1993 Volume 39 Issue 4 Pages 325-331
Ovaries having corpora lutea and uteri of 6 female Hokkaido brown bears (Ursus arctos yesoensis), which were legally killed by hunters from October to December in Hokkaido, Japan, were examined macroscopically and histologically. Four of the 6 female bears had unimplanted embryos in the uterine lumen during the period of delayed implantation between October 19 and November 20. Ovaries of bears which had unimplanted embryos contained the corpora lutea with extensive vacuolation or numerous granules in the cytoplasm of luteal cells. These bears had well-developed uterine glands which had tall epithelial cells with vacuoles in the cytoplasm of glandular cells and coagula in the glandular lumina. It is concluded that Hokkaido brown bears exhibit obligate delayed implantation and that this period of delayed implantation continues at least until late November.