Effects of inbreeding, sex and geographical region on survival in an American bison (Bison bison) population under a captive breeding program

Survival is one of the most important traits to consider for limited animal populations under conservation programs. Here, we estimate the effects of inbreeding, geographical region (Europe, North America and other) and sex on survival at six points of life in American bison populations kept in zoological gardens. The data set consisted of 3893 animals, 1812 males and 2081 females, born between the years 1874 and 2013. We analyzed survival in terms of longevity and survival of the first day after birth. We applied regression analysis with a logistic model. Longevity was examined in terms of six time categories whereas the survival of the first day after birth as a binary trait. We find that the average longevity for the studied population was 5.21 years with a standard deviation of 6.75. The longevity was significantly affected by the sex: females survived on average longer than males. The median values of longevity were considerably lower than the averages and also conditioned by the sex. The largest number of individuals was recorded in survival up to the age of sexual maturity, equal to 43.6% of males and 30.75% of females. More females survived after reaching the age of sexual maturity. Survival of the first day after birth was not influenced by the calf's sex. There was also detected that the survival of individuals decreased with an increasing level of inbreeding, under relative small pedigree parameters for the population studied. It was also demonstrated that geographical region has a significant effect on the longevity of captive bison.