140 • Technologies To Maintain Biological Diversity 



a substantial period of time requires relatively 

 large numbers of animals. Under the most 

 favorable assumptions, maintenance of 90 to 

 95 percent of the genetic diversity within a pop- 

 ulation for 100 to 200 generations would require 

 a captive population of at least several hundred 

 individuals sampled from throughout the range 

 of the species (15,27). 



Until relatively recently, zoos have not been 

 concerned with keeping representative levels 

 of genetic diversity within their exhibition 

 stock. Problems in fertility and juvenile survival 

 that often accompany exhaustion of genetic 

 diversity were simply accommodated by obtain- 

 ing new specimens from the wild. As this be- 

 came difficult, and in some cases impossible, 

 zoos began to reevaluate their role. The result 

 has been establishment of programs to main- 

 tain pedigree information on zoo animals 

 through the International Species Inventory 

 System (ISIS) and to facilitate transfer of indi- 

 viduals among zoos. These efforts help main- 

 tain genetic diversity, but existing zoos can sup- 

 port at most 1,000 kinds of terrestrial 

 vertebrates at a minimum population of 250 (2), 

 whereas an estimated 1,500 to 2,000 kinds will 

 be in danger of extinction by the year 2050 (43). 

 The magnitude of the problem will thus out- 

 run currently available facilities for captive 

 breeding. 



Recent advances in reproductive biology and 

 cryopreservation may facilitate efforts to pre- 

 serve genetic diversity. Cryopreservation refers 

 to storage below —130° C: water is absent, 

 molecular kinetic energy is low, and diffusion 

 is virtually nil. Thus, storage potential is ex- 

 pected to be extremely long. Storage in liquid 

 nitrogen ( — 196 ° C) or in the vapor above it (ca. 

 — 150° C) is a useful technique: Liquid nitro- 

 gen is relatively inexpensive, inert, and safer 

 than comparable refrigerants (e.g., liquid hydro- 

 gen, liquid oxygen, or freon). 



Storage and eventual production of live off- 

 spring from frozen semen or embryos have be- 

 come common for cattle, sheep, goat, buffalo, 

 and horse. The semen of pigs can also be fro- 

 zen. In 1982, an estimated 10.5 million cattle 

 were produced through artificial insemination 

 with frozen semen. Similarly, bovine embryo 



Photo credit American Breeders Service 



This calf, born in 1984, was conceived with semen that 

 had been frozen for 30 years. 



transfer has become commercially viable and 

 increasingly involves the use of frozen embryos. 

 Commercial use of frozen semen and embryos 

 is less common in other livestock species, but 

 acceptable results can be achieved. Frozen se- 

 men is also regularly used with poultry and with 

 some species of fish (18). 



Cryopreservation of sperm and embryos of 

 wild species has been much more limited. To 

 date, blackbuck, giant panda, fox, wolf, chim- 

 panzee, and gorilla have been produced from 

 frozen semen (7,9,38); baboon (37) and eland 

 (8), as well as mice, rats, and rabbits, have been 

 produced from frozen embryos. Procedures dif- 

 fer among species, but in theory, semen and 

 embryos from a range of mammalian species 

 can now be successfully frozen. 



The contribution of cryopreservation to the 

 maintenance of animal diversity could be 

 tremendous. Properly frozen and maintained, 

 sperm and embryos have an expected shelf-life 

 of hundreds, if not thousands, of years. Al- 

 though initial collection and preservation costs 

 may be relatively high, subsequent storage costs 

 and space requirements are low, allowing for 

 long-term maintenance of large numbers of in- 

 dividuals and gametes. 



These individuals represent a frozen snap- 

 shot of the population at the time of collection. 

 If the initial sampling of individuals is done 



