Anisotropy in the Frontal Sinuses of Bighorn Sheep and Its Implications for Understanding Pneumatization

Abstract

In bighorn sheep (Ovis canadensis), greatly expanded frontal sinuses overlie the endocranial cavity and enter the horncores. A number of bony struts further subdivide the sinuses. Previous workers have suggested that the struts function as shock absorbers and/or strengthen the sinus against the forces of head butting. Under both scenarios, it is predicted that the struts should be oriented parallel to typical loads applied to the horns. In order to test this hypothesis, skulls of male O. canadensis were CT scanned, and structural anisotropy for the struts within the frontal sinus was estimated using the star volume distribution method. Measurements were taken at the base and midpoint of the horncores. Strut orientations were highly anisotropic. Struts were oriented parallel to the long axis of the horn at its base and oriented medio‐laterally distally. These results are contrary to expectations, suggesting that sinus morphology within O. canadensis is not primarily shaped by applied loads. Instead, the orientation of the struts may simply reflect the direction of expansion of the pneumatic diverticulum during pneumatization of the skull. Funded by SICB and an NSF GRF.