Sexual Dimorphism in Skulls of the Lowland European Bison, Bison bonasus bonasus

1. What they did

Male and female European bison differ in their skulls, but earlier studies had too few skulls, or mixed bison from different breeding lines, to pin down the differences precisely. The authors set out to describe sexual dimorphism (differences between the sexes) in the skulls of the Lowland line of European bison.

They measured 600 skulls (311 male and 289 female) in the museum collection of the Mammal Research Institute of the Polish Academy of Sciences in Białowieża, Poland. The animals were from one month to 27 years old. 67 came from the enclosed bison breeding centre in Białowieża and 533 from the free-ranging population in the Polish part of the Białowieża Primeval Forest. The skulls came from dead or killed animals between 1967 and 2006.

On each skull they took seven standard measurements of length, breadth and height. They divided each measurement by the others to make ratios, picked the ratios that best separated the sexes, and built an equation (S) that gives a negative value for a female-shaped skull and a positive value for a male-shaped one. They then tested whether the difference between the sexes becomes stronger with age.

2. What they found

• Two ratios separated the sexes best: the breadth across the eye sockets (orbital breadth) compared with the length of the face (splanchnocranium), and the length of the braincase (neurocranium) compared with the basal length of the skull. A skull scores as male when the eye-socket breadth and braincase length are relatively large and the face and basal length relatively short.
• The eye-socket ratio did most of the work: it differed much more between males and females than the braincase ratio.
• Males: 229 of 311 skulls (73.6%) scored as male and 82 (26.4%) as female-like.
• Female-like male skulls were mostly young: 75 of the male skulls up to 4 years old had female features, but only 7 skulls of males over 5 years old. In males, the male score rose with age (a positive, highly significant correlation).
• Females: 230 of 289 skulls (about 80%) scored as female and 59 (about 20%) as male-like, so about one female in five had a male-like skull.
• In females, male-like skulls were spread evenly across young and old animals, so there was no link between age and score.

3. What they concluded

• In nearly all males over 5 years old, the skull shape is settled and typically male. Earlier researchers had noted the very broad eye-socket region of males, and there is even a saying that male bison have a “telescopic orbit”.
• They considered whether inbreeding explains male-like skulls in adult females. Today’s Lowland bison descend from only seven ancestors, over 80% of their genes come from one pair of founders, and inbreeding in bison of known pedigree averaged 48% over the last 10 years. The paper states that all Białowieża-line bison carry copies of the same Y chromosome, from the founder bull M 45 PLEBEJER, and suggests this may be linked to adult male skulls being more uniform than female skulls.
• However, male-like skulls in mature females do not necessarily mean anything is wrong. They may reflect natural individual variation.
• Despite long-term inbreeding, the skulls, especially of females, are highly variable. The authors see this as a hopeful sign for the future of the European bison.

6. Other technologies, skills or innovations

• A simple equation using two skull ratios that sexes most European bison skulls, especially those of adult males.

7. Implications for conservation (from the paper)

• The high variability in skull shape suggests the population is not as genetically uniform as its very small number of founders would predict, which the authors call a positive prognosis for the species.