Stable isotopes analysis
For stable isotope analysis, 1mg muskox feces and 1.0–1.5mg plant
tissue samples (extracted evenly from leaves, stems, and fruits if
available) were homogenized. Each sample was freeze-dried and then
prepared with a stable isotope ratio mass spectrometer system (IsoPrime
100; Cheadle, UK) with an elemental analyser (vario MICRO cube;
Elementar, Hanau, Germany). Purified CO2 and
N2 were used as the sample analysis gas and the isotopic
reference gases. GC column resolve CO2 from
N2 and reduction column filled with copper wires reduce
N2. All results are reported with delta notation, in
parts per thousand (‰) relative with PDB standard. Each plant sample was
repeated six times of this analysis.
δ13C and δ15N =
(Rsample/Rstandard -1) * 1,000 (‰)
The international reference materials of sucrose (ANU C12H22O11; NIST,
Gaithersburg, MD, USA) for δ13C and ammonium sulfate ([NH4]2SO4;
NIST) for δ15N were analyzed for calibration of reference gases and the
internal standard (acetanilide; Thermo Scientific). The analytical
precision was based on 10 replicate measurements of acetanilide and was
within 0.12‰ and 0.20‰ for δ13C and δ15N, respectively.
We used simmr Bayesian mixing model in the R with five plant food
sources, which were known as muskox diet (Forchhammer & Boomsma., 1995;
Gustine et al., 2014; Mosbacher et al., 2016), to estimate dietary fiber
composition in muskox diets (stable isotopes mixing models in R;
Parnell., 2019). The detailed procedure for running the simmr mixing
model is described in Swan et al. (2020).