BODIES of extinct Ice Age mammals, such as woolly mammoths, that have been frozen in permafrost for thousands of years may contain viable sperm that could be used to bring them back from the dead, scientists said yesterday.
Research has indicated that mammalian sperm can survive being frozen for much longer than was previously thought, suggesting that it could potentially be recovered from species that have died out.
Several well-preserved mammoth carcasses have been found in the permafrost of Siberia, and scientists estimate that there could be millions more.
Last year a Canadian team demonstrated that it was possible to extract DNA from the specimens, and announced the sequencing of about 1 per cent of the genome of a mammoth that died about 27,000 years ago.
With access to the mammoths genetic code, and with frozen sperm recovered from testes, it may be possible to resurrect an animal that is very similar to a mammoth.
The mammoth is a close genetic cousin of the modern Asian elephant, and scientists think that the two may be capable of interbreeding.
The frozen mammoth sperm could be injected into elephant eggs, producing offspring that would be 50 per cent mammoth.
The suggestion that it may be possible to recreate an animal that is at least part-mammoth has emerged from a study of mice by Japanese, British and American scientists.
While many types of mammalian sperm, including that of humans, can be preserved by freezing, mouse sperm is vulnerable to damage that can limit its ability to fertilise eggs when it is thawed.
A team led by Narumi Ogonuki of the Institute of Physical and Chemical Research Bioresource Centre in Tsukuba, central Japan, has demonstrated that sperm better survives freezing if testes, or whole mouse bodies, are frozen.
Even sperm taken from mouse bodies that had been frozen 15 years ago was capable of fertilising mouse eggs and producing pups, the researchers found.
The work has technical implications for the breeding of laboratory mice for medical research, but it also shows in principle that mammalian sperm can survive in a body that has been frozen for several years.
This could mean that it is able to survive in similar fashion over much longer periods, as in mammoths frozen in permafrost.
Restoration of extinct species could be possible if male individuals are found in permafrost, Dr Ogonuki said.
If sperm of extinct mammalian species, for example the woolly mammoth, can be retrieved from animal bodies that were kept frozen for millions of years in permanent frost, live animals might be restored by injecting them into oocytes [eggs] from females of closely related species.
According to Japanese and American scientists, healthy offspring have resulted from the injection of sperm into female mice from male mice that have been in a deep freeze for 15 years. The mice, as a whole, were kept frozen in temperatures of -20 degrees celsius. 

Scientists at the Institute of Physical and Chemical Research Bioresource Center which is located in Tsukuba, Japan, say that cryogenic techniques were not used and that the mice were kept in a freezer.

Scientists also say that the sperm was not alive when it was thawed out. At least one out of every five females that were injected with the sperm, gave birth to healthy offspring which are also fertile.

"Obviously, those frozen sperm were all dead in the conventional sense. Motionless sperm had no chance whatsoever of fertilising in vivo or in vitro. Nevertheless, some of these, if not all, were ... genomically intact, because they were able to produce apparently normal offspring," said the researchers.

The idea behind the experiment is that scientists say that the results now make it possible to bring back prehistoric animals or extinct animals such as the woolly mammoth, but because any fully intact body found would have been frozen for over 10,000 years and would likely be too damaged organisms and bacterias. Endangered species such as elephants could also be reproduced, but scientists also say that the chances of an experiment like that of being successful are still "slim."

"Restoration of extinct species could be possible if male individuals are found in permafrost. If sperm of extinct mammalian species, for example the woolly mammoth, can be retrieved from animal bodies that were kept frozen for millions of years in permanent frost, live animals might be restored by injecting them into oocytes eggs from females of closely related species," said Dr. Narumi Ogonuki, head of the team who made the discovery.