Sex, not familiarity, shapes social interactions in adult captive tortoises ( Testudo hermanni )

Abstract Social cohesion varies across species. Solitary animals minimise social contact to reduce costs such as resource competition, aggression, and disease transmission. Solitary Testudo hatchlings can rely on familiarity to avoid unfamiliar conspecifics. It remains unclear whether this strategy is used in adult captive tortoises and whether sex differences exist. To address these questions, we experimentally investigated a colony of captive Testudo hermanni tortoises (a vulnerable species), housed in outdoor enclosures. After a group familiarisation phase, we tested adult tortoises in a novel arena with either a familiar or an unfamiliar same-sex individual (not encountered for at least 3 months). Contrary to our expectations, familiarity had no significant effect on social cohesion measures such as inter-individual distance or facing orientation, showing that in adult captive tortoises familiarity had limited effect on social cohesion. In contrast, we found striking sex differences: males approached conspecifics quickly and remained in close proximity throughout the test, frequently displaying aggression; females initially explored the partner but then distanced themselves, with minimal aggression, similarly to what Testudo hatchlings have been observed to do with unfamiliar individuals. These results show that in adult captive tortoises adult social interactions and social cohesion are influenced by sex. This study provides insight into social cohesion and interaction strategies in tortoises, relevant for captive settings and conservation planning.