Toshio Tsubota, Yoshiyuki Takahashi, Hiroshi Kanagawa, Changes in Serum Progesterone Levels and Growth of Fetuses in Hokkaido Brown Bears, Bears: Their Biology and Management, Vol. 7, A Selection of Papers from the Seventh International Conference on Bear Research and Management, Williamsburg, Virginia, USA, and Plitvice Lakes, Yugoslavia, February and March 1986 (1987), pp. 355-358
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Tsubota et al. (1987) studied this question.