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March 19, 2018 marked the death of a northern white rhino named Sudan. He had been the last surviving male northern white rhino. Today, there are only two northern white rhinos alive, Najin and her daughter Fatu. My guest today, Thomas Hildebrandt, is part of a team of veterinarians and researchers using state-of-the-art science to try to keep the northern white rhino from extinction.

HILDEBRANDT: So we have 30 of these embryos. And we recently could show that these embryos, when we bring them back in a surrogate, then they can grow to babies. 

Welcome to People I (Mostly) Admire, with Steve Levitt.

The conversation you’re about to hear, at least for me, was incredibly fascinating; but it did get quite detailed. So let me provide a little bit of background before we jump in. Najin and Fatu, the two living northern white rhinos, live in a wildlife sanctuary in Kenya, protected 24-hours-a-day by armed guards. Although there are no longer any male northern white rhinos, researchers do have sperm samples from four males who’ve died. Unfortunately, neither of the two living rhinos are able to give birth. All hope it’s not lost, however, because Thomas and his colleagues have pioneered an I.V.F. technique for these animals, which involves a patented device used for oocyte and ovum pickup. As a little biology lesson, an oocyte is a cell in an ovary. It’s a precursor to an ovum; an ovum is a mature female reproductive cell that can create an embryo once it’s fertilized by a male cell. Where things get really crazy though is that a researcher named Shinya Yamanaka shared the 2012 Nobel Prize in Medicine for his discovery that skin cells can be reprogrammed into stem cells, and that opens up possibilities that seem like science fiction. As you can hear, Thomas speaks with a German accent. If you’d like to read a transcript of the episode while you’re listening or afterwards, you can find it on this episode’s page on our website at

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LEVITT: So you’re a leading expert on animal reproduction and perhaps the most challenging and high-profile case you’re working on is the northern white rhinoceros, a species that is on the verge of extinction, unless you and your partners can pull off something extraordinary. There are only two surviving northern white rhinos. And they’re both female, which creates some obvious problems.

HILDEBRANDT: Yes, for sure the most challenging task we took on. We worked with the northern white rhinos at the beginning of 2000 and we watched one individual after the other die. It was very depressing at that time. And in 2012 there was a big breakthrough because Yamanaka got the Nobel Prize for a technology — he turned ordinary skin cells into pluripotent stem cells that are cells which can be transformed in any kind of cell of the body, including making sperm and eggs. And that gave us a completely new hope because we have banked many individuals which were deceased — we had them in our biobank. And now we could work with these individuals again. And so a completely new program starting in 2015 developed a kind of master plan, how species can be saved under such severe conditions like the northern white rhino.

LEVITT: So in humans, we’ve gotten quite good at I.V.F., in vitro fertilization. There’s an egg and a sperm and the lab technician fertilizes the egg outside the body. And then you put that embryo back into the womb to grow. You have females and you probably have some banked sperm. In principle, you could do I.V.F. on the northern rhinos.

HILDEBRANDT: That is exactly one of our strategies. We already have generated 30 first-class embryos with this technology. However, to save a species, that really means you have to have enough genetic variation. And therefore, we need the stem cell way to actually guarantee a wide genetic diversity of the newly created population. So we hope, maybe in 12 to 15 years, we will be capable to reintroduce the first northern white rhinos back to the wild.

LEVITT: So you have just the sperm from the one male?

HILDEBRANDT: No, we have actually, we have sperm from four different males: from Saut, from Sudan, from Suni, and Angalifu. But Sudan and Saut are directly related to Fatu, so we don’t use them. So we go every three months to Kenya and perform an oocyte collection. We go through the rectum because otherwise we can’t really approach the ovaries. So it’s a completely different technical challenge in rhinos than in humans or horses.

LEVITT: So, also in humans, when you do I.V.F., you have to give all sorts of hormones. You do the same in rhinos? Or you can just go and harvest the eggs at the right time of the year?

HILDEBRANDT: No, we developed a very specific hormone protocol using similar hormones, like in humans, but our stimulation time is actually longer. So it’s a hormone which is produced in a hypothalamus, and that is in a special part of the brain, and this hormone stimulates then the pituitary gland, and the pituitary gland then gives the signal to the ovaries for more follicles. Because going every three months to Kenya is quite a challenge. It’s a very expensive operation. The hormonal stimulation has no impact on the quality of the eggs and that’s quite good.

LEVITT: And how do you figure something like that out? Trial and error or — take the northern rhino. There are obviously very few of them. You couldn’t have done very much experimentation. So, you used elephants or something to get the general idea going and then you switch it over to rhinos? I’m just trying to get a sense of how you even start to develop a protocol.

HILDEBRANDT: Actually, the elephant is a good example. I worked in my early career quite a lot with elephants. I developed the artificial insemination technique for elephants without surgical approach. And we produced more than 50 babies with this technique. But to develop something for the northern white rhinos, we started with carcasses. So if any zoo reported a dead rhino, I went there and tested the things on a dead animal. And then I optimized things until everything was perfect, then we used it on a live animal.

LEVITT: And we’re talking about this in a very scientific way, but I imagine the actual process is very hands-on. And you’re the actor, right? You’re the one who’s actually going in and taking the eggs. Could you just describe what that experience is like?

HILDEBRANDT: Yes. It’s not only the northern white rhino. We work with Komodo dragons or we work with Sumatran rhinos. Everything starts with the respect for the patient. So every step we do is so that we don’t do harm to the animal. And if I’m not really certain if this is possible, then we will not perform this procedure. So we are a team of usually five people. We have two anesthesiologists who take care of the anesthesia from the beginning to the end. The animal is pre-anesthetized, and then it receives the full anesthetic dose and then it goes down. And the procedure takes about one-and-a-half hour, and after that it stands up and starts to eat again. So then we have the team in the back which is responsible for cleaning the rectum. So the intestine is cleaned about two-meter-fifty deep inside the animal. Then after it is cleaned, we disinfect this part with a special disinfectant. Then we apply the patent device for the ovum pickup, which I developed together with a colleague. And then we have at the end of this procedure about two — two to three liter of fluid, which then has to be checked for the oocyte. And sometimes in Kenya, it gets very hot in the container where we perform that, so this last part is also very challenging. And after everything is found, we pack each oocyte in a special container, and then we organize the paperwork with the Kenyan authorities.

LEVITT: Anesthesia on animals like this must be at least as much art as science. It’s different than humans in that you don’t get to do it very often and you probably don’t have that great of information about what the animals need. Is that in and of itself a challenge?

HILDEBRANDT: Yes, you have to know that a wild patient is a kind of black box. It does not fill out a survey if there’s any underlying diseases, was the breakfast large? Was the breakfast small? So all these information are missing. So we have to be prepared for the unknown, unexpected. My colleague Frank Göritz, I work with him more than 35 years, he came up with this ingenious idea to have four different components in an anesthetic fluid and use that in a drip form, so that the rhino is anesthetized and the procedure itself is not very painful and not very challenging procedure. The ovum pickup has not much impact on the patient, and therefore the anesthetic protocol can be very gentle. However, if something comes up, the heart rate goes down, everything of these life parameters are measured. We even have the blood pressure measured. And if anything comes up, we always decide instantly to interrupt the procedure and wake up the patient.

LEVITT: I saw a video — I think it was you — performing some kind of C.P.R. on a rhino. And I have to say, it was one of the craziest things I’ve ever seen. And it seems impossible to me that it actually worked. Can you describe that event?

HILDEBRANDT: Yes. The situation was so that there was a zoo veterinarian responsible for the anesthesia, and he miscalculated the dose. We were only there for a semen collection of this bull because he hasn’t produced any baby. Then it turns out he fell into a deep sleep. And then was very clear that he is about to die. And therefore I decided to jump on the thorax, chest, and really ventilate by jumping on this gigantic body. Then my colleague went to our car and found a specific antidote. I jumped for about five minutes, but we could over bridge this time. And the animal woke up very nicely. Surprisingly, a half-year later, the two females which were not pregnant for years were impregnated by this bull. So he may was afraid to have another of these procedures. The other explanation some of my students gave is that he got a little bit brain damaged and then he liked the females. 

LEVITT: And do you think the jumping on him, that actually helped?

HILDEBRANDT: Yeah, no, you can massively move the chest of a rhino by jumping. You don’t break the ribs. That is impossible. And, oh, he woke up very fine. It is so important to have all these life parameter measured. Otherwise, yeah, anesthesia can be full of surprises. And working with the last of their kind, you should not have any surprises.

LEVITT: So, we’ve talked a lot about collecting eggs. How do you collect semen? That seems like a challenge as well.

HILDEBRANDT: You have actually three options. One, which is most common in livestock industry is you train your wild animal to an artificial vagina. And then you collect the semen out of the artificial vagina. That is nearly impossible. The other technique which works quite well is the massage. So you can massage if — an elephant bull, then you can easily massage the spot where the nerve complex for the prostate sits, and then you get an erection. And finally an ejaculation.  

LEVITT: And you’re talking about massaging from the inside — you stick your arm inside the elephant and do that. That’s interesting.

HILDEBRANDT: That sounds a little bit ugly, but avoiding a full anesthesia is always better. Elephant bulls have often big tusk and if the elephant goes down too quickly, it breaks the tusk. Doing semen collection, yeah, it’s not really a dream job for a veterinarian, but it works quite well.

LEVITT: And is that your job too or are you able to allocate that to the graduate students?

HILDEBRANDT: No, no. I developed that — my American colleague from Springfield Zoo, Dennis Schmitt, and I did a lot of these procedures. Yeah, because you also have to understand where the sensitive structures are, because if you induce some pain to the bull, then a bull can break your arm with a tail because it is very powerful or it kicks you. I lost a disc, not from an elephant bull, but from a female which I examined in such way so she was sitting on me, so that was not so nice. But coming back to the semen collection, there’s another technique, for example, in big cats and tigers. The anesthesia itself opens the sphincter which normally keeps the sperm in a duct system, and then you can get that out with a catheter. And the last one is electroejaculation. It’s a technique which is used in the livestock industry, but also in handicapped men when they have the desire to have kids. You stimulate like you would do that manually with a massaging technique. You stimulate this special nerve net, which is located next to the prostate. And then the semen usually comes out through the penis. For this technique, I developed a special probe which is optimal adapted to the anatomy of the rectum of a rhino and elephant.

LEVITT: We were talking before about collecting the eggs and you have these eggs, but right now you’re not doing I.V.F. because the only sperm you have is from essentially the father, right? 

HILDEBRANDT: No, we do I.V.F. quite successfully. Our embryos we produce are all relatively related because the mother is always Fatu and the father is in 50 percent Suni and in 50 percent Angalifu. You have to know that Fatu and Najin both have severe uterine pathology. Najin has ovarian tumor, so she is not part of the active program. Fatu has a lot of tumors in her uterus, so-called fibroma. However, the ovaries are still functional, and therefore we can harvest these eggs for making embryos in vitro. So we have 30 of these embryos. And we recently could show that these embryos, when we bring them back in a surrogate — the southern white rhino — then they can grow to babies. The embryo transfer itself is also very challenging. We tested these frozen embryo in surrogates in Kenya and it implanted and it has grown to a baby of 70 days, but then unfortunately we lost our surrogate due to a clostridium infection. A lot of rain in this region and below this surface, there were very ancient bacteria, which — because of the area where our rhinos live, it was old cattle land — and these nasty bacteria were freed and they intoxicated our rhinos and they died.

LEVITT: Hmm. Wow. Just to be clear, so, the reason you put the embryos into the southern white rhinos is because the two female northern white rhinos, their reproductive systems just don’t work. So you need to find a surrogate mother to carry these embryos. And the northern white rhino and the southern white rhino are similar enough that the biology works out?

HILDEBRANDT: Exactly, yeah. In the ‘80s, there was actually a hybrid between a southern white and a northern white rhino. So that is already an indicator that the uterine environment of a southern white rhino would be suitable for a northern white rhino embryo. We needed the proof of concept before we could touch our valuable embryos from the northern white rhino. But we will do that in the next weeks or months. And we need relatively soon a group of northern white rhinos so that we can also preserve the social heritage from Najin and Fatu because we need the transfer of the language of the northern white rhinos, the behavior.

LEVITT: You make a great point — it’s one thing to have the genetics and to be a rhino and look like a rhino. But if you don’t have a mom to teach you what a rhino does, then in some sense, you’re left with something very different than a rhino culture. What’s the timetable? How long will the two females likely live?

HILDEBRANDT: So Najin was born in 1989 and her daughter Fatu was born in 2000. Najin is 35. We hope that Najin can stay with us hopefully another five to 10 years, and Fatu can stay with us maybe another 15 years, 15 to 20 years. 

LEVITT: And the gestation period of the rhinos is quite long. You don’t get that many chances if you only have one surrogate going. Is that true?

HILDEBRANDT: Yes, therefore, we have six surrogates dedicated to the program. We have one teaser bull and six potential females. The pregnancy length is on the long end. It’s 16 months. And the baby, when it is born, is only 45 kilograms. There’s a lot of brain maturation during the pregnancy. So the baby is relatively competent when it is born, and therefore it can also quickly learn the language of the northern white rhino, the behavior, the experience, what to eat, how to avoid enemies and predators.  

We’ll be right back with more of my conversation with wildlife veterinarian Thomas Hildebrandt after this short break.

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LEVITT: You had mentioned earlier that a colleague had developed this incredible technique for taking skin cells and turning them into stem cells. 


LEVITT: The moonshot is that you’d be able to do that with rhinos. Are you working on that in parallel to the the surrogacy plans?

HILDEBRANDT: Yes. We have quite a strong team exclusively working on this aspect due to the fact that we have 12 nearly unrelated cell culture from deceased animals. So they provide our genetic backbone to actually restore also the genetic diversity in our future population. And what they already achieved is that they could transform these skin cells into stem cells using special factors. Bring back the cell into the embryonic stage. And then these stem cells can be transformed in the heart beating cell, cardiomyocytes and nerve cells, but also in the precursor cells for making the gametes, and that’s called primordial germ cells. They migrate into the gonads. The invasion of these primordial germ cells are really the base for making sperm and eggs. That is work in progress, but we already achieved a big step to make these stem cells which have the potential to turn into oocyte and sperm-producing cells.

LEVITT: That sounds like science fiction. It’s hard to believe that’s actually the state of our technology. And all of this, you’re describing this is happening right now with the northern white rhino cells?

HILDEBRANDT: Yes. In biology we have these general laws and what is working in mouse should in principle also work in a rhino or giraffe or in a whale. However, then there is a specific component which each species brings into the game.  

LEVITT: A skeptic might say, “Why do we care so much about the northern white rhino? There’s also a southern white rhino and there’s all sorts of other kinds of rhinos. Is it really important, or no?”

HILDEBRANDT: Yeah, the northern white rhino and the southern white rhinos are separated for more than 200,000 years by the Rift Valley, which is a geologic formation which rhinos can’t really cross. And that means the northern white rhino adapted to an environment which is completely different to the environment of the southern white rhinos. And the northern white rhinos were also used in the arenas of the Romans. The people from Nubia, they moved northern white rhinos to the Mediterranean sea, and then they moved them into the gladiator games; fighting gladiators in the arenas. So there are also very nice hieroglyphs of the northern white rhinos. So what we have right now, or what we observed, is the last northern white rhinos were only found in Congo and a little bit in South Sudan. But this species were very widespread over the northern part, central and northern part of Africa, and played a very important ecological role. It provides feces for a lot of insects. The feces is very poorly digested and provides an enormous nutrient base for a lot of insects. And if these insects don’t find this feces anymore, then they might lower in number, declining, and then the bats, which eat the insects, they migrate to different areas, to urban areas, and take all their nasty germs with them. We know that H.I.V. was born in Central Africa, Ebola was born in Central Africa. And by migrating bats, which have no more of the insects which live from the feces of the rhinos, then we have the next disease. So I think it is very difficult to model such changes, but we know if you take out two-and-a-half ton massive animal, which dominated this area, it will have a severe effect on the structure of the ecosystem and therefore it is very important to bring it back.

LEVITT: What would you say the closest parallel in the United States is to the northern white rhino in terms of the role it plays in the ecosystem?

HILDEBRANDT: Yeah, I would think it’s the bison. Two animals which live exclusively from plant material, and they distribute a lot of seeds. They produce fertilizer for the land, bacteria which enrich the soil. It provides the material for the insects. They make little avenues and pathways for other animals. These are very important eco factors. And if you take them out, like the bison was taken out in the beginning of the 20th century and nearly got extinct in North America, It has dramatic impact on the entire landscape and it will be not better for the next generation. So I think we should not ignore how complex nature actually is. And when we take out one element, then like a house of cards, we can collapse. We can’t see that in the beginning, but then we will see the result.

LEVITT: What’s the annual budget for saving the northern white rhino? Are you rolling in money or you scratch around doing everything you can to get just a few dollars here and there?

HILDEBRANDT: That is a very good question. Actually, when we met in 2015 with all these international experts, we found out that all our plans will be very costly and, none of us had any budget. In 2018, we all published a paper in Nature Communications, which was extremely well received in the academic world. And based on this publication, the German science ministry gave us a grant of €4 million for three years, and that was actually the beginning of a very effective research program. And that program ended in 2022. Unfortunately, Covid kicked in between, which made the work very difficult; flight tickets very expensive. The science ministry was very happy with the progress of our program, but they had no money either. So we got another 2 million until 2026. That is the deadline where the governmental money will be finished. And it is hopefully not the end of the program. Doing such a thing like rescuing the northern white rhino is very expensive —

LEVITT: Wait, this — what you just said you spent like a million dollars a year. In science, I would not call that expensive. I think a standard lab at the University of Chicago for somebody working on something nobody cares about costs over a million dollars a year. I don’t know, in the scheme of things, this is such small money.

HILDEBRANDT: Okay, yeah. The money is actually determining the speed of our progress, so if we would have more resources, that would be a game changer. We are relatively happy with what we have. We are a little bit disappointed about how money is used, for example, flying as a space tourist into space. What we could do with such money. And I think our approach repairing a habitat, a very complex ecosystem in Central Africa by bringing back a very important keystone species, the northern white rhino, is something for the next generation because Central Africa is a birthplace for a lot of nasty diseases. And if this ecosystem is more disturbed in a way that the northern white rhino is missing and there’s a shift in the society of the different species, then that would be much more expensive to fix some of these problems coming out of there. The interesting aspect is everyone we talked about this program believe we are swimming in money because we are so successful, but that’s not the case.

LEVITT: It’s interesting that your justifications for doing this are very much economic. It’s about how this will help humans. It seems like a more basic reason to do it is because it’s just the right thing to do. These are amazing animals. They’re going to disappear forever. It’s like a human responsibility to do everything we can to try to keep these species alive, regardless of whether there’s ever any direct benefit to humans. I mean, I’m sure you believe that, but have you found that message doesn’t sell?  

HILDEBRANDT: Yeah, that’s very emotional. And yeah, the human is the primary cause for the extinction of the northern white rhino, so we have the capability to rewind this process, and we should do that. But actually, this kind of position will not be successful in the funding scheme. So we have to bring arguments why this has really a priority, instead of developing a new ashtray in a luxury car or something like that. It’s really bitter that we have to argue like this and that the support is slow. We really dedicate our life to this operation. Everyone in this program works really hard, this personal engagement, because we all believe this is a very important mission and we have to be successful.

LEVITT: There are huge environmental non-profits. Have they not gravitated to your cause? Environmentalists are suspicious of technological solutions and human interventions. Have you found resistance among these groups that have a lot of money and could give you some? Do they resist you in some way?

HILDEBRANDT: Yeah, we approached many of them and haven’t really had any success. There was one very important donor. Her name is Alba Tull. She was in love with northern white rhinos and she visited us in Kenya, and she supported us with half-a-million dollars, which was really incredible how fast that went. But otherwise, this funding organization, we approached all of them and so far there’s no reflection or the return answer: that’s not their funding scheme. So it is a very well respected program, but if it comes to funding, it’s more on the side edge.

LEVITT: And in the bigger picture, it’s not just about the northern white rhinos. I assume everything you’re learning here would be widely applicable, especially the Yamanaka techniques. It seems like if you can make these amazing science-fiction transformations of cells, the application of that is so far reaching. It’s really surprising to me that project wouldn’t be the darling of the funding agencies.

HILDEBRANDT: I’ll give you another example. I’m also a scientific advisor for Colossal, the company which wants to do de-extinction in several species, including the mammoth. And they raised, in very short time, an incredible amount of money. So the mammoth is selling much better than the northern white rhino. I don’t consider that as conservation. It is a very interesting scientific exercise, but that what we try to achieve with the northern white rhino is really a conservation approach because the habitat is still there. We still have the last of their kind, two females. And we can maybe provide a new solution in 10, 12 years. People really like the story, but that does not transform into money or resources.

LEVITT: So, let’s talk about the woolly mammoths.


LEVITT: So there is incredible funding, as you said, behind this idea to bring back the woolly mammoth that has been extinct for a long time. The reason that’s being funded is because there’s a profit motive? There’s a Jurassic Park at the end of it? Why are people willing to fund that?

HILDEBRANDT: Yeah, the mammoth is an icon. Ice Age is run by the mammoth. I think the mammoth is causing more imagination than actually a rhino, even if it is a northern white rhino. I would put it side by side by flying to the moon because the mammoth is a very attractive animal from the appearance, what you can’t really say to a rhino, or at least for many people. And that is the selling point. The other point is it is the majority of the mammoths is extinct for 10,000 years; there are a small population on a Siberian islands until 3,000 years ago. But you find them as drawings and caves from the early man. And so I think, not only we are still fascinated by mammoths, our first-generation humans, they were already fascinated.

LEVITT: Is the approach to bringing back the woolly mammoth, is that the Yamanaka approach, the same one that you’re trying on the rhinos? Or something different?

HILDEBRANDT: It is a similar approach. They want to use pluripotent stem cell technology. And what they also use is another Nobel Prize winning invention. It’s a gene-editing CRISPR-Cas9. So they want to, at least the American group, wants to rewrite the genome of the Asian elephant, which is relatively closely related to the mammoth, and make it cold-resistant, hairy, and transform it with some major features to a mammoth-like creature.

LEVITT: I see. So that’s not truly bringing back a mammoth. That’s about creating something that looks like a mammoth, but really isn’t.

HILDEBRANDT: So we know exactly what made a mammoth compared to an Asian elephant. And so then the scientist at Colossal identified these differences and they plan to rewrite the genome and cell culture. So there is no animal involved and if the cells are mammoth-like cells, then they use Yamanaka technology to transform the pluripotent stem cells to make oocytes and sperm and then put them, the embryos, into real elephants or use an artificial womb, but that’s not decided yet.

LEVITT: And so the budget for this is in the hundreds of millions of dollars, right?

HILDEBRANDT: Yes. It is, yeah.

LEVITT: And is that philanthropy? Or is it someone who thinks at the end of the day, there’s a market for this? Do you know what motivates them?

HILDEBRANDT: Yeah, it’s an economic motivation because there is long-term investment into something which might be relevant for the future as a technology which can be sold. So it’s not that one person wants to see the mammoth and spend all the money on that. No, it’s really an investment group, which supports Colossal.

LEVITT: What do they expect to be paid for in the long run? Tourism or something different than that?

HILDEBRANDT: No, this gene-editing technology, safely gene editing. Because you can also treat diseases, human diseases with that.  

LEVITT: But there must be a much more direct way to work on human cells than by trying to reconstruct a woolly mammoth. There’s enormous amount of investment going on in CRISPR and this gene editing. And recently they had their first approved product for sickle cell anemia. But, I don’t know, something sounds fishy here to me that the idea behind the woolly mammoth is to —

HILDEBRANDT: To make money?

LEVITT: No, I’m sure it’s to make money. I just — I’m having trouble with their business model. And I could understand if you’re saying, “Look, we want to bring back woolly mammoths, and we’re going to start a Jurassic Park kind of thing,” and that would make total sense. But it doesn’t sound like, at least outwardly, that’s what they’re talking about. You’re working on that project, right? You’re involved?


LEVITT: Is there conflict ever between the scientists and the money people as this unfolds?

HILDEBRANDT: No, in the moment, not because the money people are really, really, laid back. They don’t really request anything. And they know also that it’s a very thin line to do something like the recreation of the mammoth, regarding ethical standards and the opinion of the wide audience. So I think they really have a high interest to develop a nice image of their company, so they’re quite successful. But on the other side, there are forces which hate biotechnology per se. And so they have to deal with that. And therefore, Colossal is supporting a lot of conservation projects besides the mammoth activity. They are also very active in helping to protect vaquitas a kind of dolphin, or a small whale in the sea on the western coast of Mexico. They are also working with the Sumatran rhino a little bit. So there are some additional activities, because they are so well equipped with money that they use this feature to develop a nice image. And I’m mainly interested on their technology progress to see if something of that can be utilized in our program. And we already found a very interesting way — because I mentioned we have these 12 cell lines, which we want to utilize to make new individuals out of these cell lines. However, 12 is not really a large number, and it might be that the genetic diversity we can achieve with that is not sufficient. Because with climate change our population which we create will be very challenged in the future with a lot of environmental changes. And therefore we need a broad genetic background. What we came up with, Colossal, is we go to the museums and to private collections, to people which have hunted northern white rhinos maybe 100 years, 200 years ago, and have that at their houses or in the museum, and then we will analyze these genetic information of these specimens and see how different these old specimens actually are to what we have in our tanks. And if it turns out that there are some genetic pattern completely missing in our current biomaterial, then we would use our biomaterial with a Colossal technique to rewrite these cells and bring the ancient material or the information from the ancient material into these cells so that the museum specimens gets a completely new function.

LEVITT: Because the amazing thing about living things is that the building blocks of the DNA is so simple that, in principle, if you can go and find this 200-year-old stuffed rhino and pull the DNA out of it, then with the right techniques, we could reproduce the DNA that rhino had.

HILDEBRANDT: Yep, yes, that exactly, that is our idea. We use information generated from the old specimen. We use computer tomography to find the most dense spot in the bone because then the DNA is best preserved in this region and then biopsy this bony material, and then sequence that information and use it for our genetic enrichment of our cell culture, and then with Yamanaka’s technique making at the end gametes out of it.

You’re listening to People I (Mostly) Admire with Steve Levitt and his conversation with Thomas Hildebrandt. After this short break, they’ll return to talk about Thomas’s experience the day the Berlin Wall came down.

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I’ve been so focused on Thomas Hildebrandt’s research that I haven’t taken the time yet to understand the path that led him to be a cutting-edge veterinarian. Let me fix that now.

LEVITT: Did you know from a young age that animal reproduction was your thing?

HILDEBRANDT: My wish for being a veterinarian I made when I was six, and I never changed it.

LEVITT: You already knew you wanted to be a vet at the age of six?


LEVITT: But you didn’t grow up on a farm, right? You grew up in East Berlin.

HILDEBRANDT: Yeah, I grew up in an urban environment, but my relatives, they lived in the countryside. From my father’s side, before he was an engineer, his family is a four-generation butcher family, and so I spent my summer holidays always in the countryside and working in this slaughterhouse and looked at the anatomy of the animals, which I found very impressive. But on the other side, I picked sick animals and raised them and release them again. So I always had the desire to be a veterinarian to do something for the animals.

LEVITT: You were in your 20s when the Berlin Wall came down. Was that a joyous event for you or frightening? What was your view on East Germany?

HILDEBRANDT: At that time, I had an experiment running in the East Berlin Zoo that was an embryo transfer in yaks, in hornless yaks. And so I stimulated the yaks and flushed out the embryos and transferred these embryos into cattle. My location where I lived was directly opposite to all the streams of people which want to see the western side of Berlin, so it was really a challenge to not going to the western side because I had to run my experiment and to go to the zoo, which really caused a lot of traffic congestion. And that I hated. Especially some of my students for these desire to go to the Western world because in veterinary medicine, we had a lot of very strong, communist people, and after the wall went down, they were all next day at the western side and asked for the 100 Deutsche Mark you got or the bananas which were thrown from the trucks. So they completely lost their philosophy. I hated that because I always have a very strong personal plan and never changed my opinion so quickly.

LEVITT: So history was being made all around you. And you were in your lab trying to make this yak experiment work?


LEVITT: And fretting about the traffic. Is that — is that what you’re saying?

HILDEBRANDT: Yeah, that was the situation. Yeah. So the yaks were stimulated. I had to flush the embryos out. And that was in strong conflict with a major movement of all hundred thousand of people went through Berlin because that was the only place where the wall was open at that time. Yeah.

LEVITT: My understanding is you were not a big fan of the East German government. You had gotten into trouble before, right? For political stuff?


LEVITT: Tell me about that.

HILDEBRANDT: Yeah, so I was always quite gifted in the school, so I had the best marks, so it was really clear that I would get a position at the University for Veterinary Medicine. However, I was a rebel, and I questioned the political decision very openly during my high school time, and that didn’t really reflect very well. And so I was banned from university for five years because of my political position, which was clearly stated as not the political position of the working class, what I demonstrate, and therefore I have not the right to enter the university. That was my resume I got from the high school. And then I worked in a dairy farm, I worked in an animal pathology to open up animals and because I found that very interesting but got very low money for that. And finally one of the professors at the pathology department recognized how skilled I was and then he helped me to enter university again.

LEVITT: It’s interesting because, looking back, those five years where you were banned from going straight into the university probably were incredibly useful and transformative for what you turned out to be. You never would have chosen it yourself. But I wonder if you wouldn’t have achieved nearly as much if you had stayed on the straight and narrow path.

HILDEBRANDT: Yeah, I have to say that it gave me really mental strength to follow a task which seems to be impossible, but it was a very thin line. I think I could not stand one more year in this hope to get finally access to the university. Partly have broken me at the, at that time. I was stuttering, I was really mentally not in the best condition. But I still had the dream to one day to study veterinary medicine. Next year I was the best student, got all these honors and everything. It was completely crazy that the year before I was not allowed to go to university. Two years later, I was the student which was shown to all these events and got all the awards. That was strange.

LEVITT: So given what you just said, I’m surprised, even given your commitment to science, that when the wall went down you weren’t right there, one of the ones breaking it down. You lost your political interest along the way?

HILDEBRANDT: No, actually not. Actually, I was also not really happy that East Germany went down in such chaos because it was my home country. I didn’t like the communists, but I liked the people. And what then happened was really a sold out of our nation, which — I didn’t really like that because the people in East Germany were different than people now in Germany. Friendship had a much higher value because nobody really had a lot of money. If you don’t deal with a secret service or with a strong communist, then the family issues were very high and had a, yeah, really high value. And that really got sold for money over the next 20, 30 years. There was also the idea of some of my students, when we discussed that before the wall went down, if he should not escape via Hungary or Czech Republic. And I never was in favor of that because I thought if you would all escape, who could do such changes? That was my opinion. I say that also quite openly, which was not well received either at that time. Then finally, East Germany was incorporated into the new Germany. And then science and traveling was suddenly possible and I went to America, stayed there for half a year at Washington, D.C., working with zoo elephants at the National Zoo. And then from there, I slowly discovered the world and worked with huge variety of different species.

LEVITT: I know that everyone in Germany has the right to see the files that were kept on them by the former secret police in East Germany, the Stasi. Have you gone to look at your files?

HILDEBRANDT: No, I haven’t. I was not brave enough because if you identify that some person on your family or your good friend had a connection to the Secret Service.

LEVITT: You just didn’t want to know? You’d rather not know?

HILDEBRANDT: No. Because, what is the value of that? No, that it might destroy inside the family, that there was this this split caused by the Secret Service. So I, no, I don’t want that.

LEVITT: I have a friend, an older man, who lived in Hungary. And he went and looked at the Hungarian version of those files. And the only thing that they had on him was that one day in high school he was whistling and a teacher asked him what he was whistling and he was whistling “Somewhere Over the Rainbow” because he had seen The Wizard of Oz, and the teacher reported him to the authorities as watching American movies. And that was the sin that was put into his file.

HILDEBRANDT: Okay. No, I’m pretty sure I had a big file — well, I still have a big file because these five years I underwent separated from university, there were always encounters from the Secret Service. So that there must be a big file, that’s for sure.

LEVITT: So do you feel personally a lot of pressure — going back to the northern rhinos — do you feel the responsibility for that species rests on your shoulders?

HILDEBRANDT: Yes. It’s a task which is really important for me. I’m now 60, and I want to solve that when I finally retire. So it is something which has to happen. And it will happen. I’m very sure because all signs, all progress we did, they pointing in the right direction. Yeah, that’s something I want to fulfill. That is really a personal mission. But it is infectious there. My colleagues think the same, so I infected them.

LEVITT: Let’s end with a prediction. Let’s say 2099, how many white northern rhinos are there on the planet?

HILDEBRANDT: I would think 50.

LEVITT: Fifty? That would be your goal? Fifty by 2099? Fantastic. Well, you and I won’t be here to to see that, but hopefully someone will be keeping track and will be able to raise a toast to all your efforts if that comes true.

HILDEBRANDT: Thank you very much. To have them, that is the biggest reward I can imagine. To see a calf of a northern white rhino, which was believed to be impossible with this material — we have two infertile females — but I think we can achieve that, and we will achieve that.

I find what Thomas and his colleagues are doing to be remarkable, and it’s pretty clear to me, based on this conversation, that Thomas is a much better veterinarian than he is a fundraiser. So if you’d like to help support the research or just learn more about it, check out the website

LEVITT: So this is the point in the show where I welcome my producer, Morgan, on to talk about listener questions.

LEVEY: Hi, Steve. So at the end of our last new episode with Conrad Wolfram, who’s a mathematician and someone who is advocating for a new math curriculum, you did a little listener poll. You asked listeners to send an email with three pieces of information: their gender, whether they considered themselves a math person, and whether they were for or against replacing the traditional math curriculum with Conrad’s new curriculum. And we got a lot of responses.

LEVITT: Oh my God. I think we must’ve gotten 500 or more responses. It was remarkable how many people wrote — and not just answered the questions, but gave really thoughtful and interesting opinions on the topic. So it really was wonderful and encouraging, the response we got.

LEVEY: So, you have tallied up the responses we got from listeners. I have not been looking at the tallied responses. So, you’re going to tell me what our listeners said.

LEVITT: I’m not going to tell you; I’m going to quiz you. First, I have to make a confession: I ran out of energy after about 300. It was a lot of work and the trends were so clear that I knew that there’s no chance that my conclusions would be different. I even tested the statistical significance of some of the conclusions and found out that they were already, with 300 observations, highly statistically significant. So I did stop data collection there. Early on, I even was thanking everyone who wrote, but my patience for that wore thin pretty quickly. So if you wrote in, I apologize if I didn’t acknowledge my thanks for you doing that.

LEVEY: Okay, let’s start this quiz.

LEVITT: What percent of the respondents were male?

LEVEY: Ninety percent.

LEVITT: No, come on. It’s always exactly the same. It’s always 70 percent. On every survey we’ve done, we’ve gotten about 70 percent male and that happened again. Okay, so now it’s a little harder. What percent say they’re math people?

LEVEY: I think overwhelmingly people who listen to the show would say they’re math people.

LEVITT: You’re more right on that than I would have been. About 70 percent of the men categorized themselves as math people, and about 60 percent of the women did. At this point, we should highlight this is not a very scientific poll, right? It’s obviously made up of people, not only who listen, but people who listened to the end and they’re willing to take the time to write it. So who knows what kind of biases are going on. This is more about interest. But I was actually heartened by the fact that so many people consider themselves math people. So at least in this very selective sample, I found that to be an encouraging result. 

LEVEY: Okay.

LEVITT: What percent of the people overall across all of these categories were in favor of adopting Conrad Wolfram’s new curriculum relative to what we’re using now?

LEVEY: I am also going to say that it’s most of the respondents, and I’ll say maybe 80 percent.

LEVITT: Ninety-nine percent.

LEVEY: Oh my gosh! Is that like one person who was not in favor?

LEVITT: There were four people in the 300 that I looked at that were not in favor of adopting this curriculum. In many ways, it’s not surprising. If you got to the end of this podcast, you probably liked it, and you probably thought this was a good idea. I’m sure the people who really hated what Conrad had to say were gone after minute seven. It was interesting because one of the very first people who wrote in was a no. And then it was 100 or 150 more before I got another no. So I hadn’t expected it to be so heavily skewed. Okay, last question. Of the four people who said “No,” how do you think it broke down?

LEVEY: Oh my gosh. I think they were all math people. And I think it was three men and one woman.

LEVITT: Almost. It was all math people and they were all men. As we’ve tried to push data science as an alternative to traditional math, we have found that almost everybody is supportive. The one set of people who really fight it are a small group of male, university-level math professors. I was just surprised that so few of the people who identified as being math people weren’t threatened by what Conrad’s really radical curriculum would imply for math. Now, I also think it’s true that the kind of people who’ve been fighting the data science movement, I don’t think very many of them listen to my podcast. I think that’s a pretty safe bet.

LEVEY: Thank you to everyone who replied to our poll. If you have a question for us, or if you have a question for Thomas Hildebrandt, we will try to get that question to him and maybe have him answer it on a future listener-question segment. Our email is That’s We read every email that’s sent and we look forward to reading yours. 

In two weeks we’re back with a brand new episode featuring ornithologist Richard Prum. Now you might think you’re not really interested in birds but if there’s anyone that can change your mind about it, I’m telling you, it’s Richard Prum.

PRUM: And then in the late ‘90s, there were some discoveries of fossil dinosaurs from China that were really revolutionary. And they had fuzzy stuff that people called dino fuzz. Some people wanted to call it feathers. And it certainly conflicted with the main hypothesis — they didn’t function in flight at all. But my theory was very congruent with that.

As always, thanks for listening and we’ll see you back soon.

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People I (Mostly) Admire is part of the Freakonomics Radio Network, which also includes Freakonomics Radio, No Stupid Questions, and The Economics of Everyday Things. All our shows are produced by Stitcher and Renbud Radio. This episode was produced by Morgan Levey with help from Lyric Bowditch, and mixed by Jasmin Klinger.  We had research assistance from Daniel Moritz-Rabson. Our theme music was composed by Luis Guerra. We can be reached at, that’s Thanks for listening.

LEVITT: So my—

LEVEY: No, don’t say that. 

LEVITT: No, it’s important. 

LEVEY: Nope. No.

LEVITT: No, no. I have to say it.

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  • Thomas Hildebrandt, head of the department of reproduction management at the Leibniz Institute for Zoo and Wildlife Research and professor of wildlife reproduction medicine at the veterinary faculty of the Freie Universität Berlin.



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