A University of Cambridge researcher has found that monkeys in Gibraltar have learned to eat soil to cope with the digestive problems caused by eating junk food.

Dr Sylvain Lemoine, a professor of biological anthropology, found that the practice known as ‘geophagy’ helps to line the primates’ stomachs. This prevents the irritation that would otherwise be caused by the calorific, high-sugar and high-fat foods left by tourists.

The research revealed that different troupes of monkeys have preferences for different types of soil, indicating that the behaviour is socially learned rather than natural. Most macaques prefer the red soil common in most of Gibraltar.

There are roughly 230 macaques in Gibraltar, the only free-ranging monkey population in Europe. While local authorities provide daily servings of fruit, vegetables, and seeds, the monkeys also routinely eat junk food, such as crisps, chocolate, and ice cream. These foods are both intentionally left by, and stolen from, visitors.

The research was published in the journal Scientific Reports in March this year.

The study shows that the monkeys learn these habits from other monkeys, rather than doing them naturally. Geophagy was more likely to occur in groups, with the majority of cases taking place when two or more macaques were together.

In a University of Cambridge press release, Lemoine said: “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”.

He added that, in the primates’ case, the “availability of human junk food could trigger this same evolutionary mechanism […] to keep consuming food that has negative digestive effects, but is as delicious for them as it is for us”.

Although tourists are told not to interact with the monkeys, this rule is often not enforced. Fast food left by visitors accounts for approximately a fifth of the food consumed by the monkeys.

Eating soil enables the macaques 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,” Lemoine continued.

The study concluded that “Gibraltar geophagy illustrates how human–primate interfaces create new challenges, shape diet, and foster behavioral traditions,” and that the behaviour “underscor[es] […] the strong influence of humans on primate behavior”.