Episode 8: The frozen zoo

San Diego is home to the world's first frozen zoo—a genetic library where scientists are racing to bank the tissues and stem cells of disappearing animals. What does conservation look like as we head into what some scientists may believe to be our next great extinction?

Photograph by Joel Sartore, National Geographic Photo Ark

San Diego is home to the world's first frozen zoo—a genetic library where scientists are racing to bank the tissues and stem cells of disappearing animals. What does conservation look like as we head into what some scientists may believe to be our next great extinction?


PETER GWIN (HOST): On the first floor of a research facility at the Safari Park of the San Diego Zoo, there are five stainless steel tanks the size of washing machines. It’s cold inside the tanks. Very cold.

OLIVER RYDER (GENETICIST): So it's a little bit like a thermos. Only the opening is big enough to insert racks that are like towers that look like apartment buildings that have multiple floors on them.

GWIN: And each floor is dotted with one hundred holes.

RYDER: And each of those holes would have a vial in it that would contain, say, one to three million cells, living cells.

GWIN: Living cells that come from animals all over the world to this facility: The Frozen Zoo. Oliver Ryder is a geneticist at the zoo. And he says there are cells from more than a thousand different species of animals frozen here.

RYDER: So you could take out those vials and you might pick up one and it's a gorilla and put it down and move to another place in the box and pull up and it would be a lemur. And put that down and pull up another box and it might be a stork or a crane.

GWIN: Next door, the whole Safari Park holds 3,000 animals. But here in one room there are cells from 10,000 animals preserved in a few freezers on wheels. It’s like a space-age Noah’s ark.

RYDER: And it's absolutely irreplaceable because, I mean, almost all the animals that are represented in it died a long time ago.

GWIN: But through their cells, their genes live on. And many of those genes come from species on the brink of disappearing. I’m Peter Gwin, and you’re listening to Overheard at National Geographic: A show where we eavesdrop on the wild conversations we have here at Nat Geo — and follow them to the edges of our big, weird, beautiful world. This week: How banking the cells of endangered animals could help researchers rescue these species from non-existence. More after this.


KATE GAMMON (WRITER): You remember that part in Jurassic Park where they open up the fake shaving can- fake shaving cream can, and they put the little vials? Yeah, It's kind of like that, but on a bigger scale.

GWIN: This is Kate Gammon, a freelance science writer. We first learned about the Frozen Zoo because of an article she wrote for Inside Science. And she first learned about the zoo at a conference where Oliver Ryder was the keynote speaker.

GAMMON: And so I got really interested in the idea of conservation biology through deep freezing and through cryobiology.

GWIN: Okay. So we got to- I've got to ask you, what is cryobiology? That's a giant word. That's kind of a scary word.

GAMMON: Sure. It's biology. So the study of life that takes place at a very, very low temperature. And the temperature that scientists have pinpointed is -196 degrees celsius. And that is the temperature where almost all processes of life stop. And you can stay there for almost ever without changing.

GWIN: So, the cells are frozen in time — at least until the researchers want time to move again.

GAMMON: You can thaw them from that temperature and they'll continue their processes of life, like jump right back into things like growing and dividing and, you know, coming back to life.

GWIN: Cue the thunder and the organ music, it’s alive! But it’s not quite Frankenstein, okay, it’s just a few cells. Still, those cells contain a lot of information. So when an animal from the zoo comes in for a check up, the vets take a sample to freeze.

GAMMON: So, you know, when they're putting in a little identification on an animal's ear, there's a little bit of skin sort of that gets clipped off that can also go straight into a test tube. But when an animal passes away, they can take things like eyeballs, internal organs, gonads. They can take sperm. After the animal dies, there’s still living sperm.

GWIN: A lot of the cell samples come from animals at the San Diego Zoo, but there are also samples from animals all over the world. And this isn’t the only frozen zoo out there, but it is the largest and the oldest.

GAMMON: It's interesting to point out that this- this whole effort was started in- in the 70s and it was based on the reproductive technologies that were emerging in humans. And so the doctor who started it, was a physician for people, and wanted to see if that you could if you could use assisted reproduction technologies in endangered species.

GWIN: So he’s really sort of considered the- the original frozen zookeeper?

GAMMON: Exactly. Yeah. The frozen zookeeper.

GWIN: His name was Kurt Benirschke and he was pretty forward-thinking guy. I mean, when he started saving the cells of endangered animals, it was like 10 years before “conservation science” had even been invented as a discipline. And it was before some of the big milestones in genetic research, like sequencing DNA, or cloning Dolly the sheep.

GAMMON: There was a poster that the researchers told me that was hanging in the office and it said, you know, "we must conserve things for reasons that we don't understand yet."

GWIN: Kate says that’s still the credo of the researchers at the San Diego Frozen Zoo.

GAMMON: And the researchers feel a real sense of a race against time where all these species are going extinct. And if we can just get a little bit of them bio-banked in these cryo-freezers, we have a hope for the future at least to understand their place in the world, if not to help other species and maybe even help these creatures return to the Earth one day.

GWIN: For more than 40 years, Oliver Ryder has been researching the DNA preserved at the Frozen Zoo. And one of his goals is to use that information to help figure out how to rescue animals from extinction.

RYDER: And I think my colleagues would certainly remember the time when we would receive a sample from the last individual of the species. And it’s a weighty responsibility.

GWIN: Yeah, I can’t even imagine. I mean, what must it be like to hold a vial that contains the last living cells of the last living animal of its kind? Especially knowing that the species was most likely pushed to extinction by humans? Oliver says, some experiences really stand out in his memory.

RYDER: One was the last male northern white rhinoceros — whose name was Sudan — died last year.

GWIN: Oliver had met Sudan years ago, when there were only a dozen or so northern white rhinos left in the wild. In 1986, he traveled to a zoo in Czechoslovakia to perform a biopsy and get a sample of skin from Sudan.

RYDER: And that sample turns out to be an exceedingly important sample.


GWIN: Here at Nat Geo, we were also following the story of Sudan.

RACHAEL BALE (WRITER, EDITOR): So this is a photo of Sudan. The last northern male white rhino, laying with his head on the ground and his keeper touching Sudan, forehead to forehead.

GWIN: Rachael Bale is a colleague of mine and she heads up our animal coverage. In October, a picture of Sudan was on the cover of the magazine. And this was important because for decades, the last wild northern white rhinos had sort of dwindled down to this small group living in a war zone. Their last habitat GWIN: was on the border between Sudan and the Democratic Republic of the Congo. And this left them vulnerable to poachers who could sell their horns on the black market. In the Congo, a group of conservationists fought to protect a tiny herd of rhinos by posting armed guards around Garamba National Park.

BALE: But eventually the fighting got so bad that it wasn't safe for them to keep their staff there anymore. They had to pull out.

GWIN: After that, the park became largely inaccessible to the rangers.

BALE: And then around 2008, northern white rhinos were more or less officially declared extinct in the wild. At that point, there were no confirmed sightings, no confirmed evidence at all of any northern white rhinos left in the wild.

GWIN: But there were a handful left in zoos; Including a female named Nola at the San Diego Zoo, and Sudan, and his relatives in the Czech Republic.

BALE: Everybody knew the northern white rhino was on its way to extinction. And people were trying desperately to make more of them.

GWIN: But, it didn’t work. The northern white rhinos in captivity were just too old to produce another calf. And then last year, Sudan got a leg infection. National Geographic photographer Ami Vitale had been following Sudan and the quest to save the northern white rhino for years.

BALE: When it became clear he didn't have much time left, they called her and she immediately flew out to document Sudan's last days.

GWIN: Sudan was old for a rhino, 45 years — actually the same age as the Frozen Zoo. In the moments before he died, he was surrounded by the keepers, vets, and armed guards who kept him alive as long as they could.

BALE: Knowing that you are actually witnessing the extinction of a species in real time. Maybe this will finally make people care. That was my initial reaction.

GWIN: Because, it’s not just this one subspecies of rhino we’ve lost. Scientists estimate a million plants and animals are threatened with extinction right now. Experts have begun to call this trend “the sixth extinction.”

BALE: Species are always naturally going extinct. But there have only been five other times in history where huge numbers of animals have gone extinct all within the same period. And right now we're in the middle of the sixth.

GWIN: Right. Sort of like we're living in a stage like the dinosaurs all disappearing.

BALE: Yeah. It's exactly like that. Except instead of it being a comet, it's us.


GWIN: All we have left of the last male northern white rhino are his cells, but Oliver didn’t just collect a sample from Sudan.

RYDER: We have living cells from 12 northern white rhinos.

GWIN: That's fascinating. I mean, you have this little immortal herd of frozen rhinos. Like what can you do with that? What’s the application of it, at some point?

RYDER: Well, okay, Peter, but I would just say that we have the cells of rhinos, we don't actually have a herd of rhinos. We have the potential for that.

GWIN: We have the potential because as long as the DNA remains intact, there’s hope for bringing back the animal. It’s a hope that, over time, feels more and more realistic.

RYDER: Over the course of the years that the Frozen Zoo has been in operation — or has been banking samples — the technologies have developed in an astounding and maybe I think quite unpredictable way.

GWIN: Researchers at the Frozen Zoo have already managed to turn some of the skin cells from Northern White Rhinos into a special kind of stem cell.

RYDER: These are cells that can make any cell in the body.

GWIN: And these stem cells… they could even make eggs and sperm, which could then be used to create an embryo, which could then be implanted in a surrogate animal. It sounds like science fiction, but at least in theory, it could work. There’s actually another subspecies of white rhino, the southern white rhino, and it’s close-enough genetically to the northern white to carry its calf. Oliver says it’s already been done in mice. A baby mouse was grown from skin cells. Of course, it will be challenging to replicate this with rhinos. But Oliver says, it’s worth a shot.

RYDER: We are facing a choice to let the northern white rhino go extinct or to try to utilize this technology.

GWIN: And the possibilities are kinda thrilling, if you let yourself imagine them. If we could bring back an animal that’s about to go extinct, it makes you wonder: could we bring back an animal that went extinct 10 years ago? 100? 1,000?

RYDER: Jurassic Park is a great story. And it's fiction. It's a story. I think that the … I'm just pausing to collect my thoughts here….

GWIN: Oliver was hesitant to go there, but it’s tantalizing to think of the amazing possibilities. I mean, if we could bring back a rhino, then why not bring back other species that have gone extinct? In 2013 we hosted an event at Nat Geo headquarters here in D.C. called “TEDxDeExtinction.” Oliver Ryder spoke at that conference and so did other researchers, like George Church from Harvard Medical School. He wants to bring back the woolly mammoth.

GEORGE CHURCH (GENETICIST): We would propose to make a hybrid elephant that has the best features of modern elephants and the best features of mammoths. We have enough of a mammoth genome and enough information in computers about many other species to know the key physiological features that we need to introduce in the elephants that they will be happy year-round in the Arctic regions of Canada and Russia.

GWIN: But that kind of talk — it really annoys some conservationists.

STUART PIMM (CONSERVATIONIST): I mean, it's total grandstanding. It's, you know, saying, look, we are technological gods.

GWIN: Stuart Pimm studies how to prevent extinctions at Duke University. And he’s not a fan of bringing back the woolly mammoth.

PIMM: Even if we brought back a woolly mammoth, what the hell would we do with it? You know, put it in a zoo, charge people 50 bucks to see it? We don't have a place to put it.

GWIN: I mean, polar bears live in the Arctic, but their survival is threatened by climate change. Wouldn’t woolly mammoths suffer the same fate? Stuart says we have to be cautious about technological optimism. He’s been called to testify in front of Congress about the potential environmental impact of new laws.

PIMM: All the times that I have testified to committees on Capitol Hill, there's always been the argument: Well, you know, if we can keep species going in captivity, maybe if we can bring them back with a Jurassic Park scenario, then, you know, we can keep on destroying the forests. And if we make a mistake, we can bring everything back. We can- You know, we can correct our mistakes with our brilliant technology. There is an enormous moral hazard in that.

GWIN: Stuart says, he worries that if we rely on a back-up plan for extinctions,
we won’t work as hard to protect animals and their habitats now, while they’re still alive.

GWIN: And returning captive animals to the wild — it isn’t easy. Stuart says it would be even more difficult to return an animal to the wild that’s been brought back from extinction.

PIMM: Well, I think the first thing is we have to have a wild into which to put it. You know, you have to say, why did it go extinct in the first place? You know, and unless those threats have been removed, then there's no point in putting it back and having a repeat performance.

GWIN: Just carving out a home for the northern white rhino will require solving a lot of complicated problems.

PIMM: You need the land. you need the place to put them. You need to make sure that they're not hunted to extinction again. You need to do something to aid the local communities so that they view it in their interest to keep these animals going, all against a background of the very difficult political challenges that all West African countries face.

GWIN: And then, will the animals be able to survive on their own? I mean, an animal isn’t just its DNA. Stuart says it’s a link in a whole interconnected ecosystem — from the specific plants it eats, to the animals that eat it. Even down to the bacteria living in its gut. DNA also can’t capture behavior, and knowledge.

PIMM: There are these various programs on TV where people are dropped into the wild and they have to try and exist. And I sometimes watch a few minutes of this with my students and we laugh ourselves silly. But the point is, that smart though we are, most of us wouldn't last a day in the wild. Because we've lost all the necessary skills to survive. It's the same thing with species.

GWIN: Because as advanced as the technology is, it’s almost entirely untested. We haven’t yet successfully resurrected a species. And rebuilding a population from scratch, well, that’s a mammoth task.

GWIN: Stuart says his fear is that at the end of the day we’ll have a freezer full of animal DNA but no animals.

PIMM: I worry when people begin to think that technical solutions are all that we need.

GWIN: He says what we need to do is to plant trees, prevent poaching, and work on saving animals before they go extinct. But it turns out, some of the most exciting innovations from the Frozen Zoo are actually just higher-tech versions of strategies that more traditional conservationists, like Stuart, are already trying. For example, in Florida, local panthers were endangered. As their numbers dwindled, they started to inbreed and they were getting sick.

PIMM: They had kinked tails. When the animals were caught and examined by vets, they had low sperm count. Some of them didn't have descended testicles. I mean, these were nightmare animals. They were genetically in really bad shape.

GWIN: The Florida government decided that the panthers desperately needed a fresh infusion of healthy genes, so they flew in a group of closely related panthers from Texas. Stuart’s research showed that once the Texas panthers bred with the Florida ones, the genetic damage was fixed and the population became healthy again. At the Frozen Zoo, Oliver Ryder’s doing something similar.

RYDER: We have species in the Frozen Zoo that are not from extinct species, but are from species that have a very limited gene pool. And if we can flow the genes from the Frozen Zoo into the living population, we can reduce the impacts of small populations like inbreeding.

GWIN: This is an unprecedented moment for the planet. And while the Frozen Zoo isn’t going to be the Noah’s ark that saves all the animals from the flood, Stuart Pimm says it’s one lifeboat. And we’re gonna need every lifeboat we can get.

PIMM: It must not, it never must not be plan A, it must be part of a comprehensive set of ideas. The most important of which are keeping species alive in the places where they live.

GWIN: As long as we keep doing that, then Stuart says the Frozen Zoo is like an insurance policy. He says it adds to our understanding of the whole problem
and might be useful in the future.

PIMM: Nobody says of medicine. We shouldn't have medical advances because they're too expensive. We say, look, let's invest in better medicine. Let's try new technologies. Let's take try new approaches. That has to be true for conservation.

GWIN: Because, just like it says on that poster on the wall of the Frozen Zoo: “You must collect things for reasons you don’t yet understand.” One day, we’ll probably be really glad we did. More after this


GWIN: While The San Diego Zoo is trying to preserve animals through their cells, Nat Geo explorer Joel Sartore is trying to photographically preserve animals by taking portraits of every species. It’s a project he calls “The Photo Ark.” We’ll put a link to some of his work in our show notes.

And you can hear more about Sudan, the last male northern white rhino, from photographer Ami Vitali. She followed his story for nearly a decade before she photographed his last moments. She wrote about that experience in the October issue of the magazine.

And all the talks from the TEDxDe-Extinction conference are available online, including the one from George Church about recreating the woolly mammoth.

All this and more can be found in our show notes. Look for them in your podcast app.

So this is our last episode of season two. I can’t believe that it’s gone so fast, but it’s been a great wild ride of cool stories, and we hope you’ve enjoyed them. We’ll be back with more soon. And the best way to know when new episodes are available is to subscribe in your favorite podcast app, so stay tuned.

I’d also like to say a heartfelt thank you and farewell to Emily Ochsenshlager and Ibby Caputo. We couldn’t have done it without you.


Overheard at National Geographic is produced by Brian Gutierrez, Robin Miniter, and Jacob Pinter.

Our senior producers are Kristen Clark and Jinae West.

Our editor is Ibby Caputo.

Our fact checker is Robin Palmer.

Our Deputy Director of Podcasts is Emily Ochsenschlager.

Hansdale Hsu composed our theme music and engineers our episodes with additional help from Nick Anderson and Interface Media Group.

Special thanks to Anny Celsi and Nicolás Dalmas Di Giovanni.

This podcast is a production of National Geographic Partners.

Whitney Johnson is the director of Visuals and Immersive Experiences.

Susan Goldberg is National Geographic’s editorial director.

And I’m your host, Peter Gwin.

Thanks for listening, and see y’all soon.