When Sudan, the last living male northern white rhino (NWR), died in 2018, it seemed like the subspecies of gentle giants was destined for the history books. However, groundbreaking biological science could bring this formerly abundant species back from the brink.
As large herbivorous grazers, rhinos have a massive role to play in their savanna ecosystems. Their feeding behaviour “mows” the grass, creating areas known as grazing lawns, which studies have shown to be preferred feeding areas for other mammals, birds, and insects. These lawns also act as natural breaks for wildfires, which are unable to burn on the areas of short grass. The rhino’s love of wallowing in mud benefits the ecosystem too, as it helps to create and maintain watering holes which are vital for all savannah inhabitants.
NWRs are a subspecies of white rhinos previously found across Central Africa. They are closely related to the southern white rhino (SWR). A subspecies is defined as genetically similar but geographically distinct from other subspecies. However, attempts to mate the remaining NWRs with SWRs have not been successful.
The two remaining NWRs, Nájin and Fatu, are a mother-and-daughter pair who live under constant armed guard in Kenya, at the Ol Pejeta Conservancy. Even if there were any males remaining, neither Nájin nor Fatu are able to become pregnant naturally; Nájin has an ovarian tumour as well as problems with her hips caused by age, and Fatu has issues with her uterus.
How can scientists recover a functionally extinct species like the northern white rhino?
In the past, the death of Sudan, the last male, would have spelt the end – especially when combined with Nájin and Fatu’s inability to have children. Scientists use the term “functionally extinct” to refer to species that can no longer produce any additional members. They will be extinct when all currently remaining members die.
The BioRescue project aims to change this. Using leading methods from veterinary science and cell biology, professor Thomas Hildebrandt, leader of BioRescue, and his team are creating “test-tube baby” rhinos, with the hopes that the first new baby NWRs might be born as early as 2024.
Before the last male NWRs died, Hildebrandt and his team collected semen samples and froze them in storage. Initially, they attempted artificial insemination, but neither Nájin nor Fatu became pregnant. So the team changed tactics and began to focus on IVF, with the aim of using the closely related SWRs as surrogates.
IVF (in vitro fertilisation) is a process by which an egg is fertilised by a sperm outside of the body – hence the name “test-tube baby.” However, if the thought of collecting rhino sperm samples sounded challenging, that is nothing compared to collecting eggs. This involves putting the female under general anaesthetic and removing immature egg cells known as oocytes.
This process took so long to develop and perfect that by the time it was ready, Fatu was the only remaining donor left. So she is anaesthetised every three months or so for Hildebrandt and his team to collect. Since 2019, the procedure has been performed just 11 times, with 164 oocytes successfully collected.
However, this raises the next issue – oocytes are cells which are not suitable for freezing, so they must be used as quickly as possible after extraction. This involves them being flown to a lab in Italy which is specialised to perform the required procedures of preparing the egg and injecting the sperm.
Using the collected eggs, this complex procedure has allowed scientists to create 24 embryos with Fatu’s eggs and two different males. Embryos can be safely frozen until Hildebrandt and his team are ready for the next stage.
Will the technology be up to the task?
One of the biggest question marks hanging over the BioRescue project has been a lack of genetic diversity. With eggs from just one female and sperm from a small handful of males, any offspring produced would be very closely related, and offspring from a match of these rhinos would be at serious risk of issues arising from inbreeding. There simply isn’t enough genetic diversity there to make a viable population.
“We knew from the beginning that advanced assisted reproduction technologies that rely on natural gametes would not be sufficient in the long run to save the northern white rhino from the brink of extinction. So it is crucial that we pursue a complementary strategy of significantly increasing the genetic diversity of gametes and of producing them in much greater numbers – which might even make it possible to create embryos from Najin through artificial gametes, something that proved to be impossible using her natural gametes. It is encouraging to see that the stem cell specialists in our consortium, from Osaka University and the Max Delbrück Center, have achieved this important milestone. It is also important to note that the plans for natural gametes and artificial gametes are not separate paths, but are interconnected and intersect at the point where in vitro fertilisation produces embryos.”Professor Thomas Hildebrandt, in a press release
As far back as 2011, scientists have shown that skin cells can be ‘reprogrammed’ to become stem cells. Stem cells are often considered to be the body’s ‘raw materials’ and have the incredible versatility to become a range of different cells.
This is where the cutting-edge science comes in.
As recently as December 2022, a team of Japanese researchers has shown that it is possible to prompt stem cells to become primordial germ cells, precursors to sperm and egg cells. They are extremely close to using skin cells that have been frozen for nearly half a century to create viable sperm and egg cells, which would drastically increase the genetic diversity available for Hildebrandt’s BioRescue project.
“Developing a culture system that delivers robust results has been extremely challenging since the precise orchestration of the specific signals required to induce the desired cellular differentiation is unique to every species. It was also necessary to confirm that the primordial germ cell-like cells were genetically identical to the cells from which they originated – this can be a daunting task,” said Katsuhiko Hayashi, a scientist from Osaka University involved in this incredibly precise and demanding process. His statement was part of a press release for the paper in Science Advances.
The cataloguing of animal matter over time means that there are samples available from eight unrelated rhinos and four of their offspring. Cell Biologist Dr Jeanne Loring, the scientist who discovered the process to coax skin cells into stem cells in 2011, notes that this would mean NWRs would have a genetic diversity greater than the existing population of 20,000 SWRs, who are “doing just fine.”
One of the keys to maturing primordial germ cells into sperm and egg cells in mice studies is the presence of ovarian tissue, so it stands to reason that this may be required for rhinos too, but this cannot be collected from Nájin or Fatu. Researchers may be able to use horse tissue, as they are the closest living relatives to rhinos, or they will need to devise a way to turn stem cells into the required ovarian tissue.
BioRescue is not without opponents
Opponents of the BioRescue project have several concerns about how and why the project is going ahead. One of their key concerns is the cost, which is extremely high. BioRescue is being funded mainly by the German Science Ministry, which awarded the project a six-year grant. The project has also received additional funding from private donors and companies. Hildebrandt is quick to point out that it doesn’t exist instead of, or at the cost of any other conservation projects, but in addition to them.
A further concern is one which comes up in similar situations, that of the “technofix,” the use of advanced technology to right the wrongs of human activity, rather than addressing the underlying causes of these negative outcomes. The concern is that efforts of conservation would decline and instead be refocused on restoration. Speaking to BBC Science Focus, Hildebrandt opposes this: “No one is advocating that. We need to be aware that this is something we cannot do routinely for each species, precisely because it is so costly.”
The process of getting from where we are today to a healthy and stable population of NWRs is a long one, and further development in the field of stem cell research is still needed, but we are closer than we have ever been to bringing a species of animal back from the dead.
NWRs have been brought to this precipice of life and death by human actions. They have been hunted and poached, and the ever-present spectre of habitat destruction has only hastened their demise. The tireless efforts of scientists worldwide have created this chance to try and right these wrongs, but it’s vital we do not get complacent.
You can keep up to date on the conservation of the northern white rhinos and their life at Ol Pejeta Conservancy on their website.
Featured Image Credit: Jan Stejskal / Dvůr Králové Zoo
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