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

> Do all European bison bulls have the same head? By measuring 300 museum skulls from Poland’s Białowieża Forest, researchers found that adult males come in two opposite shapes: long skulls with broad foreheads and narrow faces, or short skulls with narrow foreheads and broad faces. Females mostly share one shape, and the authors suggest the difference lies in the face and is possibly genetic.

Summary by Kersey Lawrence, Original Wisdom. Updated 9 November 2026. Page: https://www.originalwisdom.com/tracking-literature/2008-kobrynczuk-krasinska-polarization-skull-shapes-adult/

## Citation and access

- **Authors:** Franciszek Kobryńczuk, Małgorzata Krasińska and Tomasz Szara
- **Year published:** 2008
- **Journal or source:** Annales Zoologici Fennici
- **Full citation:** Kobryńczuk, F., Krasińska, M. and Szara, T. 2008. Polarization of skull shapes in adult Lowland European bison, Bison bonasus bonasus. Annales Zoologici Fennici 45(4): 341–346.
- **DOI:** 10.5735/086.045.0416
- **Access status:** Hosted (freely available elsewhere)
- **PDF:** https://www.originalwisdom.com/wp-content/uploads/2026/11/2008_Kobrynczuk_and_Krasinska_etal_polarization-of-skull-shapes-in-adult-lowland-european-bison.pdf

## Tags

- **Species:** Mammals > European bison
- **Region:** Europe > Poland
- **Methods and technology:** Linear regression; Pearson correlation
- **Data type:** Existing data
- **Observer qualification:** Not needed
- **Discipline:** Just interesting
- **Year published:** 2008

## Detailed keywords

- **Species:** Studied: European bison (Bison bonasus bonasus), Lowland line, adults aged 5 to 27 years. Also mentioned: hybrids of European bison and domestic cattle (earlier studies), and the Lowland-Caucasian and Pszczyna breeding lines.
- **Location and habitat:** Białowieża Forest (Białowieża Primeval Forest), Poland. Skulls held in the museum collection of the Mammal Research Institute, Polish Academy of Sciences, Białowieża. The authors describe the bison there as living in a constant habitat with identical habitats and diet.
- **Topics:** skull morphology; osteometry (bone measurement); skull shape variants; polarization (dichotomy) of skull shapes; sexual differences in skull variation; skull growth with age; face breadth and tooth rows; possible genetic origin of shape differences; inbreeding (background).
- **Statistics or models:** Perkal’s natural similarity coefficients; standardised skull size (S); classification tree (dendrite) built from distances between 12 skulls; Pearson’s linear correlation between skull size and age; linear regression of skull size on age; standard deviations of measurements. Inconsistency: the text gives the ratio of small to very large male skulls as 1:4 (9:28), but 9:28 is about 1:3.1. The methods also print “positive (–)” where “positive (+)” is meant.
- **Data type:** Existing data: new measurements taken on 300 skulls (150 male, 150 female) from an existing museum collection. Dates of collection of the skulls are not given in this paper.

## Synopsis

### 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.

## Cite this summary

Lawrence, K. 2026. Summary of: Polarization of skull shapes in adult Lowland European bison, Bison bonasus bonasus (Kobryńczuk and Krasińska et al., 2008). Original Wisdom, Tracking Literature. https://www.originalwisdom.com/tracking-literature/2008-kobrynczuk-krasinska-polarization-skull-shapes-adult/