Morphometric variation of the skull during postnatal development in the Lowland European bisonBison bonasus bonasus

We studied the variation of linear measurements and skull capacity in Lowland European bisonBison bonasus bonasus (Linnaeus, 1758) during postnatal development, and the dependencies of the parameters in relation to sex, age, and body mass of the animals. Material consisted of 599 bison skulls (310 males and 289 females), within the age range of 1 month to 21 years (males) and to 27 years (females). In the group of calves to 1 year old, no sex connected differences in skull measurements were observed, whereas the skull capacity in older calves was significantly larger (0.01>p>0.001) in males than in females. From the third year of life, most skull measurements display characteristics of sexual dimorphism. Skull development in both sexes is most intensive during the first three years of life, and slows from the age of 5. In older individuals of both sexes (≥ 6 years), orbital breadth continues growing and, in females, breadth of splanchnocranium continues increasing. Growth in a bison’s skull capacity is most intensive up to the third year of life and slows from the age of 5. During postnatal development, a bison skull grows proportionally except the neurocranium, which grows slightly slower in comparison with basal length and its development finishes earlier than that of splanchnocranium. In ontogenesis, a bison skull grows much slower compared to body mass. In relation to body mass, skull capacity and the height of neurocranium grow most slowly while orbital breadth grows most intensively. The results obtained were compared with data on skull sizes of bison born in 1930–1950 and bred in captivity and with skulls of the American bisonBison bison. Inbreeding is probably responsible for some types of phenotypic abnormalities in the skull which appear in modern European bison.