Hippopotamus (Hippopotamus amphibius and Choeropsis liberiensis) shoulder osteology and function compared with Suids (Sus scrofa, Phacochoerus aethiopicus, Potmochoerus porcus and Hylochoerus meinertzhageni) and Tayasuids (Tayassu peccary and Tayassu tajacu)
Authors: Naples Virginia L. 
Published: November 2023

Pectoral girdle and forelimb bones of the hippopotamuses, Hippopotamus amphibius and Choeropsis liberiensis were examined and compared to those of the suids, Sus scrofa, Phacochoerus aethiopicus, Potamochoerus porcus, Hylochoerus meinertzhageni and the tayassuids, Peccary tajacu and Tayassu peccari. Although no longer considered close hippo relatives, these comparison taxa share similar body shapes with hippos, making the groups ecomorphs. This study identifies specific anatomical characters among the adaptations equipping these animals to occupy partially overlapping ecological niches. Even though their habits differ (hippos are semiaquatic; pigs and peccaries are fully terrestrial) similar selective forces obtain because all three groups must accommodate the use of extremely large and robust canine teeth in intraspecific combat and self‐defense. The pygmy hippo and suoids also use the canines during feeding; all compared taxa show adaptations for support of large heads although cranial dimensions in hippos, especially H. amphibius, are maximized. The relative proportions of the forelimb girdles and limb bones differ between the hippos and among the suids compared to the tayassuids. Not only is H. amphibius far greater in body size, but this taxon also has, relatively speaking, the shortest legs. The fully terrestrial pigs and peccaries range in weight from less than 1/10th of that of the common hippo to approximately the size and mass of the pygmy hippo. Scapular glenoid fossae in both hippo taxa are rounded concavities, while the humeral heads are rounded convexities. Comparable features in suoids are more elongate although all groups show some degree of humeral head circumduction. The supraspinous fossa in the hippos is relatively much larger than that of the suoids; the latter all show more gracile scapulae than in either hippo taxon. All compared taxa have prominent scapular spines. In the hippos, the spine is a large, sharp lateral projection, recurving distally, to become a greatly ventrally expanded acromion process. In the peccaries it arises nearly from the anterior scapular border, while at the origin, the ridge is located increasingly posteriorly in the progressively larger‐bodied taxa (in order: P. porcus, S. scrofa, P. aethiopicus) until it arises directly halfway anteroposteriorly from the neck in H. meinerzhageni. In most of the suoids this feature arises as a gently sloping lateral projection from the scapular neck, without an acromion process. The exception is H. meinerzhageni in which the spinous process projects slightly farther laterally along the distal half of the bone. Both hippos show an additionally widened and thickened bulge midway along the spine, which does not extend toward the posterior border, as would a true metacromion process. In contrast, in all suoids, a metacromion process is prominent, triangular, with an often rounded, thickened and rugose posteriorly pointing projection. Although these differences are subtle, they reflect differing emphases on muscle groups that perform similar fore and aft as well as circumductive forelimb movements.