Locomotion on the edge: Structural properties of the third metacarpal in Thoroughbred and Quarter Horse racehorses and feral Assateague Island ponies

Abstract

The elongated, distally tapered limbs of horses are adapted for high‐speed locomotion. Because these traits are artificially selected for in modern racehorses, they operate at a morphological extreme with a high risk of fracture. Racehorses are subject to different training and racing regimes depending on their breed and gait, and are therefore an interesting model to examine bone functional adaptation under variable biomechanically intense conditions. This study compares bone structural properties in the third metacarpal (MCIII) of Thoroughbred (n = 9) and Quarter Horse (n = 11) racehorses, using feral Assateague Island ponies (n = 6) as an untrained/unraced outgroup, to determine whether structural properties reflect variable racing and training regimes. Geometric section properties and bone mineral densities were determined using peripheral quantitative CT at two diaphyseal sites and through the distal epiphysis. Diaphyseal strength of the MCIII in all three breeds does not differ relative to body size, but in the mid‐diaphyseal region Thoroughbreds have higher antero‐posterior relative to medio‐lateral bending strength than Quarter Horses, as well as higher bone mineral densities in left MCIII epiphyses (particularly in the lateral condyle). Interestingly, all breeds have lower bone mineral densities in the lateral versus medial condyle, an inherent structural feature that may influence predisposition to fracture when running around turns. Our results suggest that despite subtle differences in bone structure between different racehorse breeds, basic morphology of the third metacarpus is relatively similar among racing and non‐racing horses, possibly reflecting intense selection (natural and artificial) across domestic equids for similar structural features within distal limb elements.

Investigating the reliability of metapodials as taxonomic Indicators for Beringian horses

“…For example, draft horses such as Shires and Clydesdales have very large, robust limb bones to support their heavy body mass, whereas smaller horses such as (Scott 1990) and MT8 (Alberdi et al 1995) (Scott 1990) (Alberdi et al 1995 (Hanot et al 2018). Even horse breeds that are used to perform similar tasks can differ greatly from one another in their limb morphology; Thoroughbreds have much longer and more slender metapodials than Quarter Horses, though both breeds are frequently used in racing (Goldstein et al 2021). Guthrie (2003) and Barrón-Ortiz et al (2017) proposed that the metapodial morphology of extinct horses were also plastic, highlighting variation in metapodial shape for E. lambei and H. francisci, respectively.…”

“…However, modern domestic horses ( Equus caballus Linnaeus, 1758 ) show variability in limb bone morphology and structural properties as a result of artificial selection. For example, selective breeding of Thoroughbred horses for racing has resulted in longer, more slender limb bones compared to other breeds and in limbs that operate on anatomical and biomechanical extremes (Alexander 1998 ; Currey 2002 ; Goldstein et al 2021 ). Further, domestic horses vary greatly in size (Brooks et al 2010 ), which impacts the shape of the limb bones; smaller horses, such as Icelandic horses and Shetland ponies have smaller, more slender limb bones, while larger horses, such Clydesdales and other draft breeds, possess larger, very robust limb bones (Hanot et al 2018 ).…”