Gibraltar macaques eat soil to counteract the effects of a human-based diet

Gibraltar macaques that consume processed foods provided by tourists have developed a surprising strategy to cope with their effects: ingesting soil to alleviate digestive discomfort. This behaviour, known as geophagy, may act as a natural protective mechanism against a diet rich in sugars, fats and dairy products, which differs greatly from their natural feeding habits.

This is the main conclusion of a study published in Scientific Reports, led by Dr Sylvain Lemoine (University of Cambridge), with the participation of Dr Federica Landi, associated researcher at IPHES-CERCA. The research provides the first systematic documentation of this behaviour in the Gibraltar population of Barbary macaques (Macaca sylvanus) and analyses its relationship with the consumption of human-derived food in a context of intense human–primate interaction.

A response to a human-altered diet

In natural conditions, Barbary macaques follow a mostly plant-based diet consisting of fruits, leaves, seeds and occasionally insects. However, in Gibraltar these primates also consume a significant proportion of processed foods provided by visitors or taken directly from them, such as biscuits, crisps, bread or ice cream.

According to the study, up to 18.8% of feeding time is associated with tourist-derived food. This diet, low in fibre and high in sugars, fats, salt and dairy, represents a drastic shift from their natural diet and can lead to digestive disturbances. In particular, lactose (which primates stop metabolising after weaning) can cause gastrointestinal discomfort such as diarrhoea.

In this context, soil ingestion appears to be an adaptive strategy. Researchers suggest that certain types of soil, especially red clay (terra rossa), may act as a protective barrier in the gut by absorbing harmful compounds, regulating pH levels and alleviating symptoms associated with processed food consumption. It may also help restore gut microbiome balance.

A frequent behaviour linked to tourism pressure

Researchers recorded 46 geophagy events between 2022 and 2024, with an average of more than 12 events per week one of the highest frequencies ever documented in primates. This behaviour is therefore not anecdotal, but a regular part of the Gibraltar macaques’ behavioural repertoire.

The analysis reveals a clear link between human-derived food consumption and geophagy. Events increase when macaques consume more tourist food and peak during summer, coinciding with the highest visitor numbers. Groups living in heavily visited areas show significantly higher rates.

In contrast, no geophagy has been observed in groups without human contact, highlighting the key role of anthropogenic pressure.

A form of self-medication with cultural dimensions

Geophagy is widely documented across animal species and may serve various functions, such as mineral supplementation or toxin neutralisation. In Gibraltar macaques, however, results mainly support a protective digestive function linked to processed food consumption.

The study also suggests a cultural dimension to this behaviour. Different groups show distinct preferences for soil types, with most favouring terra rossa, while some display a tendency to ingest other materials such as asphalt fragments.

Furthermore, around 90% of events occur in the presence of other individuals, suggesting social learning and the potential transmission of this behaviour across generations.

An example of human impact on animal behaviour

This research highlights how human–wildlife interactions can profoundly affect animal behaviour, diet and health strategies.

It also raises important questions about wildlife management in tourist environments and the need to prevent feeding wild animals, as this can significantly alter their natural behaviour.

Reference

Frater, J., Nicourt, M., Landi, F., Maxwell, B., Thiodet, J., Mestrallet, E., Warr, S. J., Pizarro, M., Fa, J. E., Lemoine, S. (2026). Geophagy in Gibraltar Barbary macaques is a primate tradition anthropogenically induced. Scientific Reports, 16, 13139.

https://doi.org/10.1038/s41598-026-44607-0