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.