In Gibraltar, a Strange Trend Among Monkeys Confronted with Junk Food

An unexpected trend is emerging among the Barbary macaques on the Rock of Gibraltar. Likely to relieve their digestive system, assaulted by salty or sugary foods left by tourists, they are gulping down large quantities… of dirt.

Anti Junk-Food Remedy?

There are about 230 Barbary macaques (Macaca sylvanus) in Gibraltar, at the southern tip of the Iberian Peninsula. Over nearly a hundred days between 2022 and 2024, scientists recorded 44 individuals eating dirt.

This behavior is markedly more common in tourist zones and during peak season, prompting researchers to wonder whether it is linked to food discarded by visitors to the Rock, including ice cream, chocolate bars, or chips.

“We hypothesize that the junk food can be difficult to digest and disrupt the primates’ microbiome,” write the authors of the new study, published in the journal Scientific Reports.

In the absence of chemical and mineral analyses of Gibraltar’s earth, the team does not know precisely how it affects the digestive tract. They note, however, that the “terra rossa” is rich in clay and iron, and that the former can help reduce gastric acidity by binding toxins.

A Remarkable Capacity for Adaptation

According to researchers, this trend has developed too quickly to be the result of genetic or physiological changes. It would therefore be a socially learned and transmitted behavior, with groups of primates showing a preference for certain soil types.

Alongside observations of bonnet macaques in Indian urban areas, who learned to open soda bottles, and Bali long-tailed macaques, who pilfer tourists’ belongings to obtain food, these new findings once again illustrate the remarkable adaptability of these primates.

Such behavioral flexibility could prove valuable in facing the deep environmental upheavals caused by climate change.

Previously, a long-running study had revealed that Bali macaques also used stone tools as… sex toys.