Sequential Ovulation, Fertility of Polyestrus, and Induced Ovulation in American Black Bears (Ursus americanus)

Improving our understanding of the reproductive physiology of bears is critical for effective conservation and management of both captive and wild bears. Increasing our knowledge of bear reproductive physiology is of ever increasing importance due to the fact six of eight species are either threatened or endangered on a global scale, and all 8 species are threatened with extinction in at least some of their range. Bears evolved several unique reproductive physiological adaptations, including multiple paternity and delayed implantation, but other more subtle adaptations may exist as well. This thesis studied, during three mating seasons, a semi-free ranging population of the non-threatened American black bear (Ursus americanus) to better understand the subtle intricacies of the reproductive physiology in bears. This study examined equal estrus fertility and sequential ovulation. Sequential ovulation is defined as ovulations occurring during each estrus, which are temporally sequential. Equal estrus fertility is defined as equal fertility of each estrus of a polyestrous female regardless of current pregnancy status. Ovaries were examined to determine if sequential ovulation occurred in each estrus of polyestrous females. During the mating season, females were anesthetized and their ovaries were observed to document ovulatory events, which allowed accurate pairing of an ovulation with a specific estrus. The presence of both a corpus luteum (CL) and corpus hemorrhagicum (CH) in the same female as well as a recent and well developed CL in 2 of 4 polyestrous females confirms temporally distinct ovulatory events (sequential ovulation) in this study population. Breeding behavior of females was extensively observed and recorded during the breeding seasons to determine if each estrus was fertile regardless of conception status. All mating behavior was recorded until females were anesthetized for examination and embryo extraction. Matching conception to a specific estrus was achieved using paternity testing and examining embryo development. Paternity results of 9 embryos from 6 polyestrous females showed 4 embryos were conceived during the first estrus while 5 were conceived during the second estrus, clearly indicating equal estrus fertility regardless of current conception status. These results strongly indicate the potential of superfetation in the American black bear and provide insight into the complex behavioral and physiological breeding mechanisms of bears. Captive breeding programs may eventually be able to breed females with different males during each estrus to potentially increase the chances of multiple paternity. Induced ovulation has been widely assumed in bears, yet little conclusive evidence exists. The third chapter of this thesis reexamines this question by looking at both hormonal data of isolated females during the mating season and anatomical data of ovulatory events from both successfully bred and unsuccessfully bred females. The estrogen profiles showed clear indications ovulation did occur in the nonmated and isolated females. Ovulatory events also occurred in unsuccessfully mated females, which also had a higher average number of ovulatory events compared to successfully bred females. Although preliminary, the combination of these data challenge the assumption that bears are induced ovulators. Further research is needed to resolve this matter conclusively.