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Cranial asymmetry in a pocket mouse, Chaetodipus fallax (Rodentia: Heteromyidae).

Examination of a series of mammal specimens obtained through exchange from the Los Angeles County Museum of Natural History revealed the skull of an adult male San Diego pocket mouse, Chaetodipus fallax, to be markedly asymmetrical (Fig. 1). Reports of such abnormalities in the literature are rare, and no instance of cranial asymmetry among heteromyid rodents previously has been described. The atypical skull is herein characterized, and possible causative factors for this condition are presented.

The cranial abnormality of the skull is most evident in dorsal view (Fig. 1A). The rostrum deviates laterally from the midline to the right at an angle of about 33 degrees. The braincase and skull roof appear normal. The rooted, brachyodont cheekteeth are noticeably more heavily worn on the right side. The upper right incisor is shorter by 2mm than its counterpart on the left, and a lateral view (Fig. 1B) reveals that the terminal tip is blunted and unworn.

Asymmetry in the skull appears most likely to have resulted from cessation of growth of the upper right incisor, with repeated compensatory shifting of the lower jaw to permit the occlusion of both lower incisors with the single upper incisor, thus keeping them worn and beveled to a sharp edge. Apparently, the single normal upper incisor was able to serve this purpose in life. Articulation of the cleaned skull and mandible demonstrated the ease with which the incisors could occlude, and diagonal (rather than vertical) wear striations along the beveled edge of the lower left incisor are compatable with this action.


A resulting muscle imbalance from the continual forced realignment of the incisors must gradually have forced the living bone of the skull into its observed asymmetrical form, and accounted for the observed evidence of preferential mastication on the right side. Whether the initiation of this abnormality occurred early or late in life could not be determined, but the former seems most likely.

Vaughan (1961) reported an apparently comparable case of cranial asymmetry in a pocket gopher (Geomys bursarius), which he attributed to the lack of an upper incisor, and the compensatory forces of unequal development of the masseter. Such growth interruption or developmental failure of incisors probably is rare in rodents and often times may not mitigate against survivability of affected animals, especially if the events preclude completion of skull ossification.

The pocket mouse was obtained on 27 July 1976, 2.1 mi. S, 9.15 mi. E Hesperia, San Bernandino Co., California, and is deposited in the Collection of Recent Mammals, Midwestern State University (MWSU 17666).


Vaughan, T. A. 1961. Cranial asymmetry in the pocket gopher. J. Mamm., 42:412-413.


Department of Biology, Midwestern State University, Wichita Falls, Texas 76308
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Title Annotation:GENERAL NOTES
Author:Wilson, Barry C.
Publication:The Texas Journal of Science
Geographic Code:1USA
Date:Nov 1, 1991
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