Only two northern white rhinos are left. And they are...

There were fewer than 100 southern white rhinos (Ravelobensis Stups) a century ago. Today there are over 20,000. Unfortunately, this success story only extends to the southern subspecies of the white rhinoceros. With the death of the last man in 2018, the northern white rhinoceros (Ceratotherium simum Cottoni) has passed the point where it can of course be saved. With only two females left, the subspecies is now classified as functionally extinct.

This is a poignant but not entirely hopeless situation. New techniques such as in vitro fertilization (commonly known as IVF) allow us to bypass normal reproduction to produce new babies with northern white rhinos. Sperm samples from deceased men kept in biobanks solve one side of the equation, but there are no frozen stocks of northern white rhinoceros eggs to rely on so easily.

We set up the Rhino Fertility Project at Oxford University to solve this problem. We believe that by using ovarian tissue from deceased female rhinos to grow lots of eggs for fertilization in a laboratory, we have found a way to save the northern white rhinoceros – and possibly other endangered species – from extinction.

The first breakthroughs

A team led by Professor Thomas Hildebrandt from the Leibniz Institute for Zoo and Wildlife Research in Germany had a breakthrough in 2019 when it succeeded in collecting eggs from the last remaining female white rhinos from the north. After the females were treated with hormones, the immature eggs were collected, taken to a laboratory where they were matured and then fertilized with frozen sperm.

To date, a handful of northern white rhinoceros embryos have been produced this way. They are frozen and awaiting implantation in a female replacement southern white rhinoceros. The transfer of embryos to surrogates to create baby animals is well established for many species, including horses and cows, but is still in the developmental phase for rhinos.

The biggest limitation with this approach, however, is that hormonal stimulation in female rhinos produces few eggs per cycle. Not all of these eggs fertilize, and not all of them develop into an embryo. After being transferred to a surrogate mother, only a few will complete their development and become baby rhinos. As you can imagine, this limits our ability to revive entire populations, as there are only two rhinos left to collect these precious eggs from.

What if we had the means to produce more eggs? While eggs from female rhinos are in short supply, creating eggs from ovarian tissue from deceased rhinos could fill the void.

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Petri dish rhinos

As with humans, every female rhinoceros is born with thousands, if not millions, of immature ovarian follicles. In the center of each of these eggs is an immature egg, also known as an egg cell. The follicle grows over months until it is ready to ovulate. At this point, these contain liquid and secreted hormones that affect the menstrual cycle. Ovaries contain many of these immature follicles just waiting to be activated – in fact, far more follicles than are actually needed. As follicles grow and some are selected to ovulate, many are lost.

Rhinos do not go through menopause, so the ovaries of older animals still contain small follicles. Our goal is to grow these from the ovarian tissue of a deceased rhino in the laboratory. We experiment with techniques that will maximize the number of follicles we can grow in a culture dish.

Using the full potential of the rhinoceros ovaries, we want to breed as many eggs as possible. We develop our method using ovarian tissue from different rhino species including the southern white, Indian and black rhino. With all rhino species either threatened or critically endangered, this technology could help more species than just northern whites, including the rare rhinos from Javan and Sumatra.

But as you can imagine, there aren’t many rhinoceros ovaries for laboratory testing in the UK. Timely shipment of tissue from threatened or endangered species in Africa to the UK is not possible due to many legal hurdles and paperwork. The collaboration between zoos, wildlife parks and research institutes is of the utmost importance here so that we can preserve this valuable ovarian tissue.

The experience and knowledge gained by developing this technique could even be useful in conservation efforts for other species. By freezing ovarian tissue and sperm from other endangered species, and developing methods of cultivating follicles in laboratories, we could prevent further loss of some of the world’s most famous wildlife and revitalize ecosystems rich in biodiversity.

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