Directional asymmetry of facial and limb traits in horses and ponies

Current published data on directional asymmetry (DA) in horses refer to racing Thoroughbreds. The aim of this study was to identify whether horses and ponies exhibit directionality of trait asymmetries. Eleven functional (limb) and four non-functional (facial) bilateral traits were measured on left and right sides in a cohort of 100 horses and ponies using callipers. The population was investigated as pooled data and as horse (withers height >148 cm) and pony (withers height ⩽148 cm) sub-groups. Within the pooled data, functional traits were longer on the right for the third metacarpal (MCIII, 73%) and metatarsal (MTIII, 65%) bones and wider on the left for the forelimb proximal phalanx (54%), MCIII (40%), MTIII (51%) and hind limb proximal phalanx (55%). Dimensions of the intercarpal and tarsocrural joints were larger on the right side. Differences in DA were present between horses and ponies within non-functional traits, but not functional traits. The results suggest that DA within horses and ponies is more likely to be a species trait rather than one exclusive to racing as a result of pressures from directionally orientated training or from selective breeding strategies.