Macaques have learned to eat soil to avert gut irritation caused by salty and sugary snacks, researchers believe



Troops of monkeys living on the Rock of Gibraltar have learned to eat soil in what scientists believe is an effort to settle their stomachs after all the junk food they receive – and sometimes steal – from crowds of tourists.

Researchers spotted the intentional mud eating, known as geophagy, while observing groups of Barbary macaques in the territory. Monkeys that had the most contact with tourists ate the most soil and consumption peaked in the holiday season, they found.

About 230 macaques live on Gibraltar in eight distinct groups, and while local authorities provide them with daily helpings of fruit, vegetables and seeds, tourists routinely feed them snacks ranging from bags of chips and chocolate bars to M&M’s and ice-creams.

The observations don’t prove why the monkeys eat soil, but scientists suspect it has a protective effect on the digestive system. The only macaques on the rock that were not seen eating soil belonged to a group that is isolated from visitors and tourists.

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Writing in Scientific Reports, the researchers describe how the monkeys appear to learn the habit from others, with macaques favouring different types of soil depending on their troop. Most monkeys search out the terra rossa, or red clay, found across Gibraltar, but the Ape’s Den troop, which occupies the lower western slopes, favours tar-clogged soil from potholes in asphalt roads.

Humans around the world eat soil, particularly pregnant women in parts of Africa, Asia and South America, where it is consumed to help with nausea or to provide critical minerals. But the researchers saw no rise in soil-eating among pregnant or lactating monkeys, suggesting the behaviour is not driven by a need to supplement their diets.

Instead, Lemoine said the macaques seemed to eat the soil to “buffer their digestive system” against high-energy, low-fibre snacks and junk foods that are known to cause stomach upsets in some primates.

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