Geophagy in Gibraltar Barbary macaques is a primate tradition anthropogenically induced.

We report, for the first time, geophagy – the deliberate consumption of earth – in the Barbary macaque (Macaca sylvanus) population living at the human-primate interface in Gibraltar. We evaluate potential adaptive functions of this behavior in an anthropogenic context, drawing on predictions from the protection and supplementation hypotheses. Geophagy occurred at exceptionally high rates compared to other macaque species and locations, and it was more common in summer when tourist numbers peak. It was also more likely when macaques consumed greater amounts of tourist-derived food, supporting a protective function. Local ecological factors contributed as well, with the distribution of red soil (terra rossa) influencing geophagy frequency. Although the behavior was not linked to female reproductive status, supplementation cannot be fully dismissed given the very limited insectivory in this population. We propose that tourist-derived foods may disrupt gut microbiome composition, producing discomfort that individuals mitigate through soil ingestion. Geophagy is likely socially learned, as groups showed consistent preferences for specific soil types, and its practice in presence of conspecifics offered opportunities for social learning. Reports from other sites indicate that geophagy is not unique to Gibraltar, but in this population it appears to be anthropogenically induced and socially transmitted, forming a locally maintained tradition.