Monkeys living on Gibraltar have developed a unique coping mechanism for their junk food habit: swallowing soil to quell upset stomachs caused by sweet and salty snacks offered or stolen from holidaymakers.
Study Reveals Soil-Eating Behavior
A Cambridge University study found that while snacks such as chocolate bars, crisps and ice cream are “as delicious for them” as they are for humans, they have negative digestive effects for the macaques. Research indicates that eating soil may allow the primates to keep eating junk food by helping to line the gut to prevent irritation from too much sugar and fat.
Scientists also believe the dirt provides bacteria and minerals absent from their processed diet. Animals in frequent contact with tourists were observed to eat more dirt, and dirt-eating rates were seen to be higher during the peak holiday season. The researchers think the behaviour is likely to have been learned socially as different troops of monkeys have preferences for certain types of soil.
Expert Insights
Dr Sylvain Lemoine, a biological anthropologist from Cambridge’s Department of Archaeology, who led the study, said: “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. 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.”
Impact of Tourist Food
He said that foods brought by tourists in Gibraltar and eaten by macaques there are “extremely rich in calories, sugar, salt and dairy”. “This is completely unlike the foods typically consumed by the species, such as herbs, leaves, seeds and the occasional insect,” said Dr Lemoine.
“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. 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.”
Observations and Statistics
Gibraltar’s macaque population averaged an estimated 12 soil-eating “events” a week, according to the study, which is “comparable to the highest reported rates”. Three instances of soil-eating were observed directly after an animal consumed tourist food: ice cream (seven minutes earlier); biscuits (48 minutes earlier) and bread (six minutes earlier).
Gibraltar’s macaques number around 230 animals across eight stable groups that inhabit different areas of the Rock. During a total of 98 observation days between summer 2022 and spring 2024 the scientists recorded 46 dirt-eating “events” in 44 different animals. The study is published in the journal Scientific Reports.
