Image copyright Longleat Image caption Scientists have extracted eggs from rhinos at Longleat Safari park in Wiltshire

A UK zoo is taking part in a radical plan to save the world's last three northern white rhinos from extinction.

At Longleat safari park, scientists have collected eggs from southern white rhinos - a closely related sub species - to use for IVF.

The eggs will help researchers to develop the technology to help the remaining northern whites to reproduce.

A back-up plan is to mix eggs from the southern white rhinos with sperm from northern whites to create a hybrid.

It means that if the bid to produce a pure northern white rhino fails, at least some of the critically endangered animal's genes will live on.

Darren Beasley, head of animal operations at Longleat, added: "Effectively the female rhinos would act as IVF mothers, with embryos partly derived from northern white male sperm.

Image caption Sudan is the only male northern white rhino on the planet

"If the procedure works, the hope would be that southern white females would carry the developing embryos for up to 18 months before giving birth."

Experimental fertility technology may be the last hope for northern white rhinos.

The animals were once found across central Africa, but illegal poaching, fuelled by the demand for rhino horn, wiped out the wild population.

Today, there are just three of the animals left: a male, who is over 40, and two younger females. The former zoo animals, which are inter-related, are kept under tight security at Ol Pejeta Conservancy in Kenya.

However, a combination of age and fertility problems means that none is able to breed.

Dvůr Králové Zoo in the Czech Republic, which owns the animals, has now enlisted the help of wildlife fertility experts from Germany and Italy.

Image copyright Longleat Image caption The team used ultrasound to look inside the rhino's ovaries

The team believes the northern white's cousin - the southern white rhino - could be the key to saving the species.

Professor Thomas Hildebrandt, from the Leibniz Institute for Zoo and Wildlife Research in Berlin, Germany, said: "We are trying to find a solution for a very critical situation: we have a doomed species."

They have been collecting eggs from female southern whites living in zoos around Europe.

Longleat Safari Park is the first UK zoo to take part, and on Monday and Tuesday, the team managed to harvest nine eggs from three females.

The rhinos have not mated naturally with the zoo's male, which is why they were put forward to take part in the study.

Extracting the eggs required millimetre precision.

Prof Hildebrand said: "We have a two-tonne animal, and the ovaries are more than two metres inside.

"We operate on an ovary that is lying next to a blood vessel and if we poke that with our needle, there is a very high risk that the animal dies.

"We have developed a very sophisticated technique to make sure we don't do any harm to the animals."

Image copyright Longleat Image caption The scientists managed to extract nine eggs from three rhinos

The eggs have now been rushed back to the Avantea clinic in Italy, a lab that specialises in assisted reproduction in animals, where they will be prepared for fertilisation.

A rhino has never been born through IVF before, and the first aim of the project is to refine the technology using southern white rhinos.

So far the team has mixed southern white eggs with southern white sperm, and has achieved cell division in early embryos, which have been cryogenically frozen and stored. None have yet been implanted back into a a rhino.

Later this year, the researchers plan to head to Kenya to harvest eggs from the last two female northern white rhinos. The scientists believe their extraction technique is now so well established it will not put these animals at any risk.

These northern white eggs will be mixed with northern white sperm - and implanted in a surrogate southern white mother - in a bid to produce new offspring.

The fertility scientists admit the chances of success may be slim - but they are optimistic that the technology could help.

Prof Hildebrandt said: "The classical conservationists would not even call this a conservation approach, because it is so technical, so far beyond what you normally do.

"But we hope future generations will understand that this is the way to go. It is a technology that allows us to bring a species back form the brink of extinction that would normally be impossible - and that is our goal.

"We are extremely optimistic that we will achieve that."

Image copyright Longleat Image caption The scientists may create hybrids between southern white and northern white rhinos

But the scientists also have a plan B: mixing the eggs they have collected from southern whites with sperm from northern whites. This would create a hybrid species.

This would not be the first: a cross was born at a zoo in the 1970s after the two sub-species accidentally bred.

But it was little studied. And despite the fact southern and northern white rhinos are very closely related, some questions remain, such as whether a hybrid could breed to produce further offspring.

But with the northern white at such a critical conservation status, it could mean that at least some of its genetic material survives.

Dr Robert Hermes, also from the Leibniz Institute for Zoo and Wildlife Research, said: "Our great hope is to go to Africa to collect eggs from these last two northern white females and the fertilise them so we would have a pure bred northern white rhino embryo.

Image copyright Longleat Image caption The technology developed with the Longleat rhinos could also help other species

"But the last northern whites could die any time: anything could happen to them, then all their genetics would be lost. If we have at least 50% of this preserved in a hybrid - we would preserve at least half for future generations."

This rescue plan - which is also examining the role that stem cell technology could play in the future - is conservation science at its most extreme.

And some wildlife experts believe that rather than pouring money and resources into a species that is nearly doomed, more effort should be put in saving more viable rhinos species, whose numbers have plummeted in recent years as poaching has surged.

But Jan Stejskal, from Dvůr Králové Zoo, says that every effort should be made to save the last northern whites - and this could help other animals too.

He told BBC News: "Rather than conservation, I call this a rescue operation. There is no other option for these animals to reproduce.

"Of course, I have to admit we might not be successful. But on the other hand, we are gathering so much information that could be used for other species.

"I'm convinced this is worth trying, even if we don't succeed."

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