There are multiple methods to estimate sex membership of unknown individuals. The pelvis and skull are frequently examined for the expression sex-linked traits as are the humeral and femoral head dimensions. In contrast to the sacrum, the true vertebrae of the vertebral column have been investigated only rarely for possible sex differences. The axis, or second cervical vertebra, allows for the rotation of the head, which is larger in males than in females. As the head rotates, along with the first cervical vertebra, it pivots against the ossified dens located on the superior surface of the axis. The dens marks the axis as morphologically unique among the vertebrae. The axis is positioned in the axial skeleton close to other traits that have been shown to exhibit dimorphism, such as the mastoid process, gonial region, nuchal area, and the occipital protuberance. Preliminary investigation has suggested that the axis is indeed dimorphic, but whether the difference is greater in younger adults, older adults or the elderly has not been addressed. To explore which linear dimensions of the axis differ the most between females and males, and to investigate its relationship to age, 149 individuals (78 males and 71 females) from the W.M. Bass Osteological Collection at the University of Tennessee, Knoxville were measured using 13 dimensions of the axis. The three age cohorts included 30-35 (8 males, 3 females), 50-55 (36 males, 28 females), and 70-75 (34 males, 40 females) years. The results indicate that all the traits show significant differences between the sexes. Using multiple linear regression to identify beta coefficients, a discriminant function for the axis was created to estimate the sex of unknown individuals using 6 traits. The age cohorts differed only slightly in their classification rates suggesting the axis is useful in sex estimation across the lifespan.

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