Seasonal changes in the rumen microbial ecology of muskoxen

Abstract

Ruminal fermentation must respond to seasonal changes in the behavior and digestive function of muskoxen (Ovibos moschatus) as intakes of food and water rise in autumn (August - October) and decline from winter to spring (November - May). Rumen temperatures were not static but instead were punctuated by decreases of as much as 13°C below the average of 39°C. Declines in rumen temperature were associated with water ingestion while gains in rumen temperature were associated with feeding. Food intake and water turnover rates were positively correlated. Direct microscopic counts of bacteria in rumen fluid decreased from 1.8 to 0.9 X 10¹⁰·mL⁻¹ as water turnover declined from 11.1 to 7.7 kg⁻¹·d⁻¹ between October and March. The energetic cost of warming ingesta was estimated as 5 to 14% of the predicted intake of digestible energy. Concerted ingestion of food and water may allow muskoxen to substitute heat increment of feeding for the cost of warming ingesta. Gains in food intake between May and August decreased rumen osmolality (271.9 ± 16.4 vs. 245.9 ± 11.4 mOsmol.kg⁻¹) and pH (6.81 ± 0.31 vs. 6.39 ± 0.15), indicating a shift in the allostatic set point for the rumen. Rumen fluid pH was more variable in May than in August both before and after a single meal of fermentable substrate even though concentrations of fermentation acids were lower in May than in August. Allostatic set points probably alter the homeostatic range of conditions and the microbial diversity of fermentation in muskoxen. We devised an improved method to isolate rumen bacteria and to extract their DNA for measures of rumen microbial diversity.