Could “wild horses” drag you away? Quantifying muscular architecture in the forelimbs of extant, non‐domestic equids (Perissodactyla: Equidae)

Equid evolution is characterized by a high diversity of extinct species and morphologies, whereas extant equids share a superficially similar, monodactyl morphology. This inferred musculoskeletal similarity of modern equid limbs remains unexplored, and it is often assumed that domestic horse limbs are representative for wild equids (e.g., zebras, onagers, etc.). Our aim was to quantitatively describe the muscle architecture and arrangement of all forelimb muscles in extant wild Equus species to test this assumption, and investigate any differences between the species. We hypothesized that there would be subtle variation linked to locomotion on the different substrates that these species encounter. Gross dissections were performed to record muscle attachment sites and to quantify architectural metrics: muscle-tendon unit (MTU) length, MTU mass, muscle mass, pennation angle, and fascicle length; physiological cross-sectional area (PCSA) of the muscles and tendon cross-sectional area (TCSA) of the distal ligaments were then calculated. Qualitative results confirm common origin and insertion sites of all muscles across all Equus species. When normalized for size, the forelimb muscles across equids generally exhibit comparable muscular architecture and force-generating capacities. However, we observed a trend for higher force-generating potential in the distal limb flexor muscles in two species of zebra naturally found in habitats with inclined or uneven substrates. Although limited by sample size, these results indicate that scaled data for modern wild equids are generally very comparable, which may enable much smoother translation of experimental data from domestic horses into digital simulations of wild equid locomotion, including for extinct equids.