Researchers say they’ve found that frozen mice retain useable sperm, opening up the possibility of resurrecting extinct species such as the woolly mammoth.

The scientists said they developed a procedure to freeze sperm in mice and in mouse organs so that the sperm, once thawed, produces healthy offspring.

Sperm-freezing is already used for human reproduction, animal breeding, conserving endangered species and propagating animal strains used in research.

But the defrosted sperm aren’t always capable of fertilizing an egg.

In the new procedure, Atsuo Ogura of the Riken Bioresource Center in Ibaraki, Japan and colleagues said they froze parts or whole bodies of killed mice for a week or up to a year, at -80° Celsius (-112° Fahrenheit).

They then extracted sperm from the thawed mice and injected them into mouse eggs, producing healthy offspring, he said. The researchers also reported that sperm retrieved from the bodies of mice that had been frozen for 15 years yielded viable offspring.

The findings are published in this week’s early online edition of the journal Proceedings of the National Academy of Sciences.

The scientists said this approach might bring back extinct species if males of the species were found embedded in permanently frozen ground, known as permafrost. For instance, a frozen mammoth might contain viable sperm, Ogura said.

Such sperm could be injected into the egg of a closely related living species, such as an elephant, he added.

Another group of Japanese researchers calling themselves the Mammoth Creation Project have previously announced plans to try to resurrect a mammoth in a similar way, but have not announced success. Ogura, while not commenting on that project specifically, said his findings bode well for the possibility of resurrecting mammoths from long-frozen sperm.

Other experts have disparaged the idea, saying ancient DNA is typically too broken-up to be usable. They have also questioned the ethics of bringing back the mammoth, extinct for 10,000 years, since its natural habitat, too, is long gone. The Mammoth Creation Project scientists, though, say they’ve obtained permission to put their creations into a suitable sanctuary twice the size of Japan, in Siberia.

A more immediate problem with resurrecting a mammoth from sperm, Ogura said, is that a mammoth sperm and an elephant egg wouldn’t produce an actual mammoth. It would instead more likely spawn a mixed creature, or hybrid, halfway between a mammoth and a modern elephant.

His proposed solution: repeat the procedure, taking an egg from the mammoth-elephant mix and fertilizing it again with mammoth sperm. This would produce an animal 75 percent mammoth. Additional repeats would lead to better and better likenesses of the original.

“I do not know the reality of this scenario, but hope that I or any others will have a chance to try it,” he wrote in an email.

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