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Skeletal and muscular adaptations to a subterranean environment of Microtus oregoni serpens (Mammalia - Rodentia) Shaw, Dorothy Anne

Abstract

This study was undertaken primarily to examine the skeletal and muscular anatomy of a small species of field mouse, Microtus oregoni serpens Merriam, which inhabits the Puget Sound area of British Columbia and Washington. This species is largely subterranean in its habits and in an attempt to discern the degree of specialization for its habitat comparison was made between serpens and Microtus townsendi townsendi, (Backman) and Microtus longicauda vellerosa (Allen) species which burrow to a limited degree only. A detailed comparison of the external features, skeletal and muscular anatomy of the three species was made and a juvenile specimen of Microtus richardsoni richardsoni, (De Kay) was used for some comparisons. Certain parts of Microtus oregoni serpens were found to be strongly modified in a direction which seems to better adapt the animal to its subterranean mode of life. The external features which are of adaptive significance are the soft plush-like pelage, the short tail and the arrangement of the vibrissae the longest ones being furthest from the snout rather than scattered. The eyes are only about one-half the size of those of the larger species and associated with them is an interesting modification of the orbicularis oculi muscle whereby it is strongly developed doubtless as an aid in preventing dirt from entering the eyes. Microtus oregoni serpens does not have conspicuously enlarged or elongated fore-feet but among the modifications for digging which do occur are the shortening of the limbs and the inclusion of a larger part of them within the body skin. Almost the entire musculature of the anterior segment of the body is more strongly developed in serpens including the muscles of the shoulder, chest, and fore-limbs and to some extent the masticatory musculature and the musculature of the spinal column. The muscles attached to the pectoral girdle show the most marked modifications they are almost universally better-developed in serpens than in townsendi or longicauda. Correlated with a greater development of the pectoralis muscles is the development of a heel on the manubrium sterni and the sternebras in serpens, providing a larger area for muscle attachments. Microtus oregoni serpens seems to represent one of the earliest stages of adaptive specialization of a mammal for subterranean life externally approaching the condition found in specialized burrowers such as the mole but having no radical skeletal modifications.

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