Eating soil to survive tourists: an unexpected lesson from wildlife

If we saw a monkey eating dirt, we would probably think it was strange or accidental behavior. But what if this behavior were actually an adaptive response to a problem created by humans? This is the question that anthropologist Sylvain Lemoine and his collaborators, from various English, French, and Catalan universities and research centers, with the participation of researchers from the Catalan Institute of Human Paleoecology and Social Evolution (IPHES) at the Rovira i Virgili University, have posed after studying the macaques living on the Rock of Gibraltar. The results, recently published in the journal Scientific Reports, suggest that these primates deliberately eat dirt to alleviate the digestive effects of a diet altered by constant contact with tourists.The Barbary macaques, also known as the Gibraltar apes (Macaca sylvanus), constitute a unique population. They are the only non-human primates living in freedom in Europe. Although their origin is in North Africa, they have inhabited Gibraltar for centuries and today they are an inseparable part of the place's identity. Every year, hundreds of thousands of visitors come to the rock to observe them. Although feeding them is prohibited, many tourists continue to offer them biscuits, crisps, sweets, ice cream, or other processed products. In addition, these monkeys have also learned to appropriate visitors' food, and they take it from them when they have the opportunity.

At first glance, it might seem like a funny anecdote. But from a biological point of view, it is not. The natural diet of Barbary macaques is mainly based on fruits, leaves, seeds, roots, and other plant-based foods. It is a fiber-rich diet, and very different from what ultra-processed products that we humans often consume provide. The food they receive from tourists usually contains large amounts of sugars, fats, salt, and often dairy derivatives. Their digestive system has not evolved to regularly process these types of products and, as a consequence, they can have digestive alterations and changes in their intestinal microbiota that harm their health.A particular behavior

In this context, the researchers realized that the groups of macaques most exposed to tourists showed a particular behavior: they ate soil much more frequently than other populations of the same species. The practice of consuming soil is called geophagy. It may seem surprising, but it is widely documented in numerous animal species, including mammals, birds, and even some human groups. In some cases, geophagy helps obtain minerals scarce in the diet. In others, clay particles act as a kind of chemical filter capable of binding to toxins or irritating compounds present in food.The authors of this study observed dozens of episodes of geophagia among Barbary macaques. Most animals specifically selected reddish, mineral-rich clay soils. This preference suggests that they do not eat soil randomly, but rather seek materials with specific properties.

There is growing evidence that many animals instinctively use natural substances to combat parasites, infections, or digestive disorders. Some birds, for example, incorporate aromatic plants into their nests to reduce the presence of microorganisms. Various primates consume certain leaves when suffering from intestinal infestations. And some mammals select foods rich in medicinal compounds when they are sick.At this point, it is important to highlight that these scientists do not claim that geophagia is a perfect cure or that it completely eliminates the effects of processed food. What they suggest is that it could act as a compensatory mechanism that helps these animals cope with a new problem in their evolutionary history.

An after behavior

The study also provides another fascinating insight: these behaviors could be transmitted socially. Macaques are highly social animals and learn many behaviors by observing their companions. Researchers detected differences between groups in the types of soil consumed and the frequency of geophagy. This suggests that social learning could play an important role in the spread of this behavior. In other words, macaques not only adapt individually to changes caused by humans, but they can also develop their own cultural traditions to cope with them.Perhaps the most interesting conclusion is not that macaques eat soil, but why they do it. We often imagine that human influence on wildlife is limited to habitat destruction or climate change. But the effects can be much more subtle. Our daily actions can modify diets, social behaviors, learning processes, and even animals' survival strategies. The presence of entire families of wild boars in many towns in Catalonia at night, rummaging through garbage containers, is a good example.