Abstract

Traditional methods of captive breeding (i.e., the pairing of genetically compatible individuals) have limitations that inherently prevent such programs from reaching their ultimate goals of safeguarding certain species. As discussed previously,¹⁸ these limitations include:

1.  The amount and quality of physical space available to house adequate numbers of animals for maintaining genetic diversity

2.  The ability to adequately monitor and maintain the health and welfare of wildlife species in captive conditions when there is typically little or no information available on their life histories

3.  The ability to accommodate species-specific behavioral cues, which for some animals is vital for ensuring reproductive success

4.  Meeting the nutritional requirements of wildlife in captive conditions with processed, artificial diets for cost-effectiveness

5.  The ability to properly identify and treat reproductive failure, which typically is not directly attributable to a physiologic condition, but rather to a lack of information regarding the unique requirements (e.g., environmental and social) for inducing breeding activity in a particular species

6.  The ability to properly manage the genetics of small captive populations

Reduced genetic variation and deleterious inbreeding effects are significant problems facing small populations,^21,26 yet in the last decade it was estimated that 76% of the over 2700 mammals, reptiles and birds commonly housed in zoos are represented by fewer than 25 individuals.⁵ Considering the problems, limitations, and inherent risks associated with traditional approaches to captive propagation, it should not be surprising that innovative methods such as assisted reproductive techniques be considered as essential supplements to future captive breeding strategies. Certainly, assisted reproductive techniques such as artificial insemination and embryo transfer have clearly benefitted livestock production for managing the genetics and expediting the generation of valuable stock. In the case of embryo production by in vitro fertilization, over 80,000 human babies have been produced to date using this technique.

However, despite the advances in assisted reproductive technology in livestock and humans, it must be acknowledged that its practical application with regard to the preservation of rare or endangered species is regrettably premature at this time. By definition, endangered species are of too limited numbers to acquire adequate material to conduct the appropriate research. Indeed, it may be that there is insufficient time (particularly so in the case of short-lived species) to conduct investigations to disclose features specifically unique to their reproductive biologies, which are paramount for the development of effective and reliable methods for assisting reproduction. This is a particular dilemma for those species that do not have suitable non-endangered or domestic animal counterparts to serve as developmental models or, perhaps, as surrogate recipients for interspecies embryo transfer.

Interspecies embryo transfer presents an additional challenge to assisted reproduction in that its success depends on the selection of appropriate donor/recipient combinations. There have been several reports of successful interspecies transfers of embryos from rare or endangered species, including gaur²³ and banteng²⁵ embryos transferred to domestic cattle, a bongo embryo transferred to an eland,⁷ mouflon⁴ and Armenian red sheep⁹ embryos into domestic sheep, Grant’s zebra^2,17,24 and Przewalski’s horse embryos transferred to domestic horses,^17,24 and an Indian desert cat embryo transferred to a domestic cat.²⁰ However, it must be realized that despite these well-publicized successes, there have been many more failed attempts at producing viable offspring via interspecies embryo transfer. These failures have been manifested in a number of ways, including early resorptions of the foreign embryos, or mid- to late-term abortions; prolonged and complicated pregnancies in equids,^17,24 wild cattle,²³ and buffalo;^6,8 and inexplicable developmental abnormalities in aborted or stillborn fetuses in nondomestic sheep.³

The first successful interspecies transfer of an embryo collected from a zoo animal (gaur) to a domestic cow was reported in 1981 by Stover, et al. However, these investigators reported that of three pregnancies established, two were lost between 5 weeks’ gestation and near term. The placenta from the one live gaur calf born contained an abnormally low number of placentomes and abnormal histologic architecture.¹³

At the Henry Doorly Zoo, we have incorporated genome resource banking and assisted reproduction in the captive breeding program for gaur. Several investigations have been performed to develop optimal methods for cryopreserving gaur epididymal sperm collected postmortem¹² as well as semen collected from live bulls by rectal probe electrostimulation.^10,15,22 Live gaur calves have been produced by artificial insemination using thawed gaur semen¹⁰ and epididymal sperm collected and cryopreserved 27 hours after death.¹² Currently, approximately 14,000 individual straws of cryopreserved gaur sperm from 48 different gaur bulls (18 of which are no longer living) are being stored at the genome resource bank at the Henry Doorly Zoo.

Live gaur calves have also been delivered from in-vitro-produced embryos from oocytes collected postmortem¹⁴ as well as by ultrasound-guided, transvaginal oocyte retrieval.¹ In fact, a total of seven gaur calves were produced by the transfer of fresh, in-vitro-generated embryos transferred singly into 17 domestic cattle (41% efficiency). Unfortunately, all seven of these calves died within 1 week after parturition. The reasons for these losses were inconclusive; however, evidence of immunologic rejection of the gaur fetuses by the domestic cow dams was apparent.¹¹ It is also highly likely that the interspecies pregnancies were further compromised by the fact that the embryos were in-vitro derived, based on reports of intraspecies transfers of in-vitro-produced embryos in domestic cattle which indicate higher calf mortality rates as compared to pregnancies produced from in-vivo-derived embryo transfers.¹⁶

In conclusion, assisted reproductive technology is becoming an increasingly important management strategy for the conservation of nondomestic species.¹⁹ However, it is becoming evident that the greatest benefit from this technology will be in the genetic management of stable populations, rather than for the propagation of endangered species. Interspecies embryo transfer, although an attractive and exciting concept, has historically resulted in many more failures than successes. Perhaps with continued research and improved knowledge of the unique reproductive physiologies of novel species can we expect to increase the likelihood of utilizing this technique to safeguard rare or endangered species.

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