The study presents the analysis of the findings of 234 post-mortem examinations on free-ranging and captive European bison selectively culled or having fallen between 2008 and 2013 in Białowieża Primeval Forest. Pneumonia, emphysema, nephritis, bodily traumas, and intestinal lesions were observed in 106 (45.3%), 77 (32.9%), 82 (35.0%), 68 (29.1%), and 56 (23.9%) animals respectively and were the most common pathological changes. Almost half of all males (66 out of 140; 47.1%) tested showed some pathological changes of prepuce and penis, described as posthitis or balanoposthitis. Infection with liver fluke (Fasciola hepatica) and lungworm (Dictyocaulus viviparus) was observed macroscopically in 114 (48.7%) and 80 (32.9%) bison respectively. F. hepatica prevalence was associated with the emergence of other liver changes such as hepatitis and cirrhosis (P < 0.001). Similarly, the prevalence of D. viviparus coincided with pneumonia (P = 0.001), changes in the upper respiratory tract (P = 0.04), and emphysema (P < 0.001). Hepatitis, infection with F. hepatica, and pathological lesions in the male and female reproductive tracts were associated with the animals' age. Mechanical injuries, caused by other bison or less commonly by traffic accidents, were the most common cause of death of bison below six months of age. Most pathological changes were significantly more frequent in the selectively culled animals in comparison with the ones having fallen, which confirms the desirability of elimination as a tool to improve the health and welfare of the bison population and limit the number of reservoirs of invasive and possibly infectious diseases.

The European bison Bison bonasus is the largest extant terrestrial mammal on the European continent; however, the species went extinct in the wild in 1919. Restoration started in 1929 in Poland's Białowież a Primeval Forest using captive individuals sourced from zoological gardens and breeding centres. Of the 7 founders, 2 individuals contributed 85% to the genetic make-up of the lowland line of the species. The Białowież a bison population numbered 820 in 2008, but very low genetic diversity and a high level of management have raised questions as to whether it still conforms to evolutionary predictions. We tested whether the sex ratio of European bison calves conformed to the Trivers-Willard hypothesis at the population level, i.e. whether it became increasingly female-biased as bison condition deteriorated following increased population density. We found that increased population density and reduced female body mass led to increasing female-biased calf sex ratios, whereas mast years (abundant food resources) corresponded to male-biased sex ratios. Despite the high degree of inbreeding and management, European bison are still responding as expected to variations in female body condition; however, the precautionary principle cautions managers of small populations that artificial selection can alter the evolutionary strategy of wildlife even though we did not detect this in the Białowież a bison population. KEY WORDS: Body condition · Local resource competition hypothesis · Maternal investment · Reproductive success · Sex ratio manipulation · Trivers-Willard hypothesis · European bison · UngulatesResale or republication not permitted without written consent of the publisher

SummaryThe aim of this study was to find whether the coefficient of inbreeding is correlated with the incidence of cryptorchidism in the Białowieża line of bison.Our In 461 male European bison older than 1 year, there were 18 cases of cryptorchidism. In the animals born in the first decade, two autopsical cases of cryptorchidism (0.045%) were recorded, and 4 cases of cryptorchidism (0.03%) were found in the animals from the second decade. Among the bison born in the third decade, there were no cases of cryptorchidism. In the fourth decade, 7 cases of cryptorchidism (0.071%) were diagnosed, and among the bison born during the fifth decade, there were 5 cases (0.111%).The Pearson correlation coefficient suggests that, as the value of the coefficient of inbreeding rises, there is an increase in cryptorchidism cases in male European bison.