Longevity in a hunted population of reintroduced American bison (Bison bison)

Mean and maximum longevity (i.e., age-at-death) in free-ranging American bison (Bison bison) subject to natural selection is difficult to estimate in the wild; hence, data on age-at-death for free-ranging populations is sparse. I used a 20-year time series of age-at-death data from 1909 bison to estimate mean longevity of hunter-killed bison in a growing population. I tested the following predictions: (a) mean longevity of hunted females was greater than males, (b) mean longevity for hunted animals was less than those dying from natural causes, and (c) longevity did not respond to increasing population size (i.e., density independence). In support of my prediction, mean age-at-death of females (6.3 ± 3.8 (SD) years old) was greater than that for males (5.3 ± 2.7 years old); however, the difference was only 1 year. Most bison (45%) died as mature adults (4–7 years old), and few lived to become dominant or aged adults, which may have important implications for social ecology or population dynamics. Maximum longevity (i.e., oldest age-at-death) was 22 and 25 years old for males and females, respectively. The mean age-at-death of hunted females was 1.4 years younger than a small sample that died of natural causes. Contrary to my prediction, longevity increased slightly for females, but not males, with an increase in abundance. These data may be useful for better understanding bison demography and life history strategies, as well as parameterizing population models, particularly for hunted populations. Similar data from unhunted populations subject to natural selection would be informative to test the generality of these findings.