Changes of weight and size of European bison testes during postnatal development

The material for this study was obtained from culled European bison (Linnaeus, 1758) originating from both captive breeding and free-ranging populations in Bialowieza and Borecka Forests (north-east Poland), and was mostly collected from November until April. Mass difference between the right and left testis of the same individual up to 30% was considered to be physiological asymmetry, which proved typical of the species. Hypoplasia or atrophy of the testes exceeding 30% in size, were found in 37 males (13.6% of the examined animals; n = 272). Cryptorchidism was recorded in 20 bison culled in Bialowieza Forest at the age over 1 year (10.6% of individuals at the same age; n = 188). We registered the weight of testes of 259 males aged 2 months to 20 years. In the examined males, no distinct predominance in size and weight between the right and left testes was observed. The weight of testes was significantly correlated with age (r = 0.85, p < 0.0001) and body weight (r = 0.93, p < 0.0001). The heaviest testicles belonged to older and sexually mature bulls aged 6-12 years (mean 216 g) and to old ones over 13 years (mean 242 g). Two testis measurements (length and width) were taken from 136 culled males, while two additional measurements (total length with the cauda of epididymis and circumference) from 65 males. The length and width of the testes were moderately correlated with age (r = 0.31 and r = 0.20, p < 0.01) and highly significantly correlated with testis (r = 0.79, p < 0.001) and body weight (r = 0.92, p < 0.001).