ACTA THERIOLOGICA Vol. 32, 22: 373—387, 1987 BISONI AN A XCVIH Analysis of Inbreeding in European Bison Wanda OLECH Olech W., 1987: Analysis of inbreeding in European bison. Acta theriol., 32, 22: 373—387 [With 7 Tables & 4 Figs] The ancestors of the European bison population currently living in the world consisted of 12 animals only so they are closely related to each other. Inbreeding accompanying such close relationship may form the cause of decrease in vitality and consequent reduction in fertility. The present study analyses the degree of inbreeding in the European bison recorded in the Pedigree Book and determines the effect of such inbreeding on the animals' length of life and reproduction. In- vestigation was made of total of 3617 European bison of known descent on both the paternal and maternal side. The world population of European bison proved to be highly inbred; the mean inbreeding coefficient is F= 0.210. The high degree of inbreeding affects the length of animals life. The coefficient of correlation between inbreeding and life span, expressed in days, is significant (r= — 0.12±0.002), that is, the higher the inbreeding coefficient, the shorter the length of animals life. A significant correlation was also found between the inbreeding level and the percentage of deaths of young animals. Inbreeding prolongs the mean period between calvings in cows. It was not found to exert any effect on the age of first calving and the number of calves produced by a cow, and this last conclusion could form part of the success achieved by the rapid restitution of this species. [Department of Applied Animal Genetics, Institute of the Biological Basis of Animal Breeding, Warsaw Agricultural University, Przejazd 4, 05-840 Brwinow, Poland] 1. INTRODUCTION During the International Nature Conservation Congress held in Paris in 1923 a resolution was made to protect and restitute the European bison Bison bonasus (Linnaeus, 1758). At that time there existed only 12 genotypes in population of European bison (Slatis, I960), and these animals became the ancestors of several thousand European bison later registered in the European Bison Pedigree Book. As the population was so small it was impossible to avoid mating related animals, involving consequent inbreeding in their progeny. The good results obtained in the restitution of European bison show that this species is endowed with both vitality and fertility, and in addition adapts well to natural environment. Slatis (1960) analyzed inbreeding in European bison and found that it contributes to increase in the percentage of deaths of calves and young [373]