{"id":555,"date":"2026-04-22T07:20:33","date_gmt":"2026-04-22T14:20:33","guid":{"rendered":"https:\/\/scienceblog.com\/wildscience\/?p=555"},"modified":"2026-04-22T07:20:33","modified_gmt":"2026-04-22T14:20:33","slug":"gibraltars-monkeys-are-eating-dirt-to-cope-with-tourists-junk-food","status":"publish","type":"post","link":"https:\/\/scienceblog.com\/wildscience\/2026\/04\/22\/gibraltars-monkeys-are-eating-dirt-to-cope-with-tourists-junk-food\/","title":{"rendered":"Gibraltar’s Monkeys Are Eating Dirt to Cope With Tourists’ Junk Food"},"content":{"rendered":"

An adult female stops mid-path on the upper slopes of the Rock of Gibraltar, crouches against a roadside cutting of exposed red clay, and begins picking small fragments loose with a precision grip, bringing them to her mouth one by one. Seven minutes earlier, she had finished someone’s ice cream. A Cambridge University researcher notes the time, the soil type, the social context. It is, in the dry language of primatology, a “geophagy event.” What it actually represents is something stranger: a monkey developing a folk remedy for junk food, on the fly, in real time, in front of tourists who have no idea they’re watching a cultural tradition being born.<\/p>\n

A new study published in Scientific Reports<\/em> documents, for the first time, the systematic practice of deliberate soil-eating among Gibraltar’s famous Barbary macaques, a population of roughly 230 animals spread across eight groups on the Rock. The behaviour, the researchers argue, is almost certainly a response to decades of being fed chocolate bars, crisps, and dairy ice cream by holidaymakers.<\/p>\n

A Diet Problem Nobody Planned For<\/h2>\n

Gibraltar’s macaques eat well by wild primate standards. Local authorities provision them daily with fruit, vegetables, seeds and fresh water. Tourists are forbidden to feed them. But visitors have been ignoring that rule for years, and the animals know it; troops living near the cable car station and Prince Philip Arch, where footfall is highest, have worked out that the humans who smell like sunscreen tend to carry snacks. Across the whole population and the study’s observation period, just under a fifth of all macaque feeding time was spent consuming tourist-derived food: chocolate bars, crisps, biscuits, salted seeds, ice cream. “Foods brought by tourists and eaten by Gibraltar’s macaques are extremely rich in calories, sugar, salt and dairy,” says Dr Sylvain Lemoine, the biological anthropologist at Cambridge’s Department of Archaeology who led the study. “This is completely unlike the foods typically consumed by the species, such as herbs, leaves, seeds and the occasional insect.”<\/p>\n

The problem is physiological. Non-human primates lose the ability to produce lactase after weaning, which means dairy products do to macaques roughly what they’d do to a severely lactose-intolerant person. And ice cream, notes Lemoine, is “hugely popular with Gibraltar’s tourists and consequently its macaques.” Sugar and fat in concentrations the macaque gut never evolved to handle. Something, apparently, had to give.<\/p>\n

What gave was a new tradition. Between August 2022 and April 2024, across 98 observation days, Lemoine’s team recorded 46 geophagy events in 44 distinct animals. That works out to roughly 12 events per week across the population, which puts Gibraltar’s macaques in the same league as East African chimpanzees and ring-tailed lemurs in Madagascar, both of which eat dirt for well-understood reasons: plant toxins in chimps’ case, gut microbiome management in lemurs’. The difference here is that the Gibraltar macaques developed the habit not in some remote forest, but on a limestone rock covered in gift shops and selfie-takers.<\/p>\n

What the Soil Actually Does<\/h2>\n

The leading explanation is protective. Clay minerals, particularly the iron-oxide-rich terra rossa that makes up almost 83% of the observed geophagy, are known to adsorb toxins and line the gut wall, physically limiting what crosses into the bloodstream. They can also buffer stomach pH and, some research suggests, shift the composition of gut bacteria. “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,” Lemoine says. The case for this interpretation is circumstantial but coherent. The behaviour tracks tourist season almost perfectly: geophagy rates were roughly 31% lower in winter, when visitor numbers fall and junk food access drops by about 40%. The three instances where researchers managed to observe the same individual eating tourist food and then eating soil (seven minutes after ice cream; six minutes after bread; 48 minutes after biscuits) are suggestive, even if the sample size is small. And the one group on the Rock with no tourist contact at all, the Middle Hill troop, showed zero geophagy whatsoever.<\/p>\n

A mineral deficiency explanation is harder to rule out. Gibraltar’s macaques eat far fewer insects than their wild counterparts in North Africa, and some groups receive provisioned food relatively low in fresh produce. Soil might be plugging nutritional gaps as well as settling stomachs. The two functions aren’t mutually exclusive. What the data does seem to rule out is that geophagy serves reproductive purposes: in many other primates, pregnant or lactating females eat more dirt, presumably to supplement calcium and iron, but Gibraltar’s females showed no such pattern.<\/p>\n

“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,” Lemoine points out. “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.” There’s something almost painfully familiar about that. The monkeys have found themselves, through no fault of their own, in the same metabolic bind that afflicts millions of humans, reaching for a palliative because the original problem isn’t going away.<\/p>\n

A Tradition in the Making<\/h2>\n

The cultural angle is perhaps the most intriguing part. Almost 90% of geophagy events took place with at least one other macaque watching from within 20 metres, including, in some cases, juveniles observing adults closely. About 30% involved multiple animals eating from the same outcrop in sequence. And crucially: different troops prefer different soils. The vast majority of the population favours terra rossa, the reddish clay that weathers out of Gibraltar’s Jurassic limestone wherever erosion has stripped the surface. But the Apes Den troop, which ranges the Rock’s lower western slopes, has developed a pronounced taste for tar, specifically the blackish asphalt that accumulates in road potholes. That group accounts for 70% of all tar-eating events despite having access to the same red clay outcrops as everyone else. In controlled experiments where researchers laid out trays with all four local soil types, Apes Den individuals consistently went for the tar first. The other groups didn’t.<\/p>\n

“The emergence of this behaviour in macaques is both a functional and cultural one, like nutcracking in chimps, except it is driven entirely by proximity to humans,” Lemoine says. That last clause is worth pausing on. Chimpanzees developed nutcracking over millennia of evolutionary pressure in forest environments. Gibraltar’s macaques seem to have developed their geophagy tradition within living memory, as a direct response to people ignoring a sign that says “Do Not Feed The Monkeys.”<\/p>\n

Lemoine’s team surveyed researchers at 26 other Barbary macaque study sites, from Morocco and Algeria to semi-captive parks in Germany, France, and the UK. Geophagy turned up at nine of them, usually occasionally and among juveniles; nowhere else approached Gibraltar’s frequency. The closest comparison is a provisioned macaque colony in Hong Kong with even more intense human food exposure, averaging over 33 geophagy events a week. The pattern is clear enough: the more junk food goes in, the more dirt comes after it.<\/p>\n

What none of this settles is the long-term picture. If the tourists keep coming (and they will; Gibraltar saw over 850,000 visitors to the Upper Rock in 2024 alone), the behaviour will presumably persist and perhaps spread. If the macaque population changes through immigration or demographic shifts, some traditions might be lost, as seems to have happened with Middle Hill, which reportedly showed occasional geophagy in the early 2000s when it had tourist contact, and none now. “Gibraltar’s macaques are deeply entwined with human history,” Lemoine notes, “offering a striking example of a human-primate interface.” That interface is, at this point, producing primate pharmacology. The monkeys have, in their way, figured out that if you’re going to eat ice cream, you’d better have something to settle your stomach afterwards.<\/p>\n

Source: Frater et al., “Geophagy in Gibraltar Barbary macaques is a primate tradition anthropogenically induced,” Scientific Reports<\/em>, 2026. DOI: 10.1038\/s41598-026-44607-0<\/a><\/p>\n

Frequently Asked Questions<\/h2>\n

Why are Gibraltar’s macaques eating soil?<\/strong><\/p>\n

Researchers think the behaviour is a protective response to the junk food they consume from tourists. Foods like ice cream, crisps, and chocolate are high in sugar, fat, dairy, and salt but low in fibre, disrupting the animals’ digestive systems. Clay minerals in the soil may help by lining the gut wall, absorbing harmful compounds, and potentially rebalancing gut bacteria.<\/p>\n

How do we know the soil-eating is linked to tourist food rather than something else?<\/strong><\/p>\n

Several lines of evidence point in the same direction. Geophagy rates rise during summer when tourist numbers peak and fall in winter when fewer snacks are available. The one monkey troop that has no tourist contact showed zero geophagy. Researchers also directly observed three instances where the same animal ate tourist food and then ate soil within an hour. Groups with the highest junk food exposure account for the overwhelming majority of soil-eating incidents.<\/p>\n

Could this behaviour actually harm the macaques?<\/strong><\/p>\n

The dirt-eating itself appears relatively safe, though the study doesn’t rule out long-term risks. A bigger concern is the underlying cause: the junk food diet the behaviour is compensating for. High-calorie, low-fibre human foods are known to alter gut microbiome composition in non-human primates, with consequences that aren’t fully understood. The geophagy may be managing symptoms rather than solving the root problem.<\/p>\n

Is this really a “cultural tradition” or just coincidence?<\/strong><\/p>\n

The researchers make a reasonable case for culture, at least in the loose sense used in animal behaviour studies. Different troops show consistent preferences for different soil types, regardless of what’s available; the Apes Den troop favours tar, while most others prefer red clay. Almost all geophagy events happen in the presence of watching conspecifics, and juveniles have been observed studying adults doing it. These are the conditions under which social learning is thought to spread feeding behaviours through primate groups.<\/p>\n

Are tourists causing lasting harm to the macaques by feeding them?<\/strong><\/p>\n

The study suggests the dietary disruption is real and significant. Close to 19% of macaque feeding time was spent on tourist-derived food, a substantial fraction for wild animals whose gut physiology evolved for a completely different diet. Whether that translates to long-term health consequences, or whether geophagy adequately compensates, is something researchers don’t yet know. Gibraltar’s authorities prohibit tourist feeding; the problem is enforcement.<\/p>\n","protected":false},"excerpt":{"rendered":"

An adult female stops mid-path on the upper slopes of the Rock of Gibraltar, crouches against a roadside cutting of exposed red clay, and begins picking small fragments loose with a precision grip, bringing them to her mouth one by one. Seven minutes earlier, she had finished someone’s ice cream. A Cambridge University researcher notes … Read more<\/a><\/p>\n","protected":false},"author":1298,"featured_media":556,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"jetpack_post_was_ever_published":false,"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"_jetpack_memberships_contains_paid_content":false,"footnotes":""},"categories":[5,10],"tags":[],"class_list":["post-555","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-animal-human-interaction","category-health-medicine"],"yoast_head":"\nGibraltar's Monkeys Are Eating Dirt to Cope With Tourists' Junk Food - Wild Science<\/title>\n<meta name=\"robots\" content=\"index, follow, max-snippet:-1, max-image-preview:large, max-video-preview:-1\" \/>\n<link rel=\"canonical\" href=\"https:\/\/scienceblog.com\/wildscience\/2026\/04\/22\/gibraltars-monkeys-are-eating-dirt-to-cope-with-tourists-junk-food\/\" \/>\n<meta property=\"og:locale\" content=\"en_US\" \/>\n<meta property=\"og:type\" content=\"article\" \/>\n<meta property=\"og:title\" content=\"Gibraltar's Monkeys Are Eating Dirt to Cope With Tourists' Junk Food\" \/>\n<meta property=\"og:description\" content=\"An adult female stops mid-path on the upper slopes of the Rock of Gibraltar, crouches against a roadside cutting of exposed red clay, and begins picking small fragments loose with a precision grip, bringing them to her mouth one by one. 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