Monkeys ‘learn to eat soil to cope with junk food’

Research from Cambridge University suggests that monkeys on Gibraltar have developed the habit of consuming soil to ease stomach discomfort caused by eating snacks stolen from or offered by tourists. These foods, including chocolate bars, crisps, and ice cream, are highly palatable but cause digestive issues for the macaques. The study highlights how soil-eating might help the primates manage the effects of high sugar and fat intake.

Dr Sylvain Lemoine, a biological anthropologist, described the behavior as both functional and cultural, drawing parallels to nutcracking in chimpanzees. However, he emphasized that this practice is driven by monkeys’ frequent interaction with humans. Observations noted that among Gibraltar’s 230 macaques, those in close contact with tourists exhibited more soil-eating, with activity peaking during the busy holiday season.

“The emergence of this behavior in macaques is both a functional and cultural one, like nutcracking in chimps, except it is driven entirely by proximity to humans,” said Dr Sylvain Lemoine.

Scientists believe the soil may provide essential bacteria and minerals not found in the monkeys’ usual diet of herbs, leaves, seeds, and insects. The consumed soil acts as a protective barrier in the digestive tract, reducing the absorption of harmful compounds from junk food. Lemoine added that human snacks are “extremely rich in calories, sugar, salt, and dairy,” with some primates struggling to tolerate lactose in ice cream.

“We think the macaques started eating soil to buffer their digestive system against the high energy, low fibre nature of these snacks and junk foods, which have been shown to cause gastric upsets in some primates,” added Lemoine. “Soil-eating may allow them to keep consuming food that has negative digestive effects, but is as delicious for them as it is for us.”

The study found an average of 12 soil-eating instances per week among Gibraltar’s macaque population, with three cases recorded directly after animals ingested tourist food. This behavior appears to be learned socially, as different groups show preferences for specific types of soil. The findings raise questions about how human food availability might trigger evolutionary mechanisms in these primates, similar to those in humans.

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