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.

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de Mastozoología, www.mastozoologiamexicana.org to the Ilú -Tramén-tepui massif and Ptari tepui (Huber et al. 2001). The climate is submesothermic ombrophilous characterized by annual average temperatures between 18 and 24 °C and 2,000 to 3,000 mm of total annual rainfall with a weak dry season (< 60 mm / month) from December to March (Rull et al. 2013).