Thin air, icy cold and seemingly no food – yet a species of mouse can live at altitudes of over 6,000 meters in the South American Andes. The fact that a permanent population exists there is now proven by finds of rodent mummies in addition to live catches. Further studies will now clarify how the mice defy the harsh conditions in the inhospitable habitat and what they eat there.
The history of the discovery of mice on the volcanic peaks of the Andes actually dates back to the 1970s: At that time, archaeologists discovered the remains of mice while investigating ancient Inca cult sites at altitudes of over 6,000 meters. At the time, however, it was assumed that they had once been brought there by the Incas: They could have gotten there with firewood or supplies, or the rodents may even have been part of sacrificial rituals, according to speculation. However, it was simply ruled out that mice could naturally occur at these extremely inhospitable altitudes. Because there is very little oxygen in the air, the temperatures barely get above freezing and there don’t seem to be any sources of food.
But in 2020 the sensation followed: During an expedition to the Andean volcano Llullaillaco, Jay Storz from the University of Nebraska-Lincoln and a colleague discovered a mouse scurrying through the rubble at an altitude of around 6,700 meters. They were also able to capture them and assign them to the leaf-eared mice (Phyllotis). In doing so, they set an altitude record: never before had a mammal been discovered at such altitudes. However, questions remained unanswered as to the extent to which these mice really live there permanently and whether it might be a special type of altitude. In recent years, Storz and his colleagues have been looking for further traces of rodents in the heights of the Andes.
Freeze-dried height specimens
As they report, they discovered a total of 13 carcasses of leaf-eared mice at altitudes of over 6,000 meters in the summit area of three neighboring volcanoes – Salín, Púlar and Copiapó. “These are basically freeze-dried, mummified mice,” says Storz. To investigate when these animals lived, researchers subjected the remains to radiocarbon dating. It turned out that the eight animals from Salín and one from Copiapó had lived in the last few decades – at least after 1955. The four mummies from Púlar, however, were significantly older: the animals existed around 350 years ago. The results show that the presence of rodents at extreme altitudes is not a recent phenomenon. The researchers emphasize that the four mummies from Pular cannot be traced back to Inca visits to the summit. Because 350 years ago their empire had long since collapsed.
The freeze-dried mouse mummies were ideal for genetic studies. The team was able to partially sequence the genomes of the summit specimens in order to compare them with the genome of leaf-eared mice that live in lower areas of the region. It turned out that this is not a special subspecies, but rather a completely “normal” representative of the Phyllotis vaccarum species. But this result is also remarkable: This species can apparently adapt to extremely different conditions.
Permanent settlement is becoming apparent
Through the genetic analyzes the researchers were also able to reveal the gender and relationships between the animals. It turned out that two Salín specimens were closely related to each other – possibly siblings or parents and offspring. In addition, there was a balanced ratio between males and females in the mummy finds. As the researchers explain, these two findings contradict the assumption that these were animals that only made trips from deeper areas to higher altitudes. Instead, there appears to be a permanent resident population of psyllid voles at altitudes of over 6,000 meters, the scientists say.
However, questions still remain unanswered. It still remains unclear how the rodents can exist in these inhospitable heights and why they have developed this habitat. “Even at the foot of the volcanoes, the mice live in an extreme environment,” says Storz. “But on the tops of volcanoes it’s much worse. It feels like being in space,” said the scientist. Further studies will now clarify what adaptations they can use to withstand the temperatures and thin air there, as well as what they eat. They may be gnawing on some lichen on the rock.
However, the extreme habitat definitely seems to offer one advantage: the rodents are safe from predators there. Because in the lower altitudes, foxes, cats and birds of prey hunt them intensively. Perhaps the increased safety is actually the key reason for the scarab mice’s interest in one of the most extreme habitats in the world.
Source: University of Nebraska-Lincoln, specialist article: Current Biology, doi: 10.1016/j.cub.2023.08.081