Spatial strategies in non-territorial societies: How feral horses maintain boundaries with other groups

Encounters between competitive individuals or groups are common in social animals and can involve costly aggression; thus, animals often employ strategies to minimise direct conflict. However, research on whether and how animals adjust their group spatial structure when they encounter or spatially co-occur with a different group remains limited. We investigated how non-territorial units in feral horse multilevel societies manage spatial encounters with neighbouring units. We observed 25 reproductive units in northern Portugal, using drones, and employed statistical analyses to quantify spatial dynamics. We found that horses actively adjusted their spatial formations depending on the proximity of other units. Units became more circular and cohesive when other units approached, but when extremely close, they elongated their shapes to avoid boundary crossings. These adjustments indicate that horses maintain flexible unit boundaries to prevent mixing with other units. A notable exception was a particular pair of units that frequently crossed boundaries and intermixed, representing a unique social level within the horse multilevel society. Overall, our findings indicate that feral horses respond adaptively to inter-unit encounters by balancing the benefits of aggregation with the need to reduce direct overlap, providing new insights into the spatial organisation of non-territorial societies.