Growth and Sexual Dimorphism in the Skull of the Capercaillie Tetrao urogallus: A Multivariate Study of Geographical Variation

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Abstract
In autumn 1981, Capercaillie skulls (N=813) were collected from Finnish hunters. On the basis of ossification of the skull, five growth groups (from 0 to 4 yr) were formed in males and three (from 0 to 2 yr) in females. Growth groups corresponded moderately well with multivariate-estimated age groups. Of the 26 variables measured, accelerated sexual dimorphism was most obvious in the length and height growth of the beak as well as in the growth of the bony structures to which the beak is attached. Selection for the robust beak of males might be based on their need (1) to compete for females on spring leks, and (2) to effectively handle winter food of pine needles during the short days of winter. Discriminant function analysis of skull measurements revealed geographical trends over the boreal zone, which were based on variation in the cranial dimensions of the skull, especially those of the braincase. The large subspecies T. u. urogallus breeds in the north and the small T. u. uralensis in the south. The transition zone between these shows similarities with other mid-boreal zoogeographical boundaries. In addition, the effect of the large Central European T. u. major is seen in the population of southernmost Finland.