Scientists observed intentional mud eating, known as geophagy, while studying groups of Barbary macaques in Gibraltar. Nearly a fifth of all food consumed by the macaques between summer 2022 and spring 2024 was junk food from tourists, including chips, chocolate bars, M&M's, and ice-creams. Monkeys that had the most contact with tourists ate the most soil, and consumption peaked in the holiday season. The only macaques not seen eating soil belonged to a group isolated from visitors. When visitor numbers fell in winter, monkeys were 40% less likely to eat tourist food and more than 30% less likely to eat soil.
Researchers recorded 44 monkeys eating dirt on 46 occasions. In three instances, macaques ate soil shortly after being fed ice-cream, biscuits or bread. Dr Sylvain Lemoine, a primate behavioural ecologist at the University of Cambridge, said: "We think that eating this junk food disrupts the composition of the microbiome and we know that bacteria and minerals in soil can help recompose the microbiome and alleviate the negative effects." He added: "We think there's a protective effect of the soil." Macaques appear to learn the habit from others, with different troops favoring different types of soil. Most search out terra rossa (red clay), but the Ape's Den troop favors tar-clogged soil from potholes in asphalt roads.
We think that eating this junk food disrupts the composition of the microbiome and we know that bacteria and minerals in soil can help recompose the microbiome and alleviate the negative effects.
We think there's a protective effect of the soil.
There's a lot of ice-cream. They love Magnums and Cornettos. What they don't like very much is sorbet.