Polarization of skull shapes in adult Lowland European bison, Bison bonasus bonasus

1. What they did

Earlier studies of European bison skulls looked at differences between males and females, growth, inbreeding and breeding lines, but nobody had compared skulls of adult bison of the same sex. The authors asked whether the bison of the Białowieża Forest in Poland, a population living in a constant habitat with a similar degree of inbreeding, show two distinct skull shapes (polarization).

They measured 300 skulls (150 male and 150 female) of adult Lowland bison aged 5 to 27 years, from the museum collection of the Mammal Research Institute in Białowieża. On each skull they took three standard measurements:

• Basal length (Basion to Prosthion, BP): a long or short skull.
• Forehead breadth (Ectorbitale to Ectorbitale, EctEct): a broad or narrow forehead.
• Face breadth (Supramolare to Supramolare, SmSm): a broad or narrow face.

Measurements were standardised separately for males and females, and each skull was given a standardised size (S) and grouped as small, medium-sized, large or very large. Using Perkal’s natural similarity coefficients, each measurement was scored as high (+) or low (–), which sorted every skull into one of six shape variants (I to VI). They also tested how skull size changed with age in each sex.

2. What they found

Skull size and age

• Males: 9 skulls (6%) small, 54 (36%) medium-sized, 59 (39%) large and 28 (19%) very large. Females: 24 (16%) small, 44 (30%) medium-sized, 65 (43%) large and 17 (11%) very large.
• The smallest skulls in both sexes came mostly from bison aged 5 to 8 years.
• Skull size was clearly linked with age in both sexes (correlation 0.36 in males and 0.44 in females). Skull size increased faster with age in males than in females.
• None of the 21 females aged 15 to 17 had a very large skull, which surprised the authors.

Skull shapes in males

• The most common type was variant I, a long skull with a broad forehead and narrow face: 40 skulls (27%).
• Almost as common was the opposite type, variant IV, a short skull with a narrow forehead and broad face: 36 skulls (24%).
• Each had “satellite” variants grouped around it. Overall, 51% of male skulls were long and 49% short.
• Skulls within variant I or within variant IV differed little from each other, but the two variants differed greatly. The authors call this a dichotomy (polarization) of skull shapes in males.

Skull shapes in females

• One type dominated: variant II, a long skull with a narrow forehead and narrow face: 42 skulls (28%).
• The other five variants were spread almost evenly (18 to 24 skulls each), so there was no opposite set and no polarization.

Which measurement varies most

• Face breadth varied more in females (standard deviation 7.28%) than in males (5.61%). Skull length varied a little more in males (3.27% against 2.81%), and forehead breadth was similar (5.79% against 5.59%).
• Skull length and face breadth were the measurements that departed most from average in the two male types.

3. What they concluded

• Adult male bison in the Białowieża population fall into two opposite skull shapes; adult females form only one cluster.
• Variation in face breadth is what produces two dominant shapes in males and one in females. Adult females show greater individual variation in some skull dimensions.
• Changes in the shape of the skull come mainly from the face part of the skull (the splanchnocranium), whose length and breadth are tied to the rows of premolar and molar teeth.
• Because the bison of the Białowieża Forest share the same habitats and food, the authors set aside diet as a cause and suggest the differences in face structure are possibly genetic.

6. Other technologies, skills or innovations

• Standard osteometric (bone measurement) methods, following Duerst (1926) and later bison skull studies.
• Perkal’s natural similarity coefficients, which reduce each measurement to a simple high or low sign so skulls can be sorted into shape types, and a classification tree (dendrite) built from 12 male skulls, six of variant I and six of variant IV.