MADDIE SOFIA, BYLINE: You're listening to SHORT WAVE...
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SOFIA: ...From NPR.
BERLY MCCOY, BYLINE: OK, Emily, I have a critter riddle for you.
EMILY KWONG, HOST:
This early in the morning, Berly McCoy?
MCCOY: Yes. Yes.
MCCOY: But you're going to love it.
KWONG: All right. What is it?
MCCOY: What animal never grows up, is always smiling and will be the new face of Mexico's 50-peso bill in 2022?
KWONG: Never grows up - is this some kind of Peter Pan animal?
MCCOY: Yeah, some people actually call them that.
KWONG: OK. Give me another hint.
MCCOY: So there's a Pokemon named after it. It's a mob in the game Minecraft, which our editor's 13-year-old says is really cute.
KWONG: Oh, this is where my knowledge falls apart. I'm not a gamer, but I'm very intrigued. What is this?
MCCOY: I'll let Luis Zambrano from the National Autonomous University of Mexico do that.
LUIS ZAMBRANO: Axolotls look like - maybe the best description is the dragon of the "How To Train Your Dragon" without wings.
ZAMBRANO: It's like a large lizard, a fat lizard that lives under the water, but with a crown of gills behind the head and with it all the times a lovely and eternal smile in the face, basically.
KWONG: Yes. Axolotls on SHORT WAVE. Oh, I'm Googling. They do look like the dragon from "How To Train Your Dragon." Luis is right. He is correct.
MCCOY: I know. I know. And Luis says Mexico is a pretty good place to be if you want to study axolotls in the wild.
ZAMBRANO: Ambystoma mexicanum is only found in Mexico City in the lake of Xochimilco, which is a lake that is in the south part of the city.
KWONG: Wait; they're only found in one lake in the whole world?
MCCOY: In the wild, pretty much. There used to be a lot more, but over the last few decades, their numbers have dropped a lot because of lakes drying up and other reasons we'll get into.
MCCOY: So scientists are trying to learn as much as they can about their biology and their habitat to find a way to keep them from going extinct.
KWONG: Today on the show, we'll talk about the unique biology of axolotls, how scientists are trying to save them and what they can learn from studying them. I'm Emily Kwong.
MCCOY: And I'm Berly McCoy.
KWONG: And this is SHORT WAVE, the daily science podcast from NPR.
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KWONG: OK, today we're talking all about axolotls, the ever-smiling salamanders of the animal kingdom. So, Berly McCoy, what makes these critters unique?
MCCOY: A heckin' lot. To start, I think it's helpful to talk about what most salamanders grow up to be.
ZAMBRANO: They normally absorb the gills and they leave the water and go in the adulthood into the land. And axolotls decided to - don't do that. They decided to stay as juveniles and reproduce as juveniles within the water. So that's the reason they tell them that they are forever young, because, yes, they kept the youngness and still juveniles for the rest of their lives
KWONG: A forever-young salamander - that's fantastical. But scientifically, what does that even mean?
MCCOY: So in the animal kingdom, it's called neoteny. For axolotls, it means that even when they're grown up, which in this case grown up for them means they can mate, they keep some of their juvenile features.
KWONG: Oh, so do they keep the crown of gills that Luis was talking about earlier?
MCCOY: Yeah, they do. And they also keep the fin on their back that they had when they were tadpoles. And because of this, axolotls don't leave the water like other salamanders eventually do.
KWONG: Got it. OK. So do we know why they do that?
MCCOY: Well, scientists think it could have something to do with the water where axolotls live, like, being a better habitat. Maybe it had plenty of food or clean water, so they lost this metamorphosis step, although the scientists can still trigger it in the lab if they use hormones. And, Emily, axolotls do something even cooler than being the poster child for the fountain of youth.
ZAMBRANO: If you cut any part of their body, it regrows again.
KWONG: Wait. Like a whole arm or their tail? What about their head?
MCCOY: OK, they can't regrow their whole head, but they can regrow essentially everything else. And apparently it's not that big of a deal to regrow limbs because - get this, Emily - as babies, axolotls will just snack on each other.
MCCOY: Yeah, yeah. Sometimes they just take bites out of each other.
MCCOY: Some people who raise axolotls in captivity will notice it, especially if they're competing for food or really hungry - part of a tail here, a toe or two there, and it usually grows back.
KWONG: Oh, that's really intimate. I mean, how long does it take for them to regrow part of their body after their friend took a bite out of it?
MCCOY: Just a few weeks, sometimes a few months, all made possible through a massive amount of cell growth, which is pretty amazing because unchecked cell growth is usually associated with cancer, but axolotls are super resistant to cancer. It's one of the reasons scientists are so interested in understanding how their cells do what they do, both to figure out how humans might heal from injury and regrow tissue better and as a way to stop or prevent cancer by understanding how axolotls can grow this fast without forming tumors.
KWONG: Yeah, I can see why scientists want to study them. And going back to axolotls in the wild, Luis said earlier that they mostly live in Mexico City in a lake called Xochimilco.
MCCOY: Yeah. In the lake and its canal system, which used to cover a ton of land. And axolotls have lived there for thousands of years.
ZAMBRANO: Local people have been working with axolotls for at least 2,000 years. So in these 2,000 years, Aztecs and Nahuas, which were the civilizations that were here, considered the axolotls the twin brother of the most important god. So it's highly important in terms of divinities, but also it's important in terms of food. People used to eat them, and people used to use the axolotls as a traditional medicine. So, yes, it's very important for Mexican culture.
KWONG: An iconic species, clearly. And how are they doing in the wild?
MCCOY: Not great. Luis told me the first census they did about 20 years ago showed that around 6,000 axolotls live in every square kilometer. Now it's much lower.
ZAMBRANO: It decreased. Every time we have been measuring and make a census about them, it has been decreased from 1,000 per kilometer square to 100 per kilometer square, and the last one was from 100 per kilometer square to 36 per kilometer square.
KWONG: That is not good. Does Luis know what caused such a huge population drop?
MCCOY: He says it's threefold.
ZAMBRANO: One is water quality. Since this lake, these wetlands where Xochimilco is besides Mexico City, the water quality has been depleted a lot in the last 60, 70 years.
MCCOY: Mexico City has a population of over 20 million people, and pollution from Mexico City flows into Xochimilco. Also, the city pulls water away from the springs, and that water would otherwise run into the lake, so there's less water for a suitable axolotl habitat.
KWONG: Got it.
MCCOY: The second cause of axolotl decline is predators. So introduced fish species like carp and tilapia, they kill axolotls and compete with their resources.
ZAMBRANO: They were introduced for aquaculture reasons. Carp eat eggs of axolotls, and tilapia eat juveniles of axolotls. So both species are really, really aggressive with particular - with everybody, but particularly with this species.
KWONG: So there's a water quality problem. There's an introduced predators problem. What's the third thing threatening axolotls?
MCCOY: So the third thing is urbanization around Xochimilco. So some species do OK around humans, but axolotls don't seem to.
ZAMBRANO: We see that axolotls is stressed if they are close by humans. So they stress, they get ill and they die.
MCCOY: So axolotls really have a lot going against them, which could lead to their extinction in the wild.
ZAMBRANO: That is really, really concerning as a conservationist and as a Mexican because if we lose this species, we are losing part of our identity. And we lose not only that. We are losing part of our biodiversity, but also part of our identity.
KWONG: Yeah, I really hear that, how high the stakes are here. They're an amazing species and an important species culturally. So what is being done to help them out? Is anything happening there?
MCCOY: Definitely, yeah. So researchers like Luis are working with locals to create refuge areas for axolotls. And they're using historic farming practices to do that.
ZAMBRANO: The restoration program has to get rid of these three threats, and that's the reason we created this program called Chinampa Refugio.
KWONG: Chinampa Refugio - I'm familiar with these. Chinampas are floating islands that people in Mexico have used for thousands of years to grow food.
MCCOY: Yep, yep. And it turns out those chinampas make great axolotl habitats.
ZAMBRANO: One of the things we are trying to do now is to make a refuge in these canals, to say these canals have a small border to avoid that carp and tilapia getting into these canals. And also, this border helps to increase the water quality - is not only just for axolotls, but also the rest of the species, but also for farmers. And the good quality can be useful for grow better food and organic food for these local farmers.
KWONG: The program sounds mutually beneficial, in a way, for both axolotls and humans and like there's some hope for saving the axolotl from extinction in the wild.
MCCOY: Yeah, totally. And another big part of that is raising awareness, which is one of the reasons, Emily, this adorable creature is going to be appearing on Mexican currency very soon.
ZAMBRANO: We lost our vision of conservation for this species for about 70 years, and now we are reconsidering for the conservation. And one of the most important things is to put them, a figure of them, in the bill that is highly used in Mexico. The 50 pesos is one of the most used because it's one of the lowest currencies and is easier for everybody to have one.
MCCOY: Luis says even though the axolotl's only found in Mexico, he hopes people outside the country will take an interest in conserving them.
ZAMBRANO: They are as important as the polar bears or as important as the eagle or as important as the whales. So I think that although it's a responsibility of our country to try to conserve this species, it will be very, very good if the rest of the people in other countries, in other cultures also help us to try to conserve this species.
KWONG: Yeah, conservation really is a collaborative effort, and I'm really glad to know what the people of Mexico are doing to save them. Thank you for bringing this story to us.
MCCOY: You're very welcome.
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KWONG: This episode was produced and fact-checked by the amazing Indi Khera and edited by Gisele Grayson. The audio engineer for this episode was Kwesi Lee. Thanks for listening to SHORT WAVE, the daily science podcast from NPR.
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