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.