Monkeys in Gibraltar have learned to eat soil to settle their stomachs from all the junk food they consume, research suggests.
Scientists believe the dirt helps the monkeys line the gut to stop irritation from foods which are "extremely rich in calories, sugar, salt and dairy".
The soil also provides bacteria and minerals missing from junk food offered or stolen from tourists, such as chocolate bars, crisps and ice cream.
The snacks have negative digestive effects for the macaques and can cause symptoms from nausea to diarrhoea, but the food is "as delicious for them" as it is for humans, according to a Cambridge University study.
Animals in frequent contact with Gibraltar's visitors were observed to eat more dirt, with this behaviour higher during peak holiday season.
The researchers think the behaviour is likely to have been learned socially as different troops of monkeys have preferences for certain types of soil.
Experts said the soil acts as a "barrier" in the digestive tract and limits the absorption of harmful compounds.
Dr Sylvain Lemoine, a biological anthropologist from Cambridge's Department of Archaeology, added: "This could alleviate gastrointestinal symptoms from nausea to diarrhoea.
"Soil may also provide friendly bacteria that helps with the gut microbiome.
"Non-human primates become lactose intolerant after weaning, so dairy is known to cause digestive issues in monkeys, and ice cream is hugely popular with Gibraltar's tourists and consequently its macaques."
The junk diet was "completely unlike" foods normally eaten by the species, such as herbs, leaves, seeds and the occasional insect - with the behaviour being "driven entirely by proximity to humans".
Dr Lemoine said: "Humans evolved to seek out and store energy-dense fats and sugars to survive periods of scarcity, leading us to crave high-calorie junk food.
"Availability of human junk food could trigger this same evolutionary mechanism in macaques."
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Gibraltar's macaques number around 230 animals across eight stable groups that inhabit different areas of the Rock.
Scientists recorded 46 dirt-eating "events" in 44 different animals across 98 observation days between summer 2022 and spring 2024.
The study is published in the journal Scientific Reports.
(c) Sky News 2026: Monkeys eating soil to settle upset stomachs from junk food, says research
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