Gibraltar's Monkeys Are Eating Mud to Cope with Tourists' Junk Food

The Rock's Most Famous Residents Have a Problem

Gibraltar's Barbary macaques — the only wild primates in Europe and one of the territory's most iconic attractions — are in trouble. New research has revealed that the monkeys are eating mud in what scientists believe is an attempt to soothe digestive problems caused by the junk food being fed to them by tourists.

Despite clear instructions to visitors not to feed the animals, holidaymakers continue to hand over crisps, chocolate, ice cream, salty peanuts, bread and sugary snacks — food that the monkeys' digestive systems are simply not designed to handle. The result, scientists now believe, is that the macaques have developed a behaviour seen elsewhere in the animal kingdom: eating soil and mud as a form of self-medication.

What the Research Found

The study was conducted between summer 2022 and spring 2024, tracking the behaviour of Gibraltar's Barbary macaque population. The findings are striking:

• 44 monkeys were observed eating soil on 46 separate occasions
• Almost one fifth of the monkeys' overall food intake was coming from tourist handouts
• Monkeys living closest to the top of the Rock — where tourist numbers are highest — were twice as likely to consume tourist food and to eat mud
• During the high tourist season, mud consumption increased significantly; in winter, when visitor numbers drop, animals were 40% less likely to eat tourist food and 30% less likely to consume soil — a clear correlation
• In three specific cases, a monkey was observed eating mud immediately after consuming ice cream, sweets or bread — suggesting a direct cause-and-effect relationship

Why They're Eating Mud

Geophagy — the deliberate eating of soil or clay — is a behaviour documented in many animals, including primates, elephants and birds. It is thought to serve several purposes: neutralising toxins, supplementing minerals, and coating the gut lining to reduce the effects of irritating foods.

In the case of Gibraltar's macaques, the researchers believe the mud consumption is a response to the discomfort caused by greasy, salty and sugary tourist food — effectively the animals' attempt to treat their own upset stomachs.

The problem is that while the mud may reduce some of the negative effects of a bad diet, it potentially exposes the animals to soil-borne pollutants and contaminants — introducing a different set of health risks in place of the ones it is trying to address.

What Tourists Are Doing Wrong

The list of foods being handed to the macaques reads like a festival snack menu:

• Crisps
• Chocolate
• Ice cream
• Salty peanuts
• Bread
• Juice and sugary drinks

None of these items feature in the natural diet of a Barbary macaque. The local authorities do their part — providing the animals with daily nutrition through fruit, vegetables and seeds — but tourist feeding is undermining those efforts significantly, with nearly 20% of the animals' diet now coming from handouts.

The Rules Are Clear — But Ignored

Feeding Gibraltar's macaques is explicitly prohibited. Visitors to the Upper Rock Nature Reserve are informed of this rule, and signage makes it clear. The behaviour nonetheless continues, driven by the irresistible appeal of getting up close to a wild primate who will happily take food from your hand.

The consequences, however, are borne entirely by the animals. For a macaque, the short-term reward of a handful of crisps comes with digestive distress, nutritional disruption, and — as this research shows — the need to eat mud to cope with the aftermath.

A Warning for Visitors

If you are visiting Gibraltar and encounter the macaques — which you almost certainly will, as they roam freely and are famously unafraid of humans — the message from scientists and wildlife authorities is unambiguous: do not feed them. Not even a small piece of bread. Not a corner of a biscuit. Nothing.

The monkeys will approach you regardless. They are curious, bold and entirely comfortable around people. Watch them, photograph them, enjoy the remarkable experience of being in the presence of wild primates — but keep your snacks to yourself.

This article is based on reporting from The Olive Press, published April 23, 2026.