Improved conditions in zoological gardens lead to an increased life span as well as to an enhanced breeding success and decreased neonatal mortality in many species. Thus, uncontrolled breeding inevitably creates a surplus of animals, even in endangered species. As culling or euthanasia of surplus animals are legally and ethically controversial, there is an increasing demand for an effective way to control reproduction. Currently, there is no effective hormonal treatment described to terminate pregnancy in bears. Recently, pregnant brown bears were treated three times with a prostaglandin F2α analogue (intramuscular injection of 0.75 mg Tiaprost), nevertheless all animals had a normal birth (Teuschner, personal communication). Methods actual used to control reproduction in bears (e.g., testectomy, sterilization of the male, melanogestrolacetate-implants in females) do not satisfy all the essential requirements (especially reversibility, no side-effects on health and behavior). Therefore, research on new approaches to control reproduction is highly desirable.1
The aim of this study was to establish a new approach for the control of reproduction in bears. The effects of an antiprogestin, estrogens or their combination on the implantation of blastocysts at the end of the embryonic diapause and on already implanted embryos were investigated. The reproductive physiology of bears is characterized by seasonality and a period of delayed implantation between mating in April/May and implantation in November. There is convincing evidence that progesterone is produced during this period and that progesterone concentrations increase towards the end of diapause.2,3 Therefore, the application of antiprogestins may be a good choice for an effective birth control in bears. They act by a competitive displacement of progesterone at the cellular receptor.4 Thus, depending on the time of application, antiprogestins might prevent implantation or induce abortion.
In 2 consecutive yr the antiprogestin J956 was applied to 11 female bears (nine Ursus arctos, one Ursus tibethanus, one Tremarctos ornatus) living in zoos. The application was performed orally (n=4) or parenterally (n=12). J956 was administered alone or in combination with ethinyloestradiol, and before (during diapause, October) and after implantation (December) of the embryo. The effects of antiprogestin treatment were documented by ultrasonographic examination of the urogenital tract and by endocrinologic monitoring of progesterone in blood and feces.2 Oral application was inefficient (success 0/4). In contrast to oral application, none of the parenteral treated animals remained pregnant (success 12/12). Parenteral treatment with J956, with or without ethinyloestradiol, was effective in disrupting the pregnancy before implantation (success 6/6) and after implantation (success 6/6), but application 1 mo after implantation (n=2) resulted in incomplete resorption of the foetuses.
From a management point of view, there are several advantages to an interruption of pregnancy during diapause in bears. Because of their seasonality and delayed implantation, contraceptive intervention can be reduced to a single application by teleinjection. Sexual behaviour and social interactions are not suppressed; resorption of microscopically small blastocysts did not affect the health and fertility of the mother. Because of the risk of an incomplete absorption of already implanted embryos or a defective abortion of the foetuses, treatment with an antiprogestin late after implantation is not recommended.
In conclusion, interruption of pregnancy during diapause offers a new possibility for the management of reproduction of bears in captivity. Antiprogestins might be the most suitable method to prevent implantation.
The authors would like to thank Dr. W. Elger for supplying J956, Dr. Ch. Osmann, Dr. M. Martys, K. Daheim and U. Köhler for their co-operation in supplying the bears.
1. Asa CS, Porton I, Baker AM, Plotka ED. 1996. Contraception as a management tool for controlling surplus animals. In: Wild Mammals in Captivity. Kleiman DG, Allen ME, Thompson KV, Lumpki S, eds. The University of Chicago Press, Chicago, Pp. 451–467.
2. Hellgren EC, Vaughan MR, Gwazdauskas FC, Williams B, Scanlon PF, Kirkpatrick RL. 1990. Endocrine and electrophoretic profiles during pregnancy and nonpregnancy in captive female black bears. Canadian Journal of Zoology. 69: 892–898.
3. Tsubota T, Howell-Skalla L, Boone WR, Garshelis DL, Bahr JM. 1998. Serum progesterone, oestradiol, luteinizing hormone and prolactin profiles in the female black bear (Ursus americanus). Animal Reproduction Science. 53: 107–118.
4. Elger W, Beier S, Chwalisz K, Fähnrich M, Hasan SG, Henderson D, Neef G, Rohde R. 1986. Studies on the mechanism of action of progesterone antagonists. J. Steriod Biochem. 25: 835–845.
5. Göritz F, Hildebrandt TB, Jewgenow K, Wagner N, Hermes R, Strauß G, Meyer HHD. 1997. Transrectal ultrasonographic examination of the female urogenital tract in nonpregnant and pregnant captive bears (Ursidae). Journal of Reproduction and Fertility. 51: 303–312.