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

Social cohesion varies across species, with solitary animals minimising social contact to reduce the costs such as resource competition, aggression, and disease transmission. The ability to distinguish between familiar and unfamiliar conspecifics can help reduce costly encounters. While the solitary Testudo hatchlings use familiarity to avoid unfamiliar conspecifics, it remains unclear whether this strategy persists into adulthood or varies by sex. After a group familiarisation phase, we tested adult Testudo hermanni in a novel arena with either a familiar or unfamiliar same-sex individual after a period of cohabitation. Familiarity had no significant effect on inter-individual distance, facing orientation, or aggression, suggesting that adult tortoises do not modulate social cohesion based on familiarity. 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, as hatchlings do. These results suggest that while tortoises maintain a solitary strategy across life stages, adult social behaviour is shaped more by sex-specific motivations than by familiarity. By showing that sex-specific behaviours dominate adult social interactions, this study provides insight into solitary strategies in tortoises and highlights considerations important for conservation planning.