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

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.

The novel use of urinary androgens to optimise detection of the fertile window in giant pandas

“…This may also be the case in the giant panda and at the observed E/T crossover the follicles have now likely developed to the required stage and size for aromatase expression. This crossover occurs at an average of 13 days before peak E, fitting with those observations of two weeks of growth to preovulatory size in the Hokkaido brown bear (Torii et al 2020). In agreement with these observations, ultrasound imaging of follicular development in the giant panda shows 6 mm follicles at the start of proestrus (Hildebrandt et al 2006).…”

“…Follicles greater than 6 mm in diameter expressed 17α-hydroxylase in the theca interna and aromatase in the granulosa cells ( Araki et al 1996 ). Hokkaido brown bears have one major follicular wave per breeding season, where one or two follicles grow larger than 6 mm in diameter, at which point they have gained the ability for estrogen synthesis ( Torii et al 2020 ). Torii et al (2020) suggest that Hokkaido brown bear follicles take around 2 weeks to grow to a preovulatory size during the major follicular wave.…”

“…Hokkaido brown bears have one major follicular wave per breeding season, where one or two follicles grow larger than 6 mm in diameter, at which point they have gained the ability for estrogen synthesis ( Torii et al 2020 ). Torii et al (2020) suggest that Hokkaido brown bear follicles take around 2 weeks to grow to a preovulatory size during the major follicular wave. This may also be the case in the giant panda and at the observed E/T crossover the follicles have now likely developed to the required stage and size for aromatase expression.…”

Biobanks, offspring fitness and the influence of developmental plasticity in conservation biology

“…The lifespan of frozen-thawed semen within the female reproductive tract may be very short (possibly less than 4 hours in some species, and therefore if ovulation occurs 5 hours or more after insemination, fertilization would not take place. While information about ovulation status can be obtained using ultrasound scanning in readily handled domestic species, this is not practical in most wild species that, of necessity, require sedation or anesthesia for such examinations (See, for example, a recent study of brown bear ovarian dynamics ( Torii et al, 2020 ). Successful AI in the giant panda is known to depend on very precise estimations of ovarian status, either by the regular examination of cellular morphology in vaginal smears ( Durrant et al, 1998 ) or the regular (8-hourly intervals) measurement of urinary oestrogen, progesterone and luteinizing hormone ( Martin-Wintle et al, 2019 ).…”