Twenty-one biologists from seven European Union nations met at Chester Zoo at the end of last week for the first European discussions on the idea.
A liquid nitrogen zoo could help to save dozens of animals from extinction. As the human population rises, thousands of 'higher' species - mammals, birds, reptiles, amphibians and fish - already face grave danger of extinction. Countless 'lower animals' - insects, molluscs - have already become extinct, most of them never having even been known to science.
In the short term - about five years - advancing the techniques of cryobiology - the freezing of living material - could help many existing captive breeding programmes. These aim to prevent damaging inbreeding among the hundreds of individuals of a species living in zoos. Animals are prevented from mating with their relatives and carefully matched with those from other institutions. But transporting animals is expensive and causes stress. It would be better if frozen eggs, sperm or embryos could be sent.
In the longer term - 10 years or so - some scientists have a vision of genetic material being swapped within and between zoo populations and fragmented, minute populations clinging on in the wild.
A rare leopard, say, surviving in a few isolated jungle reserves would carry a small radio-collar so that its position can be constantly tracked by rangers. A female on heat would then be anaesthetised and injected with sperm extracted years earlier from a male living in another reserve.
The sperm could also be combined with eggs taken from zoo females. Fertilisation would take place in the laboratory, creating 'test tube' leopards which would be frozen, with details of the parentage of each kept on an international database. With all of these human interventions a population of a few hundred leopards split between zoos and wildlife reserves could cheat the lethal effects of inbreeding. Peter Bennett, of London Zoo's Institute of Zoology, said a large degree of intervention in wild populations would be essential if they were to survive.
So far scientists have developed techniques for freezing eggs, sperm and embryos for humans and some farm animals, but they have found it extremely difficult to freeze pig embryos. They will now have to learn how to remove eggs and sperm from a variety of species, how to perform test- tube fertilisations, how to replace embryos so that they implant into the wall of the womb and develop normally. And each technique will have to work most of the time.
Even so, a frozen zoo could never act as a safeguard against total extinction in the wild. Baby birds and mammals depend on their parents for care and information about what to eat, how to behave, what to avoid. A species confined to zoos is in danger of losing the knowledge of its proper environment, its 'culture', which must be passed from generation to generation for animals to survive in the wild.