Discovery of the world’s highest-dwelling mammal

Environmental limits of animal life are invariably revised when the animals themselves are investigated in their natural habitats. Here we report results of a scientific mountaineering expedition to survey the high-altitude rodent fauna of Volcán Llullaillaco in the Puna de Atacama of northern Chile, an effort motivated by video documentation of mice (genus Phyllotis) at a record altitude of 6,205 m. Among numerous trapping records at altitudes of >5,000 m, we captured a specimen of the yellow-rumped leaf-eared mouse (Phyllotis xanthopygus rupestris) on the very summit of Llullaillaco at 6,739 m. This summit specimen represents an altitudinal world record for mammals, far surpassing all specimen-based records from the Himalayas and other mountain ranges. This discovery 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.

Environmental limits of animal life are invariably revised when the animals themselves are investigated in their natural habitats. Here we report results of a scientific mountaineering expedition to survey the high-altitude rodent fauna of Volcán Llullaillaco in the Puna de Atacama of northern Chile, an effort motivated by video documentation of mice (genus Phyllotis) at a record altitude of 6,205 m. Among numerous trapping records at altitudes of >5,000 m, we captured a specimen of the yellow-rumped leaf-eared mouse (Phyllotis xanthopygus rupestris) on the very summit of Llullaillaco at 6,739 m. This summit specimen represents an altitudinal world record for mammals, far surpassing all specimen-based records from the Himalayas and other mountain ranges. This discovery 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.
high altitude | Andes | range limits | hypoxia | Phyllotis T he 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 (1)(2)(3) 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 5,200 m to 5,800 m above sea level (4)(5)(6). 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, we organized a scientific mountaineering expedition to survey the rodent fauna of Volcán Llullaillaco and the surrounding altiplano/ Puna de Atacama of northern Chile. Llullaillaco (6,739 m) is the second-highest active volcano in the world and straddles the border between Chile and Argentina. Our trapping results challenge current thinking about physiological and ecological constraints on the altitudinal range limits of mammals and indicate that the world's highest summits are not as barren as once believed.
We captured the 6,739-m specimen of P. xanthopygus on the very summit of Llullaillaco (24°43.235′S, 68°32.208′W) (Movie S2). 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 specimenbased records for this species are from 5,182 m in the Himalayas (US National Museum 198648 and 198649), credible sightings at 6,130 m were reported from a 1921 Everest expedition (12).
Phylogenetic analysis of cytochrome b (cytb) sequences corroborated the species identifications of our record specimens of P. limatus and P. xanthopygus and revealed close relationships with conspecific specimens from elsewhere in northern Chile, northern Argentina, and southern Peru (Fig. 2). The summit specimen (GD 2097) groups with those of previously collected altiplano specimens of P. xanthopygus rupestris (13). Moreover, the cytb haplotype of this summit specimen is identical to that of another P. x. rupestris specimen (LCM1780) collected at Toconao, Chile, a 2,500-m locality ca. Our capture of P. x. 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.
Our discoveries prompt many evolutionary and ecological questions. Given the exceptionally broad altitudinal range of P. 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 acclimatization (physiological plasticity)? Given that mice inhabiting the upper reaches of Llullaillaco are living >2,000 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.

Materials and Methods
Specimen Collection. We captured mice using Sherman live traps, except for the specimen from the Llullaillaco summit, which was captured by hand (Movie S2). We killed mice in the field and prepared them as museum specimens, all of which are housed at the Colección de Mamíferos of the Universidad Austral de Chile, Valdivia, Chile. Tissue samples from Argentinian and Peruvian specimens were obtained as loans from the collections of Centro Nacional Patagónico, Puerto Madryn, Argentina, and Louisiana State University Museum of Natural Science, Baton Rouge, LA.