Nutritional development of feeding strategies in arctic ruminants: digestive morphometry of reindeer, Rangifer tarandus, and muskoxen, Ovibos moschatus.

Reindeer have been classified as intermediate feeders and muskoxen as grazers based on differences in digestive morphology and consumption of fibrous plants. We hypothesized that the digestive morphology of young (< 2 months) reindeer and muskoxen anticipates transitions in diet and determines the feeding strategy of each species at adulthood. We compared structural morphology and rates of cell division in the rumen, abomasum, duodenum and liver of reindeer and muskoxen as neonates (1 day old), during the transition from milk to forage (30-60 days old) and in adults (> 7 yr). Development in utero provided the neonate with a functioning mucosa of the gastric abomasum and duodenal mucosa with high surface enlargement for digestion and absorption of concentrated milks. Transition to forage was preceded by changes in ruminal papillae structure that increased surface area and likely contributed to active fermentation by 60 days of age. The abomasum also increased in acid-secreting parietal cells during the transition to forage, which may enhance digestion of plant and microbial proteins. Rates of cell division also indicated a sustained differentiation of tissue structure during the transitional period. Young arctic ruminants expressed digestive structures that preceded full function, which indicated the ultimate feeding strategy of each species. For example, the rumen of young muskoxen had thick cornified epithelia and muscle layers that would provide ruminal mucosa with better protection from fibrous abrasion and enhance motility of bulky diets. Conversely, young reindeer had more complex papillary shapes in the rumen and more foliate villi in the duodenum, indicating a greater absorptive capacity of these structures than in muskoxen. Ontogenetic programs, therefore, play the primary role for digestive development of reindeer and muskoxen and determine the nutritional strategies of adults.