A comparison of body condition and reproduction of caribou on two predator-free arctic islands

Abstract

To examine the relationships between body condition, pregnancy, and population dynamics in predator-free caribou (Rangifer tarandus groenlandicus), 147 females of the rapidly increasing Southampton Island herd were collected during the spring (n = 100) and autumn (n = 47) from 1988 through 1991 to assess their reproductive status and body condition (i.e., fat and muscle reserves). Caribou had been hunted to extinction on Southampton Island by 1953. Forty-eight animals were transferred from Coats Island to Southampton Island in 1967. During this study, caribou population density on Southampton Island was low and range quality high in both summer and winter. Results were compared with previously published data from the winter food limited Coats Island herd. On Southampton Island, 1-year-old or older caribou showed no annual or seasonal variation in body condition, and the pregnancy rate was exceptionally high (97%; n = 93). All 21 yearling females and 69 of 72 older females were pregnant. Southampton Island caribou were lighter and had less fat in autumn than Coats Island caribou, but Southampton Island caribou were both heavier and fatter in spring. This result suggests that the autumn "set point" in body composition (the maximum amount of body reserves animals attempt to acquire by autumn) may not be fixed as is usually assumed but may vary in response to winter feeding conditions. Pregnancy in yearlings does not preclude pregnancy when the animals are 2 years old. The ability to maintain large body reserves into the late stages of pregnancy could explain this finding. Finally, comparisons with other studies suggest that body condition and pregnancy rate, particularly for yearlings, are at least partly density dependent in predator-free caribou populations.