Studies on the morphology of burrowing scuirids with special reference to the forelimb of Marmota and Cynomys [Études de la morphologie des scuiridés fouisseurs avec une référence spéciale du membre antérieur de Marmota et Cynomys].

Résumé

The present investigation restricts itself chiefly to a study of the forelimb morphology of the woodchuck (Marmota monax rufescens) and the prairie dog (Cynomys l. ludovicianus). The gross anatomy of the pectoral appendage of these two mammals is described and figured in detail. The pectoral morphology of the woodchuck and prairie dog is contrasted with that of less fossorial sciurids and particular attention is given to those muscles and bones of the forelimb which have special adaptive significance. A detailed discussion of the burrowing habits of Marmota and Cynomys follows the morphological section of the thesis and precedes the summary. Fourteen preserved specimens and forty skeletons representing four genera, six species, and nine subspecies of sciurids were examined. Specimens prepared for dissection purposes were either embalmed with a standard embalming fluid or were preserved in 8 per cent formalin. All the bones and muscles described in this study are illustrated in the twenty-four plates appended to the thesis. Both Marmota and Cynomys are well adapted externally for burrowing. Their short legs, stout forepaws, and broad manus armed with long claws make their forearm an effective excavating tool. The claws of Cynomys are relatively longer, more slender, and less worn than those of Marmota. Both genera have a stocky, fusiform body and short external ears. The tail of Marmota and Cynomys is short, the latter having one of the shortest tails in proportion to body size of any of the sciurids. The hind foot is modified to throw dirt backward and the toes and sides of the pes are fringed with hairs making for a broad plantar surface. Osteological adaptations in the pectoral appendage of Marmota and Cynomys include a broad scapula with a prominent spine and well developed acromion and metacromion processes affording a considerable surface for the attachment of the powerful shoulder muscles. The clavicle is well developed in both animals, compensating for the great freedom of movement permitted at the shoulder joint.; In burrowing mammals, bones of the fore-arm are generally short in contrast to the appendicular bones of cursorial and arboreal species. This reduction in length is usually more obvious in the distal bones of the arm or leg. Cynomys and Marmota are no exception to this rule although the distal bones of the prairie dog are proportionally longer than those of the woodchuck. This shortening of the bones of the forearm make them appear relatively heavy in proportion to their length. The well developed processes and ridges of these bones afford an excellent surface for the origin and insertion of hypertrophied muscles. Of the sciurids studied, the olecranon of Marmota and Cynomys is considerably longer in proportion to the total length of the ulna. This permits considerable strength to be exercised at the sacrifice of speed during the digging operations. It is of interest to note in this connection that the length of the olecranon in proportion to the ulnar length is significantly less in Cynomys than in Marmota. Possibly this is a reflection of the soil texture encountered by these two mammals in burrowing. A prominent pair of sesamoid bones appear on the palmar surface of each metacarpo-phalangeal joint. Between, and protected to a considerable extent by these sesamoids, pass the strong flexor tendons to the digits. A prominent falciform furnishes additional bony protection to the all important flexor tendons of the manus. It is very difficult to compare the muscular effectiveness of different species of mammals by means of a gross dissection study alone. Even if the size of individual muscles could be compared accurately by volumetric means it would be misleading, as A. B. Howell has indicated, to estimate the strength of a particular muscle solely in terms of its cubic capacity. Not only does