The environmental limits of Animal life have always fascinated biologists, and new discoveries about organismal adaptability continually force us to revise our assumptions about such limits. At high altitude, endothermic Vertebrates are forced to cope with a combination of environmental stressors, the most salient of which are the reduced partial pressure of oxygen (hypoxia) and freezing temperatures. Nonetheless, numerous alpine Mammals and Birds have evolved physiological capacities for meeting such challenges and are capable of surviving at surprisingly lofty altitudes so long as food is available. Upper altitudinal limits of wild Mammals are generally thought to fall in the range 5200 m to 5800 m above sea level. Such limits are surely dictated by food availability in addition to physiological capacities for tolerating hypoxia and extreme cold. The altitudinal range limits of alpine birds and mammals are often not known with certainty, due to scanty survey data in inaccessible highland regions, and many published records in the scientific literature are surpassed by sightings reported by members of mountaineering expeditions.
Motivated by reported sightings of Mice living at record altitudes, Jay Storz of the School of Biological Sciences at the University of Nebraska-Lincoln, Marcial Quiroga-Carmona of the Instituto de Ciencias Ambientales y Evolutivas at the Universidad Austral de Chile, Juan Opazo, also of the Instituto de Ciencias Ambientales y Evolutivas at the Universidad Austral de Chile, and of the Millennium Nucleus of Ion Channels-Associated Diseases, Thomas Bowen of the Department of Anthropology at California State University, Fresno, Matthew Farson of the Modoc Medical Center, Scott Steppan of the Department of Biological Science at Florida State University, and Guillermo D’Elía, again of the Instituto de Ciencias Ambientales y Evolutivas at the Universidad Austral de Chile, organized a scientific mountaineering expedition to survey the Rodent fauna of Volcán Llullaillaco and the surrounding altiplano/Puna de Atacama of northern Chile, the results of which were published in a paper in the Proceedings of the Natonal Academy of Sciences of the United States of America on 25 June 2020.
On a 2013 mountaineering expedition to Volcán Llullaillaco, Matthew Farson and Thomas Bowen filmed a Mouse (identified as Phyllotis spp.) scurrying across a snowfield at 6,205 m above sea level, an altitude that exceeds existing records for wild Mammals. This sighting motivated a subsequent high-altitude trapping expedition in February 2020, led by Jay Storz, Marcial Quiroga-Carmona, and Guillermo D’Elía. During this expedition, they live-trapped Rodents from ecologically diverse sites on the altiplano and puna spanning over 4300 m of vertical relief.
On Volcán Llullaillaco, Stortz et al. live-trapped Rodents in and around Aguadas de Zorritas (4140 m to 4360 m), base camp at Ruta Normal (4620 m), base camp at Ruta Sur (5070 m), high camp at Ruta Sur (5850 m), and the volcano summit (6739 m). In total, Stortz et al. collected museum voucher specimens of 80 Mice representing four species: Andean Altiplano Mouse, Abrothrix andina, Altiplano Laucha, Eligmodontia puerulus, Yellow-rumped Leaf-eared Mouse, Phyllotis xanthopygus, and Lima Leaf-eared Mouse, Phyllotis limatus. Stortz et al. collected Eligmodontia puerulus and Abrothrix andina at maximum altitudes of 4099 and 4620 m, respectively; these altitudes approximate or exceed previous records for these species. Stortz et al.'s altitudinal records for Phyllotis limatus and Phyllotis xanthopygus (5070 and 6739 m, respectively) far exceed existing records for both species.
Stortz et al. captured the 6739 m specimen of Phyllotis xanthopygus on the very summit of Llullaillaco. This summit specimen represents an altitudinal world record for Mammals, far surpassing all specimen-based records from the Himalayas and elsewhere in the Andes. An extensive review of published accounts indicates that the Large-eared Pika, Ochotona macrotis (Lagomorpha), was the previous record holder. Although the highest specimen-based records for this species are from 5182 m in the Himalayas, credible sightings at 6130 m were reported from a 1921 Everest expedition.
Phylogenetic analysis of cytochrome b sequences corroborated the species identifications of Stortz et al.'s record specimens of Phyllotis limatus and Phyllotis xanthopygus and revealed close relationships with conspecific specimens from elsewhere in northern Chile, northern Argentina, and southern Peru. The summit specimen (GD 2097) groups with those of previously collected altiplano specimens of Phyllotis xanthopygus rupestris. Moreover, the cytochrome b haplotype of this summit specimen is identical to that of another Phyllotis xanthopygus rupestris specimen (LCM1780) collected at Toconao, Chile, a 2500 m locality about 180 km northeast of Volcán Llullaillaco. Similarly, two other specimens ofPhyllotis xanthopygus rupestris collected at different altitudes (GD 2082 at 4406 m and GD 2095 at 5069 m) on Volcán Lullaillaco share identical cytochrome b haplotypes with a specimen (LCM1737) collected at the mouth of the Loa River on the Pacific coast, about 400 km northwest of Volcán Lullaillaco. Thus, not only does Phyllotis xanthopygus rupestris range from sea level to the crest of the Andean Cordillera at 6739 m (the broadest altitudinal distribution of any mammal), but individuals found at opposite extremes of this vast range share identical cytochrome b haplotypes.
Stortz et al.'s capture of Phyllotis xanthopygus rupestris on the summit of Llullaillaco suggests that we may have generally underestimated the altitudinal range limits and physiological tolerances of small Mammals simply because the world’s high summits remain relatively unexplored by biologists. The upper range limits of many Vertebrate taxa are not precisely demarcated, and putative altitudinal records for many taxa exist as unverified sightings or reports in mountaineering expedition accounts rather than as voucher specimens in museum collections.
Stortz et al.'s discoveries prompt many evolutionary and ecological questions. Given the exceptionally broad altitudinal range of Phyllotis xanthopygus, have Mice from the high Andes evolved genetically based adaptations to hypoxia that distinguish them from lowland conspecifics? To what extent is the ability to tolerate such a broad range of environmental conditions attributable to acclimatisation (physiological plasticity)? Given that Mice inhabiting the upper reaches of Llullaillaco are living more than 2000 m above the upper limits of green plants, what are they eating? Such questions can be answered by future mountaineering expeditions in the Humboldtian tradition that combine high-altitude exploration and scientific discovery.
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