Grasping performance in primates does not align with preferred substrate use.

Arboreal locomotion presents considerable mechanical challenges, requiring animals to maintain stability on narrow supports. While some species rely on gait adjustments, others use grasping autopodia to counteract toppling torques. We investigated how substrate size affects grasping force in strepsirrhine primates-a lineage regarded as a model for early primates and known for fine-branch arboreal locomotion. Using a custom apparatus, we measured in vivo grip strength across three substrate diameters (small, medium and large) in 11 species. In both hands and feet, grip strength peaked on medium-sized substrates-those allowing optimal digital wrapping-and declined on small and large diameters. These patterns remained significant after controlling for phylogeny, body size, sex and age. Despite weaker performance on small substrates, strepsirrhines commonly navigate thin terminal branches in nature, suggesting an ecological mismatch between peak grasping performance and substrate use. This implies that powerful digital grasping may be less critical for arboreal stability than often assumed. Instead, whole-body mechanics and precise limb placement likely compensate when grip is reduced. Rather than maximizing force, the primate hand appears adapted for versatility-supporting the broader principle that evolutionary success often reflects functional adequacy and adaptability over specialization for force production.