Monitoring follicular dynamics using ultrasonography in captive brown bears (Ursus arctos) during the breeding season.

Artificial insemination (AI) may be a useful tool in the reproductive management of endangered animals, including bears. To establish an AI program for bears, we investigated follicular dynamics using weekly transrectal ultrasonography in six captive brown bears. Along with ultrasonography, we monitored plasma progesterone (P) and estradiol-17β (E) concentrations. Furthermore, two bears were administered a gonadotropin releasing hormone (GnRH) agonist to induce ovulation on the first day on which the largest follicle reached more than 10.0 mm in diameter. Brown bears showed two patterns of follicular development in the early and late periods of the breeding season. In the early period (May to mid-June), multiple follicular waves were observed; namely, many follicles developed, and the largest follicles grew to less than 6.0 mm in diameter then regressed. In the late period (mid-June to July), one or two follicles grew to greater than 6.0 mm in diameter and developed as dominant follicles. Moreover, the growth rate of the largest follicle in the late period was faster than that in the early period of the breeding season. One bear with a follicle of 13.1 mm ovulated spontaneously, and one bear ovulated when the follicle was 10.2 mm in diameter after GnRH agonist treatment. Plasma E concentrations increased and showed peaks five to seven days before the largest follicles reached their maximum size. Plasma P concentrations increased on the day the corpus luteum could be detected using ultrasonography. This is the first study that showed there are two patterns of follicular development in brown bears. Furthermore, the largest follicle reaching greater than 10.0 mm in diameter could be an indicator of the appearance of ovulatory follicles.