Morphological Variation in Gaited Horse Breeds

• We collected 32 body measurements from 1,682 horses of gaited and nongaited breeds.
• We performed principal component analysis to assess morphological patterns unique in gaited breeds.
• Gaited breeds are significantly different in two body shape patterns.
• Gaited horses have proportionally longer front limb segments.

Gaited horses, renowned for their smooth gaits, are anecdotally noted to have proportionally longer hind limbs than nongaited breeds of the same height. However, gaited horses perform a wide spectrum of different gaits that we hypothesize may require diverse skeletal adaptations. To investigate the contribution of morphology to gait ability, we collected body measurements from gaited and nongaited individual animals and principal component analysis was conducted on 32 body measures for a set of 1,682 horses. Principal component (PC) 1 captured 65.3% of the trait variance, whereas PC2 comprised 6.6% and PC3 3.2% of variance in the data set. All body measures correlated positively with PC1 which quantifies a horse's overall body size. In contrast, PC2 quantifies body thickness. Principal component 3 represents a pattern primarily involving skull thickness and lower limb length. Because of the presence of sexual dimorphism and unequal sampling across sexes, we generated a pruned data set of 432 individuals with equal numbers of gaited and nongaited horses within each sex class. Analysis of variance and pairwise correlations were conducted to identify differences in the PC scores and measurements (normalized by wither height) due to sex, gait phenotype, breed, and age. After accounting for these fixed effects, gaited horses were significantly different from nongaited horses in PC2 and PC3 (P < .0001). Comparisons of individual measurements demonstrate that gaited horses have smaller eye and jaw widths, proportionally longer front limb segments and thinner lower limb circumferences (P < .05). This is the first study to identify different proportions in the front limb segments associated with gait.