Discussion
Our results showed that the microbiome of muskox adults and calves have
similar levels of alpha-diversity, at phylum and family level, although
the calves showed higher variance values. The adults and calves had
different bacterial communities even if the calves showed more diverse
composition within the group compared to the adults. There was an
apparent difference in the relative abundance of two Firmicutes
families. The diet analysis indicates that the adults and calves had
common dietary plant species. The different bacterial communities
between the female adults and calves, which share similar diets, suggest
that the gut microbiome in the calf group is still unstable and not
fully colonized despite the dietary similarity. After birth, calves
receive the gut microbiome from mothers and begin to form their own
independent gut microbiome (Barko et al., 2018). Muskox calves graze
from one week after birth, following mothers to choose dietary plants;
it was also observed during our study period that the females and calves
were foraging together. Diet analysis results also confirmed that the
adults and calves had the common plants. Considering the breeding cycle
(birth around March or April; Adamczewski et al., 1994), the calves in
this study are assumed to be three or four months old. Thus, we predict
that the calves may not finish constructing their gut microbiome by
August.
The major dominant phylum of our fecal sample was Firmicutes. At the
family level, Ruminococcaceae and Lachnospiraceae, which belong to the
class Clostridia, phylum Firmicutes, were dominant to occupy more than
80% of total abundance. Ruminococcaceae and Lachnospiraceae were
reported to encode carbohydrate-active enzymes for glycoside hydrolases
and carbohydrate esterases in herbivores (Wang et al., 2016).
Ruminococcaceae was also know to affect the secondary metabolite
synthesis to get involved in host immunity, such as antibiotic
biosynthesis (Gosalbes et al., 2011) by producing SCFAs (short chain
fatty acids) for lipid metabolism and digestion (Morrison & Preston.,
2016) and by detoxifying the plant secondary metabolites (Kohl et al.,
2014). Lachnospiraceae was reported to produce SCFAs for metabolism (Hao
et al., 2017; Vacca et al., 2020), but also digest lactose by converting
lactate into butyrate (Meehan & Beiko., 2014). In our results, adults
had more Ruminococcaceae and less Lachnospriaceae than calves. Such
differences could be related to the microbial functions for host
digestion and metabolism, depending on their need. We infer that the
differences could result in the differential needs for digestion between
the adults and calves since calves were still relying on the breast milk
during the sampling period.
Predicted microbial functions indicates that energy metabolism was the
most dominant (adults: 6.63%, calves: 5.88%). We found pathways to
help digestion of dietary fibers for carbohydrates (carbohydrate
metabolism, adults: 0.11%, calves: 0.13%) and lipids (fatty acid
metabolism, adults: 0.41%, calves: 0.33%; glyoxylate and dicarboxylate
metabolism, adults: 0.76%, calves: 0.65%; fatty acid biosynthesis,
adults: 0.60%, calves: 0.57%).
Our microbial results correspond to the previous studies on large
herbivores (Table 1). In the previous muskox studies, Firmicutes was the
most dominant phylum (74–83%) and Ruminococcaceae, Lachnospiraceae
were the most dominant families (47–65% and 13–16%, respectively)
(Salgado-Flores et al., 2016; Bird. 2019). In Svalbard and Norwegian
reindeers, phylum Firmicutes and family Ruminococcaceae, Lachnospiraceae
were also abundant (Zielińska et al., 2016; Sundset et al., 2007;
Gruninger et al., 2014). From previous studies and our PICRUSt2 results,
we estimate that Ruminococcaceae and Lachnospiraceae could promote
cellulose metabolism for herbivores.
From the stable isotopes analysis, we found that adults and calves
shared similar diet plants with E. nigrum and S. glaucabeing the dominant species. In previous studies, S. glauca were
known to be a main food source of muskoxen, especially during the summer
(Gustine et al., 2014; Thing et al., 1987), which shows a relatively
high digestibility for muskoxen (Staaland & Olesen 1992). E.
nigrum has not been previously considered an impotant plant despite the
potential vegetation records in a newly introduced area (in westeren
Alaska, Ihl & Klein., 2001). Our findings suggest that muskox adults
and calves are dependent on the similar plant items and E. nigrumcan be additionally consumed by muskoxen at a specific time period in
our study site. Considering the recent increase of E. nigrum with
Arctic warming
(Bråthen
et al. 2018), it can become an important food sources for muskoxen.
In conclusion, our findings may provide ecological information for
understanding the host and microbial interactions and shed light on the
microbial functions for digestion in herbivores. In future studies, it
will be interesting to analyze the detailed microbial functions in
adults and calves related to the digestion and immune functions.
Furthermore, it is necessary to record the diet compositions of the
large herbivore animals and monitor their dietary changes since the
Arctic ecosystem is rapidly changing with global warming.