Visitor impacts on social dynamics and space use in captive Western lowland gorillas (Gorilla gorilla gorilla) in a naturalistic zoo exhibit: A new methodological approach

Abstract

The welfare of zoo animals has come under increasing scrutiny, yet the effects of visitors on captive species, particularly in relation to environmental factors and individual variability, remain insufficiently studied. This study introduces a novel methodology to assess how visitors influence the social and spatial dynamics of Western lowland gorillas (Gorilla gorilla gorilla) in naturalistic enclosures. We conducted daily observation sessions to track the positioning of gorillas relative to one another and their environment. Key visitor-related factors such as mere presence, density, movement and activity along two independent paths, adjacent to the enclosure were recorded and related to the gorillas' spatial distribution. Two indices – the Retreat Index (RI) and the Exposure Tolerance Index (ETI) – were developed to quantify each individual’s tendency to retreat from exposed areas or tolerate visitor observation. For the first time, Ripley's functions were employed to assess changes in distribution patterns among individuals in response to visitor presence. Our results show that visitor pressure significantly reduces inter-individual distances, a trend further amplified by environmental factors such as wind, temperature, and humidity. Among visitor-related variables, movement of new visitors, rather than mere presence or crowd size, was the primary factor triggering behavioural changes. Social network analysis revealed increased interactions, particularly between females and their offspring, along with enhanced comfort and resting behaviours in the presence of visitors. Additionally, we observed a dichotomy in the gorillas' behavioural response, with less pronounced reactions in areas where visitors were less visible. Ripley's functions identified substantial aggregation of gorillas in visitor-influenced scenarios, in contrast to a near-random distribution when visitors were absent. This pilot study establishes a foundation for developing a comprehensive methodology, applicable across various zoo settings, to evaluate the impact of visitor-induced disturbances on gorillas, offering fresh insights into the welfare of captive animals.