Animals dying peacefully of old age is rarely newsworthy, but the death of Nola the northern white rhino at the San Diego Zoo on Sunday made international headlines. Nola was one of only four northern white rhinos left on Earth; her death is a real-time window on extinction.
In fact, that extinction has been happening for a long time. Starting in the early 20th century, poachers in search of rhino horns hunted the species from the thousands to the hundreds to, by the 2000s, the tens. The last sighting of a wild northern white rhino in Africa was in 2006.
Nola died on Sunday morning, when a lingering infection caught up with her. “Nola’s condition worsened and we made the difficult decision to euthanize her,” said the San Diego Zoo in a statement. “We’re absolutely devastated by this loss.” And now there are three—all too old or ill to reproduce, all under armed guard at a Kenyan conservancy.
Extinctions aren’t sudden catastrophes, despite the showiness of famous meteor strikes. They are long, slow declines. You can see them coming from years away.
“We feel really bad it’s happening on our watch,” said Andy Blue, associate curator of mammals for the San Diego Zoo, a few weeks before Nola's death. The zoo’s world-famous conservation program has rescued other species, like the California condor, from the brink of extinction, and it had been trying to breed northern white rhinos since Nola first arrived in 1989. But the reproductive habits of the northern white rhino proved tough to make it happen in captivity.
As Nola and her male companions, Angalifu and Dinka, aged past their reproductive years, scientists saw one last ray of hope. Before and after their deaths, the researchers studying them froze samples of their cells. If the northern white rhino has a future, it’ll start in a lab.
Nola’s history was surprisingly cosmopolitan. She began her life in the savannas of Sudan. Captured in 1975, she then spent two decades at the Dvůr Králové Zoo, now in the Czech Republic. (Whatever you think of captivity, her capture likely saved her from intense poaching in Sudan.) As strife engulfed the native habitat of northern white rhinos and poaching went unchecked, conservationists realized the population of northern white rhinos in zoos—then a non-whopping 15—might be the animals’ only hope for survival.
So in 1989, Dvůr Králové loaned Nola to the San Diego Zoo Safari Park. The zoo was already famous for successfully breeding a close relative, the southern white rhino. The northern and southern versions are either are subspecies or different species, depending on who you ask, but the southern white rhino is not endangered.
Nola settled in as the reigning queen of the South Africa exhibit in San Diego Zoo Safari Park. “She’s one of the favorites,” Blue told me. “She’s reminds of me a big dog”—a 4,000 pound-big dog. Nola liked apples and back scratches. Because her toenails grew unusually fast, she was the only rhino who got regular pedicures. In her later years, she spent most of her day plonked down next to a pond, her massive head resting in the sand.
But in her earlier years, Nola was getting busy—or, rather, zookeepers were busy trying to get her to get busy. A 20-year-old male northern white rhino, Angalifu, joined Nola in San Diego in 1990. The two mated but never conceived. Zookeepers tried keeping the two rhinos alone, with a third rhino, with a herd of female southern white rhinos, but nothing worked. “We honestly tried several different ways as far as herd management and eventually got to at where we are now,” Blue says.
By the time Angalifu died last year, both he and Nola were too old to reproduce. But his death, like Nola’s, was symbolic. “Our CEO met with us we and we talked about what it would take to make one last-ditch effort in these cases,” says Blue. “He literally went around the table and asked each team for commitment.” The teams all said yes.
If studying big, charismatic animals like the rhino is one end of biology, then way on the other end is the study of individual cells. That, among other things, led to the discovery of an infertility treatment called intracytoplasmic sperm injection, a difficult in vitro fertilization technique. If any northern white rhinos are born in the future, they will likely be the result of ICSI.
The San Diego Zoo saved the sperm and testicular tissue of of Angalifu and Dinka after their deaths. With only about an ejaculation’s worth of sperm from each rhino, that sperm is preciously rare. “ICSI is most conservative use of sperm. You inject a single sperm in an egg,” says Barbara Durrant, the zoo’s director of reproductive physiology. “In normal IVF, you need to put tens of thousands of sperm per egg, and in artificial insemination, you would need millions.” Nobody has done IVF with rhinos before, so Durrant’s lab will need to figure out the exact conditions—temperature, reagents, hormones—that coax rhino sperm and egg to combine in a petri dish. Biologists will them implant the embryos into surrogate southern white rhinos.
If the researchers run low on rhino sperm or if they are unable get viable eggs, they may turn to an even more experimental technique. Scientists could conceivably take skin cells saved from rhinos and transform them into stem cells, which can, in turn, differentiate into any type of cell, including eggs and sperm. Combining those eggs and sperm into new embryos shuffles the genes, which is preferable to simple cloning, since clones are of course all genetically identical.
All of this, if it even works, is likely to take decades and millions of dollars. Is it worth it? What complicates the answer is that northern and southern white rhinos are almost identical. “I can’t tell the difference, and I’ve seen a lot of rhinos,” Blue says. A controversial 2010 paper, which claims the northern and southern white rhinos are different species, notes small differences in the shapes of their heads. In the wild, the rhinos lived in different habitats and may have had different diets, but raising them in captivity basically erases all that. In one case, a northern and southern white rhino even mated to create a hybrid baby rhino, who grew up into adulthood but did not seem to be fertile.
To be clinical about it, the difference comes down to DNA. The northern and southern rhino populations have not exchanged DNA in the wild for millions of years, so they’ve drifted apart genetically. “They represent unique genetic components,” says Durrant, “And we want to preserve those genes.” That’s why some conservationists have argued that if you really want to preserve northern white rhinos you should hybridize them with the southern variety. It saves the genes, packaged inside a new type of animal.
But would that actually “save” the northern white rhino? As biologists get better at creating embryos in the lab, researchers have begun talking about “deextinction,” the resurrection of long-dead species like the wooly mammoth. Critics call it “self-gratifying,” arguing it is more about making humans feel good than for the good of the deextincted species. A resurrected species wouldn’t have a natural habitat, for one thing, and would lack the genetic diversity that would otherwise help a population resist disease.
The northern white rhino faces the same problems if scientists are to resurrect them from cells in a lab. But the San Diego Zoo is committed for now. “No one wants to give up,” says Blue. “There’s always hope, until we don’t have any other options.”
Rhino lovers have a particular reason to be optimistic. A century ago, just twenty southern white rhinos were left in the world. Thanks to an intensive captive breeding program centered at the San Diego Zoo Safari Park, the southern white rhino is back, living in the wild. One day, decades into the future, the northern white rhino might return to the savannah.