Data for : Relative threat modulates the effect of familiarity in territorial encounters among social fish

Territorial behaviour is costly, and territorial defence is expected to scale with threat levels posed by intrusions. This threat can be a function of multiple factors, including the number of intruders, familiarity with the intruder, and proximity of the encounter to the territory. Here we tested how these factors interact to shape territorial defence in the group-living cichlid Neolamprologus multifasciatus. In field experiments, residents presented with familiar or unfamiliar pairs positioned at natural intergroup distances from their territory boundaries, responded aggressively only to unfamiliar pairs, demonstrating the “dear enemy” effect. In contrast, single males placed at intergroup distances elicited no aggression, while single males placed on territory borders elicited intense aggression, in both cases regardless of familiarity. A laboratory assay confirmed that visual cues are sufficient for discrimination between familiar and unfamiliar individuals: focal males displayed more aggression toward unfamiliar males across a glass divider than toward familiar males. Together, these results demonstrate threat-sensitive expression of territorial behaviour and conditional sensitivity to the familiarity of the intruder. By showing that spatial positioning, intruder number, and familiarity can combine to alter perceived threat, we highlight the multi-dimensional nature of territorial aggression and the sophisticated mechanisms underpinning territorial behaviour.