Monitoring follicular dynamics to determine estrus type and timing of ovulation induction in captive brown bears (Ursus arctos)

Abstract

It is important to understand ovarian physiology when developing an artificial insemination (AI) protocol. Brown bears (Ursus arctos) have a breeding season from May to July, although the type of estrus (polyestrus or monoestrus) is still contested. The present study aimed to define the ovarian dynamics, including follicular waves and ovulatory follicle size, and estrus type in brown bears. Six brown bears were used for ovarian ultrasonography; four were observed between April and October (before the start and after the end of the breeding season) and two in June (breeding season). In addition, we attempted to induce ovulation by administering a gonadotropin releasing hormone (GnRH) agonist. We observed follicular development in April in four bears, but follicles did not develop to greater than 6.0 mm in diameter until May. Thereafter, a group of follicles developed to more than 6.0 mm and grew as dominant follicles, except in one bear. After ovulation and subsequent corpus luteum (CL) formation, the follicular waves disappeared. Furthermore, in three bears treated with GnRH, follicles between 8.2 to 11.2 mm in diameter at the time of treatment ovulated and formed CLs. In two bears, follicles between 5.8 to 8.8 mm ovulated spontaneously within the observation interval. Our results suggest that brown bears may be monoestrous animals. Therefore, AI can only be performed once during the breeding season. Our results also suggest that dominant follicles larger than 8.0 mm are a suitable size for inducing ovulation.

Keywords: Brown bear; Estrus type; Follicular dynamics; Ovulation induction; Ultrasonography.

Fig. 1.

Images recorded from ultrasonography of an ovary in Bear C. Panels A to C show the same follicle (gradually increasing in size), which was identified and traced by its position in the ovary and location relative to other follicles (arrow head). These observations were done in May, the second and third observations were undertaken four and nine days, respectively, after the first. Panels D to F are images of the same follicle in Panels A to C. The dominant follicle (D) ovulated and a corpus luteum (CL) with cavity was identified in the same location (E; arrow). Later, a CL filled with luteal tissue was identified (F; arrow). Scale bar: 10 mm.

Fig. 2.

The follicle growth profiles of Bears A to D (those observed from April to October), along with changes in plasma steroid hormones, vaginal cytology, and vulva score. The top panels show the total follicle number in each animal. Follicles and corpora lutea (CLs) diameters that could be traced by ultrasonography are shown in the second row of panels. Minor wave follicles (Minor F) and major wave follicles (Major F) are follicles that developed to less than 6.0 mm in diameter and those that developed to greater than 6.0 mm, respectively. The changes in plasma estradiol-17β (E₂) and progesterone (P₄) are shown in the third row of panels. The bottom panels show the cornification rate (Corn.) and the vulva score (Vulv.). Bears C and D were administered gonadotropin releasing hormone (GnRH) to induce ovulation. The broken lines indicate the date of GnRH administration. The asterisk for Bear B indicates unilateral (right side) observation of the ovary due to a technical problem.

Fig. 3.

The follicle growth profiles of Bears E and F (those observed in June), along with changes in plasma steroid hormones, vaginal cytology, and vulva score. The top panels show the total follicle number in each animal. Follicles and corpora lutea (CLs) diameters that could be traced by ultrasonography are shown in the second row of panels. Major wave follicles (Major F) are those that developed to greater than 6.0 mm in diameter. The changes in plasma estradiol-17β (E₂) and progesterone (P₄) are shown in the third row of panels. The bottom panels show the cornification rate (Corn.) and the vulva score (Vulv.). Bear E was administered gonadotropin releasing hormone (GnRH) to induce ovulation. The broken lines indicate the date of GnRH administration.

Fig. 4.

Scatter diagram and linear relationship for follicular diameter in the minor waves (open circles, n = 21) and major waves (closed circles, n = 15). Data from previous observations (n = 3) [10] were combined with those obtained in the present study. The day on which each follicle had the largest diameter was defined as Day 0.