Gibraltar's Macaques: Self-Medicating with Soil to Combat Junk Food Effects

A recent study conducted by researchers from the University of Cambridge has revealed that Barbary macaques living on the Rock of Gibraltar have developed a habit of consuming soil, known as geophagy, to alleviate digestive issues caused by junk food provided by tourists. The study monitored approximately 230 macaques across eight distinct groups from August 2022 to April 2024. It found that these monkeys, which are the only free-ranging monkey population in Europe, consume soil more frequently in areas with high tourist traffic, particularly during peak holiday seasons.

Daily MailScientific AmericanMonkeys ‘self-medicate’ to help them digest tourists’ junk foodGibraltar monkeys 'eat soil to calm stomachs after junk food'ReutersThe GuardianGibraltar Macaques Eat Soil to Counteract Tourist Junk Food Effects

The macaques' diets have shifted significantly due to the availability of human snacks, including chocolate bars, crisps, and ice cream. These foods are high in calories, sugar, and fat, contrasting sharply with their natural diet of herbs, leaves, and seeds. The study indicated that nearly 20% of the monkeys' food intake consisted of junk food from tourists. Researchers observed that the frequency of soil consumption peaked during summer months when tourist numbers were highest, with monkeys in tourist-heavy areas being 2.5 times more likely to consume these unhealthy snacks.

monkeys: Tourist snacks are making the Rock of Gibraltar's monkeys sick, and their solution will make you laughThe IndependentThe GuardianGibraltar Macaques Eat Soil to Counteract Tourist Junk Food EffectsWIONMonkeys ‘self-medicate’ to help them digest tourists’ junk foodGibraltar monkeys 'eat soil to calm stomachs after junk food'Monkeys learn to self-medicate with soil to help them digest junk food

Dr. Sylvain Lemoine, the lead author of the study, explained that the consumption of soil may serve as a protective mechanism for the macaques' digestive systems. The soil is believed to help buffer the negative effects of high-energy, low-fiber foods by lining the gut and potentially providing beneficial bacteria and minerals absent from their processed diet. The study documented 46 instances of geophagy among 44 individual monkeys, with some cases occurring shortly after the monkeys consumed junk food.

Gibraltar Macaques Eat Soil to Counteract Tourist Junk Food EffectsMonkeys learn to self-medicate with soil to help them digest junk foodBBC Science FocusPeople Magazinemonkeys: Tourist snacks are making the Rock of Gibraltar's monkeys sick, and their solution will make you laughMonkeys ‘self-medicate’ to help them digest tourists’ junk foodThe Independent

The researchers noted that geophagy appears to be a socially learned behavior, with different macaque groups exhibiting preferences for specific types of soil. Most macaques favored the local red clay, known as terra rossa, while one troop preferred soil from potholes. This behavior is indicative of a developing animal culture shaped by their interactions with humans, akin to other learned behaviors observed in primates, such as nut-cracking in chimpanzees.

Monkeys ‘self-medicate’ to help them digest tourists’ junk food

While the study highlights an adaptive response to dietary changes, it raises concerns about the long-term implications of human food provisioning. Dr. Paula Pebsworth, a primatologist, emphasized the need for effective management strategies that focus on reducing or eliminating the feeding of human foods to the macaques. Additionally, there are risks associated with soil consumption, particularly as much of it is found near busy roads, which could expose the monkeys to pollutants.

Gibraltar Macaques Eat Soil to Counteract Tourist Junk Food Effects

The findings from this study underscore the complex relationship between humans and Gibraltar's macaques, illustrating how anthropogenic influences can shape animal behavior and culture. As these monkeys navigate the challenges posed by their changing diets, their practice of geophagy serves as a fascinating example of self-medication in response to human impact.

BBC Science Focus