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On the peaks of the Andes, life seems to be a fiction. There is nothing. Huge volcanic peaks that are over 6,000 meters high. They are just a world of stone, ice and snow.
Between 1970 and 1980, archaeologists climbed some of these volcanoes and found rocky mounds built by the Incas, the empire that ruled South America until the early 16th century. They are easy to spot and have even found mummies attributed to Capac Cocha, a ceremony that involved child sacrifice. Among these ruins they also found the remains of some mice. They speculated that these rodents had been transported – perhaps accidentally – by the indigenous people themselves. They didn't believe they would arrive alone.
In 2011, an anthropologist and a doctor found a naturally mummified mouse on one of these Andean peaks, about 6,200 meters above sea level. Three years later, they accidentally discovered one of these rodents among the rocks and ice; and in 2016, a mountaineer saw a rodent hiding in its lair in Llullaillaco, a 6,739-meter-high northern massif on the border of Chile and Argentina, the world's second-highest active volcano.
But it wasn't until 2020 that Bolivian mountaineer Mario Pérez Mamani noticed the presence of a rodent.
“A mouse!” he pointed out to his partner, mountaineer and University of Nebraska biologist Jay Storz, who, exhausted and dazed from the altitude, barely noticed the small animal quickly hiding under a rock. But the American managed to catch it with his hand. It was a Phyllotis vaccarum, known as the long-eared mouse, which belonged to the family Sigmodontines (Sigmodontinae), a subfamily that is thought to have reached South America more than 3 million years ago, when only a few islands connected the north and south of the continent. These small animals had managed to colonize impossible heights. There was life at the top.
Jay Storz and Mario Pérez Mamani descend from the summit of Cerro Vicuñas, which was examined without finding mice.
How they got to heaven
This encounter prompted Storz and other biologists to investigate the presence of mice. Between 2020 and 2022, 24 volcanoes emerged in the Andes, in Chile, Bolivia and Argentina. They found 13 dead individuals of this rodent on three of these peaks: on the Chilean volcanoes Púlar (6,233 meters) and Copiapó (6,052 meters) and Salín (6,029 meters).
“We found life on the tops of very few of them,” admits the Nebraska biologist. “And the long-eared mouse is the most extreme of all.” According to him, it is the only animal found above 6,000 meters, although there were also remains of other rodent species at altitudes above 5,000. They all belong to the Cricetidae family and were separated from their distant European relatives about 18 million years ago.
There used to be a hypothesis that the Incas had something to do with the arrival of these little mice on the peaks, even that some sneaked into the firewood they were carrying to make a fire. “It always seemed crazy to me, because no mouse can survive the journey with a few sticks on its back,” says Guillermo D'Elia, a biologist at Austral University and one of the co-authors of the study on the long-eared mouse published in the journal Current Biology.
Naim Bautista searches for mummified mice in the Copiapó volcano, 6,052 meters above sea level.Jay Storz
The scientist was skeptical and used radiocarbon testing to date the age of these little hairy mummies: most were from around 1950, with one from around 350 years ago. That is, much younger than the fallen Inca Empire. This study joins others “that suggest that mice live at these altitudes,” says D'Elia. “They are few, but they exist.”
The evidence shows that they get there on their own. The mummies they found were half male and half female, as would be the case in a “natural population,” he says. In mammals, males tend to reproduce in order to reproduce, but in these peaks the sex of the mice found is the same: “If they simply came from above, you would expect to find more males than females,” he speculates. “They make people think that these are natural populations,” he notes.
Tame the peaks
There is still a lot to know about this species. For this reason, biologists continue to work to obtain more information about these rodents, but the mission is arduous. Storz was an experienced mountaineer and when he received rumors of a mouse with the world record height, he became interested in investigating the matter. Her two passions intersected.
Each scan lasts three weeks; requires acclimatization to the low oxygen levels at 6,000 meters. Arrival at the peaks must be before midday in order to descend a few hundred meters to the base camp. Or risk storms. Once at the top, “you usually have time to hug your partner, take a summit selfie and say goodbye, because it is dangerous to stay at such altitudes for so long,” warns the American. “But we also have to look for the mummies.” They press the time. “In the cases where we found stuffed bodies, they were visible and we didn’t have to look as closely,” he says. And no matter how small it may be, there is always a chance of finding P. vaccarum.
The mountains of western Bolivia tend to be snowier than those of northern Chile and lie in the driest desert in the world, the Atacama. But beyond that, “at first glance there is no difference between the volcanoes in which we find mummies and the others,” admits the biologist. “We don't understand why we found it on some peaks and not others,” except when there is a lot of snow and they don't expect to find life. They also found corpses of long-eared mice on peaks that had no archaeological remains. “It's a mystery why mice climb some mountains and not others,” he admits.
Jay Storz conducts excavations to find mummified mice on the 6,233-meter peak of the Púlar volcano.Mario Pérez Mamani
Breathing is difficult at the top. Exhaust. The mind becomes clouded. It's difficult to think clearly, but the views are impressive. Further down – at around 3,500 meters – in the dry puna of Atacama, “it is the best place in the world to look at the night sky,” admits Storz. “It has a charm; It’s not for everyone, it’s very strict, almost Martian.”
Two large camelids, vicuñas and guanacos, live there, surrounded by foxes and flamingos in the lagoons. The vegetation is sparse: there are cacti and yaretas, a bush that looks like moss. Up to 5,000, the environment is “a little friendlier,” explains D’Elia. “They can imagine what they are allowed to eat and what they are not allowed to eat.” Until then, there will be plenty of mice. At this altitude the cold is severe at night and the lack of oxygen is already noticeable. “You get dizzy when you bend over and stand up quickly,” he describes. “Your mind and body work in slow motion.” It is difficult to fall asleep deeply at night. The wind twists and bends the bushes.
These little mice hide among the vegetation and rocks and are more active at night. Researchers set traps to catch and study them. In places like the Casiri Lagoon at 4,800 meters on the border with Bolivia, “everything is very primitive and rudimentary,” he says. “They are very remote places, you don’t see many people,” apart from the occasional local with his cattle. The roads are bumpy, unclear and slow. “And there comes a time when there is nothing left: sand, volcanic ash, stones, snow and ice,” he continues. Sulfur fumaroles sprout from the cracks and the ground turns yellow.
Scientists have seen tracks of a puma, the big cat of the Andes, at an altitude of 5,200 meters. “It's impressive that they go so high, I imagine they hunt vicuñas and guanacos,” says Storz. The shy Andean cats also live there, although he has never seen one. “But it’s possible they saw us,” he says.
There's nothing to eat
The absence of predators could be one of the reasons why the mice reach such high altitudes, the researchers speculate. Although at 6,000 meters they “obviously have to deal with other problems,” the American admits.
D'Elia is not surprised that mammals are the highest living vertebrates, while there are no lizards or reptiles due to different physiological boundaries. What's strange is that it's a small rodent: “The closer you get to the poles or to high altitudes, the larger the animals get,” he says, “because they are more 'energy efficient.'” But not in the Andes.
Biologists assume that rodents live there all year round and do not migrate. In the laboratory studies, they sequenced the genomes of all 13 mice bodies. All belonging to the same species, they compared them with individuals from the highlands and at sea level and found little difference between them. “It's like it's the same large, widely distributed population,” says the scientist.
The desolate landscape at the summit of the 6,029 meter high Salín volcano.Jay Storz
Knowing that these rodents live at such altitudes allows them to explore the limits of vertebrate life. In Santiago, at the University of Chile, they catch some of these mice at lower altitudes. They use controlled experiments to try to find out how they evolved to cope with such extreme conditions by comparing them with individuals from “extremely related” species that do not reach this size.
But just because they are nearly genetically identical doesn't mean there aren't differences, warns D'Elia. More research needs to be done, but he speculates on the possibility that they have a gene “that allows hemoglobin at altitude to capture oxygen more efficiently.” And another mystery remains: what do they eat when there is nothing upstairs? They only identified lichens, very resistant organisms consisting of a fungus and an algae or cyanobacteria. Analysis of the stomach of a mouse they caught in Llullaillaco reveals some plants from the Chilean highlands.
With their findings, they initially broke the altitude record of the long-eared pika (Ochotona macrotis) – which is most closely related to hares and rabbits – which was found at an altitude of 6,130 meters in the middle of Mount Everest. “There are still areas that are little explored, not only the Chilean Andes, but also the Peruvian, Ecuadorian and Argentinian Andes,” says D'Elia. “There was a 'final frontier' of exploration waiting to be visited.”