3.3 Diet of the summit mouse
One of the most puzzling results of the metagenomic and metabarcoding analyses is the predominance of Erythroxylaceae (the coca family) and Amaryllidaceae (the garlic family) in the stomach, cecum, and all 12 of the independently analyzed sections of the colon (Figure 3). Two representatives of Erythroxylaceae, Erythroxylum argentinum andE. cuneifolia , exist at elevations below ~2000 m far to the east of Llullaillaco, but they do not occur in Andean desert or dry puna habitats. Coca was widely used throughout the Incan empire and is still used in indigenous Quechua and Aymara communities and, occasionally, by mountain climbers. Upon summiting a particular peak, there is a custom (especially among Argentine climbers) of leaving offerings to Pachamama, an Andean “Earth mother”, Gaia-type deity. Such offerings are left at the base of rock piles called ‘apacheta’ that serve as summit markers. A typical offering to Pachamama is a sprinkling of coca leaves or a small bag of such leaves at the base of the apacheta. This custom provides a ready explanation for the predominance of Erythroxylaceae in the gut contents of the summit mouse, which had presumably encountered just such an offering on the summit of Llullaillaco. The presence of garlic in the stomach contents of the summit mouse has a similar explanation. In the Argentine province of Salta (where the western portion of Llullaillaco is located), garlic is a traditional folk remedy for altitude sickness. Argentine climbers are known to chew cloves of garlic during their ascent. As is the case with any unchewed coca that climbers possess upon reaching the summit, it is also customary to leave leftover cloves of garlic at the base of the summit apacheta. The predominance of both coca and garlic in the gut contents of the Llullaillaco summit mouse suggest that climbers’ offerings to Pachamama on the summits of high Andean summits may sometimes serve as unintentional offerings to opportunisticPhyllotis mice living in an extremely food-scarce environment.
Aside from Erthroxylaceae and Amaryllidaceae, we also detected DNA representative of several plant families such as Fabaceae, Malvaceae, and Poaceae, that occur at high elevations at the base of Volcán Llullaillaco and in the surrounding Altiplano (Arroyo et al., 1988, 1998; Luebert & Gajardo, 1999; Marticorena et al., 2004). Within Fabaceae, the herb Astragalus pusillus was documented at elevations up to 4300 m on the flanks of Llullaillaco (Marticorena et al., 2004). Within Malvaceae, several perennial herbs such asCristaria andicola , Nototriche auricoma , and N. clandestina occur at elevations between 4000-4500 m (Arroyo et al. 1998) and Cristaria andicola was documented as the most abundant plant species in the diet of Phyllotis at another altiplano study site in northern Chile (López-Cortés et al., 2007). Within Poaceae (=Gramineae), bunch grasses in the genus Calamagrostis (recognized as Deyeuxia in Arroyo et al. [1998] and Luebert & Gajardo [1999]) occur above 4000 m (Marticorena et al., 2004) and are also known to be included in the diet of Phyllotis from the Chilean Altiplano (López-Cortés et al. 2007). Although all of these plants seem plausible as potential sources of food for the summit mouse, there are no records of vascular plants or other vegetation above 1988, 1998; Luebert & Gajardo, 1999; Marticorena et al., 2004; Storz et al., 2024; Vimercati et al., 2019), although it is also true that botanical surveys typically do not venture above such elevations, so we should be careful about interpreting absence of evidence as evidence of absence.
Given that sequences representative of Fabaceae, Malvaceae, and Poaceae were detected in the gut contents of the mouse captured at 6739 m, far above the apparent elevational limits of those plant taxa, there are three possible explanations to consider: (1) plant material is carried upslope by the wind and accumulates in sufficient quantities on the lee edge of ridge lines and snowdrifts to provide a source of sustenance for high-elevation mice (‘Aolian deposits’; Antor, 1995; Spalding, 1979; Swan 1961, 1992); (2) the plants in question actually occur at much higher elevations than previously thought (though they may be scarce and cryptic); or (3) the mouse was not a full-time summit resident, but rather a transient sojourner that had simply consumed the plant material at or near the base of the mountain some days prior to its capture. The former two hypothesis cannot be rejected, since few systematic plant surveys have been performed on Llullaillaco or other >6000 m volcanoes (Storz et al., 2024). In assessing the plausibility of the third hypothesis, it is important to note that the ~1.6 km elevational distance between the summit of Llullaillaco (6739 m) and the apparent vegetational limit (~5000-5100 m; Storz et al., 2024) translates into a linear distance of ~5 km from any side of the volcano. Summiting the volcano from the vegetation limit is roughly equivalent to a direct-from-basecamp ascent, a feat that only the most elite mountain climbers could accomplish in a single day. We cannot rule out the possibility that mice undergo upslope/downslope dispersal on a seasonal basis, but such movements could certainly not occur on a daily basis. Moreover, in addition to the live capture of theP. vaccarum specimen on the Llullailllaco summit at 6739 m, video records and identification of active burrows of P. vaccarum between 6145 – 6205 m on the same volcano, and the discovery of desiccated cadavers and skeletal remains of numerous P. vaccarum on the summits of four different >6000 m volcanoes in the same mountain chain (Halloy, 1991; Steppan et al., 2023; Storz et al., 2020, 2023, 2024) provide a consilience of evidence suggesting that these extreme high-elevation mice are representative of resident populations. We think it is more likely that potential food plants exist at higher elevations, although they must be scarce and patchily distributed. The plausibility of this hypothesis is supported by the surprising discovery of bryophytes growing in association with active volcanic fumaroles near the summit of Volcán Socompa (Halloy, 1991), a 6051 m volcano located 47 km northeast of Llullaillaco along the Argentina-Chile border.
Primer set ITS detected sequences from families of two lichen-associated fungi, Phaeococcomycetaceae y Parmeliaceae (phylum Ascomycota), indicating that Phyllotis feeds on saxicolous lichen, as has been documented in other arctic and alpine mammals during periods of food scarcity (Conner, 1983; Seward, 2008; Richardson & Young, 1977). During the Arctic Winter, cricetid rodents such as snow voles (Chionomys nivalis ) and northern bog lemmings (Synaptomys borealis ) feed on tundra lichen (Richardson & Young, 1977). Likewise, during Winter months in the high alpine, North American pikas (Ochotona princeps ) feed on lichens under the snowpack (Conner, 1983). Terricolous, arboreal, and saxicolous lichens are an important component of the winter diet of Caribou (Rangifer tarandus ) in the northern Holarctic (Seaward, 2008) and arboreal lichens are an important component of the winter diet of Yunnan snub-nosed monkeys (Rhinopithecus bieti ) in montane coniferous forests at elevations serve as a seasonal or short-term supplement to the normal diet of many mammals living in arctic and alpine environments, their low nutritive value suggest that they are unlikely to represent a year-round dietary staple for small mammals like Phyllotis that have high metabolic demands.
We found no strong support for the arthropod fallout hypothesis, as we did not detect arthropod DNA in the gut contents of our summit mouse. In contrast to high-elevation pikas on the Quinghai-Tibetan Plateau that feed on yak feces, we found no evidence for interspecific coprophagy in our high-elevation Phyllotis , as indicated by the absence of metagenomic sequence reads and COI barcodes matching vicuña, guanaco, or any other potentially co-distributed mammals. The absence of such sequences also constitutes absence of evidence for scavenging.