Hope is in sight for the most endangered mammal on Earth after scientists used an IVF technique to create hybrid rhino embryos.
Frozen sperm from male northern white rhinos – none of which remain alive on the planet – was thawed and used to fertilise eggs donated by females from a closely related sub-species.
The eggs successfully developed into early stage embryos, called blastocysts, ready for implantation into a rhino womb.
It is the first time the assisted reproduction technique, adapted from horses and cattle, has ever been used on rhinos.
The researchers hope it will enable them to pluck the northern white rhino from the brink of extinction.
Only two of the animals remain alive, protected around the clock by armed guards on a Kenyan conservation park, and both are female.
At a later stage the plan is to repeat the procedure using eggs from one or both of the surviving females.
The resulting embryos would then contain all the genes necessary to produce a new generation of northern white rhinos (NWRs).
Professor Thomas Hildebrandt, from the German Leibniz Institute for Zoo and Wildlife Research in Berlin, said: “These are the first in-vitro (laboratory) produced rhinoceros embryos ever. They have a very high chance to establish a pregnancy once implanted into a surrogate mother.
“Our results are solid, reproducible and very promising. Now we are well prepared to go to Kenya and collect oocytes (eggs) from the last two NWR females in order to produce pure NWR blastocysts where both eggs and sperm are from NWR.”
The last surviving male northern white rhino, named Sudan, died earlier this year at Ol Pejeta Conservancy in Kenya, aged 45.
His daughter and granddaughter, both of whom shared his home, are now the last two remaining members of the sub-species left alive.
However scientists have a stock of stored frozen sperm from male northern white rhinos that could rescue the animal from extinction.
Using an IVF method called Intracytoplasmic Sperm Injection (Icsi), members of the international team injected NWR sperm directly into eggs donated by female southern white rhinos.
The southern white rhino is still quite numerous, with an estimated 21,000 of the animals living in South Africa today.
Female southern white rhinos could act as surrogate mothers for a fledgling NWR population, say the scientists whose results are reported in the journal Nature Communications.
To remove the eggs, the researchers patented an almost two metre long (6.5ft) needle device guided by ultrasound.
More than 20 egg collections were performed among southern white rhinos from European zoos.
Several embryos have been frozen for future implantation into the wombs of surrogate mothers.
The procedure for carrying out the embryo transfers is still being developed.
Professor Cesare Galli, from the Avantea research laboratory in Cremona, Italy, which specialises in veterinary IVF, said: “In our lab we were able to develop procedures to mature the oocytes, fertilise them by Icsi and culture them.
“For the first time we had rhino blastocysts – an early stage of an embryo – developed in-vitro, similarly to what we do routinely for cattle and horses.”
Steven Seet, spokesman for the Leibniz Institute, said the scientists were in a “race against time”.
He added: “This research is groundbreaking. We are witnessing the development of a method that can help to compensate the negative impact of humans on nature.”