Gibraltar Macaques Eat Soil to Ease Junk Food Digestion

Barbary macaques on the Rock of Gibraltar have begun eating soil to alleviate digestive discomfort caused by junk food scavenged from tourists, according to a study by researchers at the University of Cambridge. The behavior, known as geophagy, was documented among the territory's approximately 230 wild monkeys, Europe's only free-living primate population, during observations from summer 2022 to spring 2024.

The study, published in Scientific Reports, analyzed 98 days of fieldwork across nine locations on the limestone outcrop. Researchers recorded 46 instances of soil consumption involving 44 individual macaques, averaging about 12 events per week per monkey. This rate is among the highest observed in primates. Monkeys in areas with frequent tourist contact, such as the summit trails, exhibited the behavior most often, accounting for 72 percent of incidents in three specific groups. In contrast, an isolated troop with no access to human-provided food showed no geophagy at all.

Junk food from visitors constitutes nearly 19 percent of the macaques' overall diet, far exceeding their natural intake of herbs, leaves, seeds and insects. Tourists, despite prohibitions, offer or leave...