tradition” created by living in a human-dominated environment.
“Foods brought by tourists and eaten by Gibraltar’s macaques are extremely rich in calories, sugar, salt and dairy,” said Dr Sylvain Lemoine, a biological anthropologist from Cambridge’s Department of Archaeology, who led the study, published in the journal Scientific Reports.
“This is completely unlike the foods typically consumed by the species, such as herbs, leaves, seeds and the occasional insect.”
“Humans evolved to seek out and store energy-dense fats and sugars to survive periods of scarcity, leading us to crave high-calorie junk food,” said Lemoine.
“Availability of human junk food could trigger this same evolutionary mechanism in macaques. 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”
Dr Sylvain Lemoine
Gibraltar’s macaque population averaged 12 soil-eating “events” a week, at the high end of recorded geophagy frequency in primates, and comparable to chimpanzees and lemurs, which eat dirt for minerals and to mitigate against plant toxins.*
Geophagy is common in some human cultures, often associated with nutrient intake during pregnancy. However, researchers found no increase in macaque soil-eating during pregnancy or lactation, suggesting it is not driven by a need to supplement.
“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,” said Lemoine.
“Our findings are more supportive of this protection hypothesis. The consumed soil acts as a barrier in the digestive tract, limiting absorption of harmful compounds. This could alleviate gastrointestinal symptoms from nausea to diarrhoea. Soil may also provide friendly bacteria that helps with the gut microbiome.”
“Non-human primates become lactose intolerant after weaning, so dairy is known to cause digestive issues in monkeys, and ice cream is hugely popular with Gibraltar’s tourists and consequently its macaques,” said Lemoine.
A few instances of observed geophagy came shortly after researchers had watched the same macaque consume bread and ice cream.**