A Quantitative Analysis of the Manus Musculature in Tapirs (Perissodactyla: Tapiridae)

The distal limb of many hooved mammals (ungulates) has become highly specialised, with tendonised muscles and elongate bones and ligaments. Several clades of ungulates retain fleshy, muscularised distal forelimbs; these include hippopotamuses, rhinoceroses, and tapirs. Of these species, tapirs (Tapiridae: Tapirus) represent the most plesiomorphic manus anatomy for its higher taxonomic group (Perissodactyla); the tetradactyl manus of tapirs is reminiscent of the earliest members of the lineages leading to modern horses (equids) and rhinocerotids. Within the tapir manus, osteological evidence indicates clear differences in load distribution, digit use during locomotion, and phylogenetic signal in the shape of certain bony elements. To date, no quantitative investigation has ever been performed to explore differences in the muscular anatomy of the tapir distal forelimb (manus). Here, we conducted a comparative muscle architecture quantification of the muscles which are intrinsic to the tapir manus, across all four extant species (Tapirus indicus, T. bairdii, T. pinchaque, T. terrestris). Despite limited sample sizes, we observed notable variation across the different species with regard to the force-generating potential of the muscles (based on physiological cross-sectional area, PCSA) and the shortening range of each muscle (based on fascicle length). High force-generating capacities were recovered for the interosseus muscles (preventing hyperextension) for the third and fourth digits, as may be expected for a mesaxonic manus such as that of Tapirus. Our results also indicate subtle differences in patterns of force-generating potential in the interosseus muscles between specimens housed in captivity and those from the wild, specifically living in upland rainforest and exhibiting ranging behaviour up and downhill on a regular basis. These data offer tantalising insights into the variation in the force-excursion relationship in the muscles of the ungulate manus, providing both qualitative and quantitative information for veterinarians, biologists, and palaeontologists investigating perissodactyl locomotor anatomy and evolution.