CLIMATE

Chilean Mouse is the World’s Highest-Dwelling Mammal, Study Shows

ANTOFAGASTA – A team of researchers from Chile and the United States captured a mouse at the summit of dormant volcano Llullaillaco in the Antofagasta Region. The discovery of this mouse at 6,739 m (22,109 ft) was record-breaking: it marks the highest-dwelling mammal, surpassing all previous altitude records notched in the Himalayas and other mountain ranges, including the Andes. The team’s report was published on July 16 in the Proceedings of the National Academy of Sciences of the United States of America.

In February 2020, a scientific mountaineering expedition in the Antofagasta Region, led by J.F. Storz from the University of Nebraska and researchers from Universidad Austral de Chile, discovered the highest-dwelling mammal, a yellow-rumped leaf-eared mouse, Phyllotis xanthopygus rupestris, at an altitude of 6,739 m (22,109 ft) at the summit of Llullaillaco, a dormant volcano.

Along with this mouse, researchers collected several dozen others from a range of ecologically diverse altiplano and Puna de Atacama sites covering more than 4,300 m (14,000 ft) in elevation change. The mice have been retained as museum specimens and housed at the Colección de Mamíferos of the Universidad Austral de Chile, Valdivia, Chile. 

The phylogenetic analysis suggests that the volcano summit mouse is identical with other specimens found at a lower altitude, e.g., at Toconao, Chile, a 2,500 m locality about 180 km north-northeast of Llullaillaco and at the mouth of the Loa River on the Pacific Coast. It indicates the widespread presence of this particular mouse species from sea level up to the crest of the Andean cordillera at 6,739 m. This is the broadest distribution seen for any mammal altitude-wise.

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Llullaillaco is the world’s second-highest active volcano lying at the border between Chile and Argentina. According to researchers, the discovery of mice at such high altitudes challenges previously-assumed limitations on mammals’ physiological and ecological altitude ranges. It also confirms that the world’s highest summits are not as uninhabited as once thought.

At high altitudes, P. xanthopygus rupestris is capable of surviving despite the presence of freezing temperatures and reduced oxygen pressure (hypoxia). The possibility of special genetic adaptations in these mice from the high Andes needs to be investigated. Dr. Storz’s team plans to look for genetic changes that might have equipped these animals to survive at high elevations. Another unanswered question is the type of food available for mice at the upper reaches of the volcano. No doubt, this will be the subject of a future high-altitude expedition.

“It’s so amazing that they’re up there,” Graham Scott, a physiologist at McMaster University in Hamilton, Canada who was not involved in the study, told Science News. “Understanding how these and other animals live under low-oxygen conditions could help humans to overcome diseases that cause reduced oxygen levels,” he says.

Sightings in 2013 Foreshadowed This Year’s Findings

According to Science News, the official highest-dwelling mammal record was previously held by the large-eared pika, Ochotona macrotis, reported at an altitude of 6,130 meters (20,111 ft) during a 1921 Mount Everest expedition. That record was already in jeopardy as of 2013. When American climbing buddies Matt Farson, an emergency medicine doctor, and Thomas Bowen, an anthropologist, stumbled upon a yellow-rumped leaf-eared mouse while searching for the best route up Llullaillaco.

As reported by National Geographic, at an altitude of 20,340 ft, Farson, “bleary from the lack of oxygen – and genuinely concerned he might be seeing things” caught one on camera. “I hadn’t seen an animal since 16,000 feet … so to see something up there just made no sense,” Farson said.

As National Geographic further noted, Farson wasn’t aware of the import of his sighting until he told Bowen. “It struck me that this was probably a major finding,” Bowen said.

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