The Sirenia along with the elephants and hyraxes form the Super‐Order Paenungulata. They are the only fully aquatic mammalian herbivores and are represented by three species of manatees (Trichechidae), the dugong and the now extinct, Steller's sea cow (Dugongidae). Sirenians are non‐ruminant herbivores that have specialized hind‐gut fermentation in a large paired caecum at the juncture of the large and small intestines. The stomach is relatively small and is characterized by a unique cardiac gland which contains most of the digestive‐enzyme secreting cells. Little is known of the digestive physiology of sirenians. The metabolic rate of the manatee is very low in comparison to other mammals and may even represent the lowest weight‐specific metabolic rate for any mammal known. Metabolic rates for Dugong have not yet been determined. Foods and feeding habits of these species are reviewed both qualitatively and quantitatively. Manatees consume approximately 896 of their total body weight in aquatic plants daily, whereas the same value for the dugong is about 14%. Digestibilities of aquatic plants vary from 45 to 70% for manatees and a single in vitro measurement for Dugong was 83% for sea‐grass. The chemical composition of various plants consumed by wild and captive sirenians are presented and their value in the nutritional ecoloev of the different species is discussed.

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The digestive efficiency of polar bears (Ursus maritimus) feeding on their principal prey, the ringed seal (Phoca hispida) is high. Apparent digestibilities of 83.7 ± 10.3% for dietary protein, 97.3 ± 2.1% for dietary fat, and an assimilation of dietary energy of 91.7 ± 5.8% were found for the five diets. Presence of hair and bone in the diet significantly reduced the apparent digestibility of protein N. Gastrointestinal transit times were affected principally by the fat (energy) content of the diet and varied between 12.3 ± 1.9 to 38.0 ± 8.0 h. The results explain the apparent preference for seal blubber by polar bears in the arctic as well as demonstrate that any seal > 1 month of age would satisfy the bear's daily energy requirements. The limiting factors in energy intake at a given feeding are the stomach capacity of the bear and the relative proportion of blubber and meat ingested.