Assessment of reproduction of brown bears in Sweden using stained placental scars

Abstract

The Swedish brown bear Ursus arctos population is protected, but managed with legally defined hunting seasons. Management decisions (e.g., hunting quotas) are frequently changed and should be based on knowledge about demographic parameters, but collecting sufficient data in the field is time consuming and expensive. An efficient method to collect data on reproductive output could be counting placental scars in the uteri of female brown bears, because hunters in Sweden are required to collect samples (including reproductive organs) of harvested bears and submit them to the authorities. We assessed the reliability of placental scar counts to determine reproductive performance by counting the number of young with female radio-collared brown bears and comparing that with placental scar counts after those females had been harvested. We found that staining uteri improved the detection of placental scars. The differences between number of scars detected before and after staining the uteri, increased significantly with female age. The number of placental scars and number of observed cubs-of-the-year accompanying females corresponded well 2 and 3 years after birth; relatively small deviations between them might have occurred because of early cub mortality prior to the observations after leaving the den. Placental scar counts can provide accurate information on age of primiparity, evidence for reproductive aging (senescence), and reproductive productivity, and therefore inform decisions regarding adaptive management, sustainable hunting, and conservation.