Last summer, scientists reported finding the world’s highest-dwelling mammal, a yellow-rumped leaf-eared mouse, which was seen scampering among the upper reaches of Llullaillaco, the world’s highest historically active volcano, straddling Argentina and Chile.

It’s incredible that anything could live that high, at 20,340 feet—there is no vegetation, and seemingly nothing to eat. Here, at the edge of the Atacama Desert, there is little rain, and temperatures sometimes plunge below minus 75 degrees Fahrenheit.

“It’s hard to overstate how hostile an environment it is,” says Jay Storz, a biologist at the University of Nebraska, Lincoln, and a National Geographic Explorer.

Intrigued by the discovery, Storz organized an expedition to the volcano in February specifically to search for rodents. And rodents he found. In fact, he encountered another yellow-rumped mouse even higher than previously sighted, atop the very summit of Llullaillaco, at 22,110 feet—breaking the record announced just last year.

The research, described in a study published this week on bioRxiv, where papers can be seen before peer review, is the beginning of a scientific quest to understand how these animals adapt to and survive such grueling conditions. The results could help us better understand how other creatures adapt to extremes, and could even have medical applications for humans coping with low levels of oxygen, for example due to disease, exertion, or altitude sickness.

Most of the mice, which belong to four different species, were caught using small traps during the expedition in February, so the animals could be further studied. But on the summit of Llullaillaco, Storz caught the mouse by hand, just as he was arriving. It was a lucky break, as you can only stay on the summit for a few minutes, due to low oxygen conditions and the possibility of violent storms.

“Nobody expected mice to be living that high,” Storz says. “And it turns out they get as high as you can possibly get.” Storz’s climbing companion, professional mountaineer Mario Perez-Mamani, captured the moment on video.

Mighty mouse

The yellow-rumped leaf-eared mouse (Phyllotis xanthopygus), is a known species that lives in the foothills and mountains of the Andes, and also can be found as low as sea level. (Related: Meet the animals that thrive in extreme mountain conditions.)

That means the mouse has an unprecedented elevation range of more than 22,000 feet. “That wide of a range is extraordinary,” says Scott Steppan, a mouse expert and biology professor at Florida State University. “No other species does that.”

On the February expedition, Storz and colleagues also found a Lima leaf-eared mouse (Phyllotis limatus) at 16,633 feet, far surpassing the known record for this species. The other two encountered species were found near or at their previously known altitudinal maximum.

In all, the expedition suggests “we’ve probably underestimated the altitude limits and physiological capabilities of lots of animals, just because the summits of the world’s highest peaks are relatively unexplored by biologists,” Storz says.

It all started in 2013, when American climbers Matt Farson, an emergency medicine doctor, and anthropologist Thomas Bowen spotted what was later assumed to be as a yellow-rumped leaf-eared mouse on the volcano. A later 2016 expedition, including Steven Schmidt from the University of Colorado, Boulder, found another mouse in the same location and collected a DNA sample near its burrow, confirming that is was a Phyllotis xanthopygus, which was announced in late June 2019 at the annual meeting of the American Society of Mammalogists in Washington D.C.

Added intrigue

Large-eared pikas, the previous record-holder, have been observed at 20,100 feet. There have also been sightings of yaks and blue-sheep around 20,000 feet, but this is outside of their known habitable zone. In this case of the leaf-eared mice, these individuals are thought to be part of established populations.

The finding adds intrigue to Llullaillaco, which is home to the world’s highest archeological site, a cache of nearly perfectly-preserved Inca mummies, discovered in 1999 by National Geographic Explorer Johan Reinhard. Reinhard also noted rodents at high elevations, though he at the time assumed they followed the climbers and survived off their food. Llullaillaco also has one of the world’s highest-elevation lakes, and resilient, almost otherworldly microbes.

The discovery raises many questions. How do the mice survive at such high elevations, where it is incredibly cold and there is less than half the oxygen found at sea level? And what do they eat?

The animals might eat bits of detritus that are blown up by the wind, but these don’t seem very substantial, says Storz, who studies deer mice, which also span from sea level to elevations above 14,000 feet; they are basically the North American equivalent of leaf-eared mice, he says.

View Images The yellow-rumped leaf-eared mouse has an elevation range of more than 22,000 feet, the world’s largest for any mammal by far. Illustration by Historic Collection, Alamy

These animals are able to survive at high altitude through “a whole suite of physiological changes,” such as slower muscle metabolism and a specialized cardiovascular system. (Related: Humans 'domesticated' mice 15,000 years ago.)

To be continued

Storz plans a return visit to better understand their ability to withstand such extreme lifestyles. Specifically, he plans to put live mice in metabolic chambers to measure their VO2 max, an indicator of oxygen consumption, and to perform other tests. The work has received funding from the National Geographic Society and the U.S. National Institutes of Health, as better understanding adaptations to high altitude life is “potentially relevant in treating a number of human diseases that relate to... problems with oxygen delivery and oxygen utilization,” he says.

These include heart disease and lung conditions including emphysema or chronic obstructive pulmonary disease. The results could also aid doctors in treating altitude sickness and coping with life at high altitude or elsewhere where there are low levels of oxygen.

The finding is “completely unexpected, and one, therefore, that deserves critical research on this animal as well as focused field research in other similar areas around the globe to parallel cases, like the Himalayas,” says James Patton, an emeritus professor at the University of California, Berkeley, who wasn’t involved in the research.

Patton marvels that a mouse could survive here, and is excited to hear more about how this is possible. “Amazing, to put it lightly.”