Growth pattern analysis of the internal cranial structure in the male Hokkaido brown bear (Ursus arctos yesoensis) by computed tomography.

Forty male brown bear heads were non-destructively analyzed by computed tomography. Skull, frontal, and cranial cavity volumes were measured, and growth curves and regression lines were constructed. The morphological dynamics of the frontal sinus were also examined. Skull volume increased rapidly from 0 to 11 years of age, then grew more slowly until 21 years, when it plateaued. The frontal sinus also increased rapidly from 0 years, reaching approximately 300 mL by 10 years, after which growth ceased. The frontal sinus-to-skull ratio rose quickly from 0 to 4 years, then increased more gradually until 10 years, and then plateaued. Morphologically, the frontal sinus developed posteriorly to cover the anterior part of the cranial cavity, extended ventrally in the anterior lateral part of the cranial cavity and the orbital region, and protruded into the zygomatic process of the frontal bone. The cranial cavity increased slowly from 0 years, with continued growth beyond 11 years at a reduced rate. The cranial cavity-to-skull ratio decreased because the skull volume significantly increased. Both the frontal sinus and cranial cavity volumes strongly correlated with skull volume, and a well-fitting regression line was obtained. This means that the volumes of the frontal sinus and cranial cavity correlate more closely with skull volume than with age. This is the first report on the non-destructive analysis of the changes in volume and morphology of the frontal sinus and cranial cavity in male brown bears.