4. CONCLUSIONS
A combination of metagenomic, metabarcoding, and stable isotope data provided new insights into the diet of Phyllotis mice living at extreme elevations that far surpass known vegetation limits. Stable isotope data revealed that Phyllotis vaccarum maintains a mainly omnivorous diet in all elevational zones, and elevational variation in diet reflects variation in vegetation composition and the extent to which the mice rely on animal prey. Estimates of trophic position based on isotopic data indicated that mice collected near apparent vegetation limits (~5100 m) on the flanks of Llullaillaco rely more heavily on animal prey than mice from lower elevations. Metagenomic and metabarcoding analyses of gut contents from the mouse from the summit of Llullaillaco (6739 m) revealed a strictly herbivorous diet. The absence of animal DNA suggests that mice at extreme elevations do not subsist on wind-blown arthropods or other animal material. The detection of DNA from lichen-associated fungi indicate that Phyllotis mice living above known vegetation limits may supplement their diet with saxicolous lichens, as observed for other arctic and alpine mammals during periods of food scarcity in the winter. However, measured δ15N levels indicate that lichen is not an important dietary staple in mice native to any of the surveyed elevational zones. The metagenomic and metabarcoding data also produce a scientific conundrum: the gut contents of a mouse captured at 6739 m elevation contained DNA from several families of native plants that are not known to occur above elevational distributions of both the plants and the mice. It is possible that some of the plants identified in the diet of the summit mouse exist at higher elevations than previously supposed, but they exist beneath the snowpack or in other cryptic microhabitats.