The two major birth defect issues on bison since 1995 are male genital malformations and underdeveloped facial bones. This indicates that bison fetuses are being exposed to biologically significant levels of environmental toxins. Hoy has photographed many large male bison with no scrotum and even more with misaligned hemiscrota. A major concern is when these animals have malformed scrotums they aren't always able to produce viable sperm.
Estrogenic and anti-androgenic toxins can cause birth defects that affect reproduction. The major defects of those hormone disrupting toxins is small or undescended testes. Exposure to toxin mixtures that chelate minerals and disrupt normal gene signaling, as well as disrupting hormones is changing the bison and it's not for the better.