Preservation isn’t unknown in the world of zoology—the concept has been around for decades. But as species continue to disappear as quickly as they have in recent years, learning about the methods people use to preserve animal life can offer hope.

It’s a well-known fact that the last male of the northern white rhino subspecies passed away in 2014, leaving only two surviving females. In recent years, scientists at the Ol Pejeta Conservancy in Kenya have been developing new ways to preserve this species.

Najin, the mother, and Fatu, the daughter, are the last two remaining. This leaves the subspecies functionally extinct, meaning they are no longer able to reproduce. Before Angalifu, the last living male, passed, scientist Carly Young collected semen samples  from him and stored them using a process called cryopreservation.

Cryopreservation—the process in which biological material is preserved using liquid nitrogen—has been successfully used many times in the past, both to transport samples over long distances and to store them for long periods of time.

The BioRescue consortium, led by the Leibniz Institute for Zoo and Wildlife Research, collected oocytes from Fatu, the younger of the two northern white rhinos. Oocytes, or immature eggs, mature into ova, which—when combined with sperm cells—can form an embryo.

The process used to create an embryo is called intracytoplasmic sperm injection, or ICSI. This procedure has worked successfully on kangaroo eggs and sperm cells, so the idea of the same process working on a different mammal isn’t far-fetched.

Unfortunately, Najin is too old to support an embryo, and neither she nor Fatu is healthy enough to carry one. So scientists with BioRescue turned to the possibility of a surrogate. Surrogacy isn’t new in the animal kingdom—it has been studied for years, long before the dire situation of the northern white rhinos.

The idea is for a female southern white rhinoceros to carry the embryo to term and give birth to a northern white rhino.

Some have reasonably questioned these efforts, as the concept of harming or endangering surrogate rhinos has raised ethical concerns. The morality of subjecting an animal to a process that could potentially harm it has led some reporters to question BioRescue’s methods.

After a thorough ethical evaluation by the University of Padua in Italy, it was determined that no harm was caused to the surrogate southern white rhino mothers. Although this process has not yet succeeded, there is still much hope for the northern white rhino.

Proof of pregnancy has been successfully achieved in southern white rhinos, though none of the attempts with northern white rhino embryos have yet worked.

In the future, the ultimate goal of these efforts is to reintroduce the northern white rhino to the world—not as a memory of what once was, but as a living, thriving species once again.