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What Colorado's Tiniest Creatures Tell Us About Life at the Highest Elevations

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Hosted by writer Emma Atkinson, RadioEd is a triweekly podcast created by the DU Newsroom that taps into the University of Denver’s deep pool of bright brains to explore the most compelling and interesting research coming out of DU. See below for a transcript of this episode.

Show Notes

If you’ve hiked above the tree line of a mountain in Colorado, you’ve likely come across a couple fuzzy little critters making their homes among the rocks. Deer mice, in particular, are native to North America and are often found at the peaks of the Rocky Mountains. 

As you stand there at the summit, gasping for air after a long hike, you might have realized that the little mice don't look tired or breathless at all. They scurry around between the rocks, little balls of energy. 

So why is it that, at one of the highest elevations humans can reach, you’re so worn out, while the mouse is fine? 

In this episode, Emma speaks with Jon Velotta, assistant professor of evolutionary biology at the University of Denver, who studies how these mice have adapted to the high altitudes at which they live.  

Jon Velotta is an assistant professor of evolutionary biology at the University of Denver, where he studies how animals adapt to extreme environments. Velotta’s research blends evolution with the fields of physiology and genomics. His ongoing work includes how mice have adapted to the extreme cold and low oxygen conditions of high-altitude, and how fish have made the evolutionary transition from saltwater to freshwater. 

Jon Velotta and his student researchers sit atop Mt. Blue Sky, formerly known as Mt. Evans.
Jon Velotta and his student researchers sit atop Mt. Blue Sky, formerly known as Mt. Evans.

More Information: 

Jonathan Velotta’s Google Scholar webpage 

Physiological and genomic evidence that selection on the transcription factor Epas1 has altered cardiovascular function in high-altitude deer mice,” Jonathan Velotta et al 

Elephants have evolved to be tuskless because of ivory poaching, a study finds,” NPR 

The Genetic Basis of Chronic Mountain Sickness,” Roy Ronen, Dan Zhou, Vineet Bafna and Gabriel G. Haddad 


Emma Atkinson (00:05): 

You're listening to RadioEd, the University of Denver podcast. I’m your host, Emma Atkinson. 

Imagine with me for a moment: You’re at the top of a fourteen-thousand-foot mountain, a 14er, as we call them in Colorado.  

Sound of hiking boots on the trail. 

You’ve been hiking for hours and have finally reached the summit, a windy and treeless expanse. Maybe you had to scramble up some rocky terrain to get there. You’re breathing hard and shivering—hiking at such a high altitude puts a strain on your body. It’s beautiful up there, the vast vista of the Rocky Mountains stretching out before you, but boy, was it tough getting there. 

You go to take a step, your feet sore inside your hiking boots, but before you can set that foot down, you see movement. You stumble aside, not sure what you’ve just seen, but then—there it is again! A flash of soft brown fur, accompanied by a long, thin tail and a set of adorable large ears. You have just seen a deer mouse. 

And as you stand there, gasping for air, you realize that that little mouse doesn’t look tired or breathless at all. It scurries between the rocks, seemingly a little ball of energy. 

So why is it that, at one of the highest elevations humans can reach, you’re so worn out, while the mouse is... fine?  

Jon Velotta (01:19): 

So I guess if you're in Colorado, it's—the summit of a fourteener is 60% of what is available at sea level. So every breath you take, you're only taking 60% of that oxygen. And so it's incredibly difficult to breathe. So for folks who have been there, you know that when you get out of your car, or whatever, if you drive to the summit of Mount blue sky, or if you hike up it, you're gasping for breath. And you're maybe feeling lightheaded and dehydrated. And so you probably don't want to spend a whole lot of time up there.  

Emma Atkinson (02:00): 

Jon Velotta, who you just heard, is an assistant professor of evolutionary biology at the University of Denver. I sat down with him to chat about his research involving tiny mammals like deer mice living at high altitudes and how it relates to evolution and adaptation. 

Jon Velotta (02:14): 

But animals make a living there permanently. They live there all year round, and they're not making choices to do that. And we've talked about why, which is that it's a relatively predator-free environment so that means better living situation. But also, it's so incredibly difficult to live there. So we want to know how, and the how is not straightforward, because it involves basically every aspect of an animal's ability to get in and use oxygen 

Emma Atkinson (02:55): 

Velotta and his research assistants are studying how the deer mice native to North America adapt to high-altitude environments, like the one at the top of Mt. Blue Sky just an hour west of Denver in the Rocky Mountains. 

One of Velotta’s [workers], Kelsey Hunnicut, says she really loves working with the little critters and studying how their genes have evolved. 

Kelsey Hunnicut (03:13): 

What I mean by that is the genetic changes specifically that occur, that allow these animals to adapt to these really extreme environments. In some of my past work, the things that allow different populations of mammals to diverge and remain separate and form new species. And adaptation is sort of a similar line of thinking: how can we take these populations that can live at lower altitudes and at higher altitudes and what changes in the genome can allow them to establish at these higher altitudes? And then what allows them to remain there? Is sort of the things that motivate me. 

Jon Velotta (03:55): 

We want to understand the adaptations that animals have to extreme environments. And at altitude, right, there's this pressure of low oxygen, because there's really low pressure. And the animals that live there have to deal with that all the time. And mice that live at high altitude have to deal with that literally 24/7. And they also have to deal with cold. So it's also very cold up there. And so they're one of the only things that has to do this all the time. And also, they're active during the winter. So they're just extremely subject to the extremes of altitude. And so we want to know how they do that. Because there's not very many animals that are adapted to live in these places. 

Emma Atkinson (04:56): 

Deer mice are a tasty snack for owls and other large animals that live at or below the tree lines on mountains, so living at the highest altitudes makes it possible for these little critters to eke out a living, so to speak.  

And for the mice living at altitude, it’s all about oxygen. 

Mammals combine oxygen with food to make energy. Less oxygen means less energy. So when you’re summitting a fourteener, your body has to work that much harder. You breathe more heavily, you get dehydrated. And your body makes more red blood cells.  

Now, when you have too many red blood cells floating around in your veins, it can lead to some serious problems. For one, it can make your blood thick and viscous, like molasses and that’s as bad as it sounds. 

Jon Velotta (05:38): 

There's all these other things that happen too. You go up to high altitude, and all the blood vessels, the vessels that carry blood through your lungs, start to constrict. They get smaller and tighter, and nobody wants that. And the reason is because that actually limits the amount of oxygen that you can get into your body. And it causes a lot of problems downstream, and it could lead to acute mountain sickness kinds of things like hypoxic pulmonary edemas, which is just basically fluid in your lungs. And it can also lead to chronic disease. 

Emma Atkinson (06:23): 

People who live at high altitudes are prone to these problems. In the Andes, humans who live way up there have what’s called chronic mountain sickness. The hallmark symptom is something called polycythemia (say that five times fast), and it can cause pulmonary hypertension and stroke. For those among us who aren’t MDs: that’s not good. 

But here’s the interesting thing about those high-altitude mice Velotta is studying: They don’t have any of those issues. 

Jon Velotta (06:49): 

We're studying mice, because we know that mice essentially don't get chronic mountain sickness. And so we know every one of those things I just told you doesn't happen to mice. They don't breathe faster when they're at altitude, they actually breathe slower and deeper, which is a way more effective way of breathing; They don't have more red blood cells, they have just as many red blood cells as anyone would at sea level; They don't get high blood pressure when they respond to oxygen. And so we have this model for this animal that doesn't get chronic mountain sickness and that's only part of it. And we sort of want to know how they do that. 

Musical interlude  

Emma Atkinson (07:50): 

Researchers know that these mice are different, genetically speaking. They’ve adapted to live at higher altitudes. 

So let’s talk evolution and adaptation. 

When we think of organisms evolving, we might think of the journey from bumbling neanderthals—think the Geico caveman—to our current human selves. It took thousands of years to get to where we are today, driving cars and watching Netflix and competing in Olympic sports. Okay, not all of us will compete in Paris this summer, but evolution will have helped those athletes get there. 

Evolution is an umbrella term for the change in anything, living and nonliving, over time. Adaptation is a type of evolution. Adaptation is what happens when features of organisms change, allowing them to survive and reproduce better than the organisms that don’t have those features. 

Jon Velotta (08:37): 

That's the process of natural selection, or Darwinian evolution. 

Emma Atkinson (08:42): 

Velotta explains that while we may think of “survival of the fittest” as the primary way that organisms change over time, it’s not the only form of evolution. 

Jon Velotta (08:51): 

Evolution doesn't need natural selection, it can also occur randomly. So features of organisms, the genes of organisms can change, just by chance. That is also a process of evolution. It's random or neutral because it doesn't involve features that make them better for their environment, but it involves change. And so essentially, anytime we're talking about change in an organism, across generations, we have evolution, whether or not that evolution has occurred, because that feature makes them more fit for their environment, we say that is whether or not that feature evolves by adaptation, or natural selection, or not. So adaptation can be like a feature, and adaptation to an environment could be like, cheetahs run extremely fast to catch prey. That's an adaptation to their lifestyle. But adaptation can also be a process. So it can be the process of adaptation, whereby the genes that provide cheetahs with extreme speed, change over time, and eventually you get an extremely good runner.  

Emma Atkinson (10:13): 

Okay, that is so interesting. So adaptation is a kind of evolution. 

Jon Velotta (10:17): 


Emma Atkinson (10:18): 

Wow, that's so interesting, because I think we often think umbrella term “evolution” when we think about survival of the fittest, survive and advance. So it really is adaptation that is more specific, more granular. 

Jon Velotta (10:30): 

Yeah, adaptation really is what we think drives a lot of biodiversity and change. So the reason that animals – I keep saying animals get my biases out of my mind – the reason that organisms look the way they do, are where they are, do the things that they do, are because of the process of adaptation by natural selection, evolution by natural selection. But that's not necessarily the case. Some animals look the way they do just because of randomness.  

Emma Atkinson (11:06): 

In 2021, researchers conducting genetic analysis on elephants in Mozambique’s Gorongosa National Park found that the elephants had done something extraordinary: in response to increased poaching driven by the increased value of ivory during a civil war, tuskless female elephants passed on tusklessness to their offspring at a remarkable rate.  

One of the biologists who worked on the project called the rate at which the elephants evolved—adapted—to be tuskless, which happened over 15 years, quote, “astonishing.” Velotta says it’s one of his favorite examples of “only the strong survive,” and something that he talks about in his classes. 

Jon Velotta (11:43): 

That is a direct result of natural selection from poaching. Because if you don't have tusks, then poachers are not likely to go after you. And thus, the ones that do not grow tusks survive and pass those tusklessness genes to their offspring. 

Emma Atkinson (12:03): 

Another fascinating example of modern-day human-driven adaptation is the recent changes seen in lizards in Puerto Rico. 

Jon Velotta (12:11): 

Lizards normally live in trees and they climb around on bark. And so they have these grippy toe pads and stuff, but the lizards in cities, so like in Puerto Rico, this anole, the Puerto Rican crested anole, lives in the cities. And they've evolved these, wider, different kinds of toe pads like toes, essentially lizard toes to climb up smoother surfaces. And they've actually been able to show that the ones that live in cities are better able to use these smooth surfaces, because think about a lizard trying to climb like a handrail. So it has morphological adaptation that allow it to live in, San Juan versus the forests outside of San Juan. 

Emma Atkinson (12:56): 

So let’s get back to Velotta’s research. I imagined him running around after little mice at the top of a mountain, but that’s not really how it goes down. 

Jon Velotta (13:04): 

A lot of it's in the lab. So we've been working with colonies that are derived from wild populations. And that's a lot of what we do. And it's because when we're studying evolution, we really want to keep everything constant. And so if we bring animals in from the wild and study them there in the lab, then we're keeping the temperature constant. And then what we can do is simulate high altitude. So we have hyperbaric chambers, it sounds fancier than it is, it's really just a tube that we hook up to a vacuum, and the vacuum lowers the pressure. And that's what high altitude is. That's why it's low oxygen is because it's lower pressure. And so we just put them in a pressure tube, and we simulate high altitude. 

Emma Atkinson (13:59): 

The researchers do get the chance to get out of the lab and into the mountains sometimes, though. 

Jon Velotta (14:03): 

We go in the summer, usually, and we drive up Mount Blue Sky. There's a road, highest auto road in North America. And there's a field station up there, of course from DU, there's an old astronomical observatory, and there's an A-Frame, which is a kind of hut, basically cabin, where astronomers used to live, and they don't live there anymore. And I don't even know if the astronomers at DU actually use that observatory anymore, but they almost never use the A-Frames and so we go in there, we bring mice sometimes and we do our little experiments in there. And then there's the field station at 10,000 feet at Echo Lake.  

And then we can get animals from below altitude, and a lot of them come from Nebraska, which is just historical because of people who have been studying them who are at the University of Nebraska. And so they're easy to catch. And then we're also getting mice now from Michigan, low altitude in Michigan to do some comparisons. And we've studied another species that's closely related to the deer mouse that live only at low altitude. So, yeah, it's not a lot of extremes. I mean, it's extreme when you go up to altitude, it's hard. The first night, usually, at the field station is sleepless, and it takes a little while to acclimate. But it's not as extreme as you're thinking. 

Emma Atkinson (15:37): 

You know you found that mice have adapted to these high altitudes and that's happened over time. When people have to adapt to high altitudes, it's usually people flying into Denver, it’s people who want to hike a fourteener. So it's much more...they have less time to adapt. Has your research helped you understand how people can adapt to high altitudes more quickly? 

Jon Velotta (16:03): 

We know how individuals, we as in other people, know how individuals are able to adjust to high altitude. And so I won't say adapt. Because as an evolutionary biologist, we like to reserve adaptation for the process by which evolution happens through natural selection. So that cannot happen on a single individual’s time frame. That's a generational process. And so adjust, acclimate, acclimatize – there's a number of phrases. But we know a lot about how that happens from human studies, from animal studies. So our research really helps us understand that generational adaptation in the evolutionary Darwinian sense.  

But what's interesting is that there are humans that are adapted to high altitude in the evolutionary sense, more so than in the acclimatory or adjustment sense. And so those are people that live at extremely high altitudes, in three places around the world: the Tibetan Plateau, which is in China, the Andes, and in high altitude in Kenya. And actually, the interesting thing is that those three groups of people have very different adaptations to high altitude. So our research informs that way more than it informs how you or I, if we were to just go up to Mount blue sky, how we would fare. We would fare not well, but these mice that live there, and these humans that are on the Tibetan Plateau, for example, they fare much, much better. And we see similar sorts of adaptations that mice have to what humans have, but slightly different. So that suggests that evolution targets the same basic processes. But because mice and humans are separated by millions and millions of years of evolution, they do it in a different way. But we're all still mammals, so there's only so many processes that natural selection can work on, essentially. 

Sound of hiking boots on the trail. 

Emma Atkinson (18:48): 

So the next time you’re up above the treeline, watch where you’re stepping. 

Jon Velotta (18:52): 

You could be—you're walking around on the summit and rock scrambling and kicking a boulder and there's a mouse probably right under your feet. Not to scare them or anything, because they're not going to come out of their hidey hole. But you can look up there and it seems so desolate and devoid of life. And yet there are animals up there existing. 

Emma Atkinson (19:33): 

A big thanks to our guest, DU assistant professor of evolutionary biology, Jon Velotta—and his post-doctoral colleague, Kelsey Hunnicut. More information on his work is available in the show notes.  

If you enjoyed this episode, I encourage you to subscribe to the podcast on Apple Music or Spotify—and if you really liked it, leave us a review and rate our work. It really helps us reach a larger audience—and grow the pod.  

Joy Hamilton is our managing editor. Madeleine Lebovic is our production assistant and musical genius, James Swearingen arranged our theme. I'm Emma Atkinson, and this is RadioEd.