The only wild primates found on the European continent eat soil to buffer their digestive system from low-fibre junk food like crisps, chocolate, bread, and ice cream, according to a new study.
The team of scientists who brought out the research said it is the first time that they witnessed and documented this phenomenon known as ‘geophagy’ — the deliberate consumption of earth —among the populations of Barbary macaque (Macaca sylvanus) living in Gibraltar, a British overseas territory located off the southern tip of Iberia.
The study said a survey of 26 other Barbary macaque sites showed that while geophagy exists elsewhere, the Gibraltar population is unique in its extreme frequency and its specific link to human diet.
The findings published in Nature Scientific Reports state that high energy tourist-derived food comprising high calories, sugar, salt, and dairy consisted nearly 18.8 per cent of the monkeys’ diet which lacked essential minerals. This was likely causing stomach indigestion and irritation such as nausea and diarrhoea.
So, to buffer against health issues, soil is seen to act as a barrier in the digestive track, creating a lining in their gut to limit absorption of harmful compounds. The scientists said soil may also be serving to supply gut friendly bacteria to improve the gut microbiome. Geophagy acts as a form of “medication” to absorb toxins, adjust gut pH, or mitigate digestive discomfort, they noted.
The analysis was conducted between August 19, 2022, and April 12, 2024, across five field seasons with 98 observation days where scientists recorded 46 geophagic events in the macaque populations.
Of the 46 events, the macaques consumed red soil (terra rossa) outcrops in 38 (comprising 82.6 per cent of the cases), followed by three instances of yellow soil and black soil (6.5 per cent each) and two cases of tar (4.3 per cent).
Researchers also documented some instances of geophagy soon after these macaques consumed tourist-derived food. They noted, “In three of the 20 cases (15 per cent), geophagy directly followed consumption of biscuits which occurred 48 minutes earlier. Consumption of dairy ice cream was observed seven minutes earlier and bread six minutes earlier.”
Macaques in areas with higher tourist flow — that is Gibraltar Rock, were 2.5 times more likely to consume junk food compared to animals living in other areas. “Middle Hill, the only group without tourist contact and no longer consuming human food showed none, but this could be due to lower observation time than in the touristic groups,” the study said.
Local officials provide these macaques with daily stock of fruits, vegetables and water in assigned feeding spots and tourists are forbidden to offer any food.
But the increase in geophagy aligned with peak tourist season. Geophagy in summer was highest at 56.5 per cent and reduced in winter at 39.1 per cent.
These frequency of geophagy was exceptionally high with 12 instances per week, a rate identified as ‘very frequent’ among the highest recorded for the Macaca genus and exceeding high in wild populations.
The team noted, “For example, in a population-level investigation of salt lick visitation in Amazonian large-bodied mammals, using camera-traps, it was found that agouti, paca, collared peccary, Brazilian porcupine and tapirs visited salt licks between 24.2 and 15.4 times per 100 camera nights, corresponding to 1.7 to 1.08 times per week. Interestingly, the same study found that red howler monkeys were the most selective species with a mean frequency visit of 8.2 visits per 100 camera nights.”
It added however that Gibraltar geophagy rates remain much lower than the extreme rates observed in snowshoe hares in Alaska that visit salt licks nightly between 3 and 9 times per night.
The study found little support for the supplementation hypothesis, as geophagy did not increase during nutritionally demanding periods like pregnancy or lactation. The paper also said this behaviour is a socially learned habit specific to certain populations or groups, not necessarily tied to a biological need.
Nearly 90 per cent of geophagy events occurred in the presence of other macaques, providing ample opportunities for infants and juveniles to observe and learn the behaviour, the study said.
In conclusion, the paper said the research highlights the need to consider both ecological pressures and cultural processes in primate foraging within changing environments. “Future research integrating chemical, mineralogical, microbiome, and long-term social data will clarify the physiological benefits of soil ingestion and track its transmission, persistence or loss across populations,” it said.
The paper added that the geophagy in Gibraltar is not merely a feeding anomaly, but a functional and cultural response to an anthropogenic landscape, underscoring the adaptive flexibility of Barbary macaques and the strong influence of humans on primate behaviour.