Gibraltar Macaques Project
Frater, J., Nicourt, M., Landi, F. et al. Geophagy in Gibraltar Barbary macaques is a primate tradition anthropogenically induced. Scientific Reports 16, 13139 (2026).
Abstract: 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.
Jeanne Thiodet (2025). Impact of Anthropogenic Pressure on the Behaviour of Barbary Macaques (Macaca sylvanus) in Gibraltar. Master Thesis, University Paris Sorbonne Nord
Abstract: Wildlife tourism is an increasingly widespread practice throughout the world. Primates are particularly popular with visitors because of their behavioural and physiological proximity to humans. In this study, we investigate the impact of anthropogenic pressure from visitors on the behaviour of Barbary macaques (Macaca sylvanus) in the Upper Rock Nature Reserve in Gibraltar. We collected data for 2 months from January to March 2025, on the number of tourists, vehicles, human-macaque interactions and noise intensity in 10 different sites, and on the impact of these anthropogenic pressures on the behaviour of 47 individuals (self-centred behaviour, agonistic behaviour and affiliative behaviour). The results showed a heterogeneous spatialisation of anthropogenic pressure according to the different sites. We showed a negative association between the number of visitors and the number of yawns, and a positive association between the number of vehicles and the number of yawns, as well as a positive associative trend between self-scratching and the number of visitors. These results suggest that certain self-directed behaviours, such as yawning and self-scratching, may be sensitive to specific indicators of anthropogenic pressure, while affiliative and agonistic behaviours appear to be less directly affected, highlighting a possible modulation linked to other factors (hierarchy, intra-group pressure, etc.). By highlighting the diversity of behavioural responses to gradients in tourist numbers, this study highlights the need for differentiated management of sites according to their level of exposure, in order to preserve the well-being of the macaques and promote sustainable cohabitation with visitors.
Eliott Mestrallet (2025). Behavioural Adaptations and Tourist-Macaque Interactions: A Comparative Study of Social Learning in Gibraltar's Barbary Macaques Master Thesis, University Paris Saclay, University of Cambridge
Abstract: In Gibraltar's Upper Rock Nature Reserve, Barbary macaques (Macaca sylvanus) are daily exposed to high levels of human presence, particularly through tourism. This study investigates how macaques from four social groups navigate such anthropogenic pressure, combining systematic observational data with a controlled problem-solving task. Behavioural observations were collected using a structured location sampling protocol, revealing group-level differences in interaction rates, food-related behaviours, and pillaging tendencies. Groups situated near high-density tourist areas showed more frequent and prolonged interactions, especially when food was involved. An experimental task using a transparent pencil case filled with peanuts allowed for individual-level assessment of cognitive engagement. Results showed considerable variability in task interaction and success, which could not be explained by group origin or sex. Instead, behavioural indicators of stress, emerged as reliable predictors of problem-solving efficiency, with higher stress associated with longer latencies to success. These findings highlight the behavioural plasticity of Barbary macaques in human-modified environments, as well as the role of emotional state and social context in shaping their responses. Rather than displaying uniform strategies, individuals and groups differ in how they approach human artefacts and manage cognitive challenges. This study contributes to our understanding of primate adaptability in anthropogenic landscapes and underscores the need to account for emotional and ecological variables in conservation and wildlife management strategies.
Rosalind Wippell (2025). POPULATION SCALE AND INTER-INDIVIDUAL, SPATIAL PROXIMITY VARIATION IN BARBARY MACAQUES (MACACA SYLVANUS). Undergraduate dissertation, Archaeology Department, University of Cambridge
Abstract: Proximity may be used as a proxy when investigating social relationships between primates. Using social network analysis and generalised linear mixed models, this study investigated the variation in inter-individual proximity in five groups of Barbary macaques (Macaca sylvanus) and whether this was influenced by seasonality, grooming and rank differences in the groups. This has not previously been explored in Gibraltar.This study found that grooming and proximity were associated in most groups, indicating that those in closer proximity were more likely to groom. In some groups, male-male dyads were shown to be more spread than female-female dyads in winter compared to summer, and most groups showed males to be more spread than females overall, likely reflecting kinship between philopatric females. Rank difference had a significant effect on proximity in one group, where higher food competition may have resulted in stricter hierarchies. Group differences in spread were shown to reflect home-ranges and the importance of the anthropogenic environment in which the macaques live.
Thiodet-Barbier, J. & Lemoine, S. (2024). Extractive foraging adaptive strategies to an anthropized environment in Barbary macaques (Macaca sylvanus). 36th Meeting of the Societe Francophone de Primatology. Toulouse, France
Anthropized environments significantly affect the foraging behaviour of non-human primates, prompting adaptive strategies to exploit new resources, often of human origin. The Barbary macaque (Macaca sylvanus) in Gibraltar, which frequently interacts with tourists, provides an ideal case study for investigating these adaptive behaviours. This study focused on the macaques' extractive foraging abilities, particularly their use and manipulation of plastic bottles containing food or liquid. We conducted experiments with 46 individuals across six groups varying in their degree of contact with humans, presenting them with six conditions of open/close and solid/liquid/empty transparent plastic bottles. Results indicated a higher interest in bottles containing solids and liquids compared to empty bottles, with intragroup and human pressures influencing this interest. Closed conditions posed greater challenges, reflected in longer extraction latencies. Males were more likely to extract solid and liquid by biting to create holes, while females and individuals under higher social pressure preferred quicker, less labour-intensive methods. Smell was a crucial cue, especially for empty bottles, with males relying on olfactory cues more than females. Our findings highlight the Barbary macaques' behavioural flexibility and the impact of human interaction on their foraging strategies. These insights are essential for understanding primate adaptation in anthropized environments.
Frater, J. & Lemoine, S. (2023). The human-primate interface of Gibraltar: relevance and potential for ethnoprimatology and evolutionary studies. Primate Society of Great Britain, Spring meeting 2023, Cambridge, UK.
Gibraltar is hosting a population of eleven groups of free-ranging Barbary macaques (Macaca sylvanus) originating from North-Africa, and managed by the local authorities. The so-called "Apes of the Rock" constitute the main touristic attraction of this territory, and thus correspond to an important source of income. However, the macaques can also be a source of conflict with local inhabitants. This human-primate interface is inequal across the Rock, with some groups being in intense contact with tourists while others are not. This differential in human pressure, the presence of several neighbouring groups, and the accessibility of habituated non-human primates offer a unique opportunity for ethnoprimatological and evolutionary studies. Despite such potential, no long-term studies have yet been installed. The Gibraltar Macaques Project aims at filling this gap. Precisely, through the combination of historical, archival and ethnographical resources with newly and planned collection of behavioural, hormonal, isotopic and genetic data, we aim at bringing an integral perspective on this population. Potential for ethnoprimatological studies includes questions related to the anthropogenic effect on in-group behaviour, in particular levels of cooperation and competition, on between-group interactions and on population dynamics, as well as on landscape usage and thermoregulation. Questions related to the perception by humans of the macaques, in terms of heritage, nuisance, attraction and conservation are of particular interest. Evolutionary studies would benefit from this site by using this interface in a comparative perspective with other wild and semi-captive populations of Barbary macaques, but also in a broader sense, with other non-human primates. Particular questions include the reconstruction of the past population history, including group fissions, but also how territoriality, out-group hostility, and in-group cooperation are expressed in this high density and peculiar population, and how these behaviours vary from other populations and from other non-human primates.
