The incidence of, and relationship between, distal limb and facial asymmetry, and performance in the event horse: Asymmetry in the event horse

Identification of influential conformational traits is an important aspect in choosing the most appropriate horse for a specific discipline with regards to both performance potential and career longevity. Symmetry of bilateral traits, both functional and non-functional, demonstrates the ability of an individual to display their genotypic quality through their phenotype. External trait symmetry has been linked to effective functioning and health of a number of internal organ systems. The identification of a relationship between bilateral trait symmetry and performance in the equine athlete could highlight important conformational indicators of potential future performance. Using Invicta metric callipers and a tape measure, direct measurements of 15 bilateral traits, functional (limb) and non-functional (facial), were obtained from 18 advanced event horses (AdE) and 15 event horses proven incapable of reaching advanced level (NIE). Tests of difference were performed to determine differences in asymmetry between the two populations for individual traits and for identification of the mean total asymmetry (MTA) of the two populations. The AdE group demonstrated statistically lower asymmetry for a number of functional (metacarpal length and width, P≤0.05; fore proximal phalanx length, P≤0.05) and non-functional (nostril length, P≤0.001; pinna length, P≤0.05) traits. Furthermore, the AdE group demonstrated significantly lower MTA (P≤0.001) than the NIE. Where significance was not identified, all but one functional trait still demonstrated a trend for lower asymmetry in the AdE group. From the results, it is suggested that asymmetry levels have future potential to be used as an indicator of performance potential. The size of the mean asymmetry values being considered, further imply that the differences between the performance of the two groups cannot be as a result of limb biomechanics alone and that other more intricate biological disruptions, for example neurological development, may also be reflected by the trait asymmetries.