Animals that eat only animal prey are sometimes called strict or obligate carnivores to distinguish them from facultative carnivores that eat mostly animal prey but also consume nonanimal foods. Felids are strict carnivores and, in the wild, obtain most of their food by predation on the tissues of mammals, birds, or fish. Their domestic representative, the cat (Felis catus), differs in several respects in its metabolism and nutrient requirements from the domestic dog (Canis familiaris), a canid that is a facultative carnivore. It is presumed that these differences are an evolutionary consequence of their respective ancestral diets. The cat has a higher dietary requirement for protein because it has only a limited ability to regulate nitrogen losses in the urine losses that are of little consequence when nitrogen (protein) intakes are high, as they would be when whole animal prey are consumed. Further, the cat is particularly sensitive to a deficiency of the essential amino acid arginine a deficiency that results in toxic levels of ammonia in the blood but which is unlikely when whole animals are consumed. The cat also has a dietary requirement for taurine, a sulfur-containing amino acid that can be synthesized from methionine by the tissues of most other mammals after weaning. An experimental taurine deficiency results in central retinal degeneration and cardiomyopathy in the cat. Fortunately, taurine requirements of felids can be met by consuming whole animals.
Carbohydrates are rarely consumed by wild felids, and the domestic cat has evolved with the ability to derive most of its needs for blood glucose from specific amino acids by gluconeogenesis. The dog, by contrast, is more omnivorous and derives most of its blood glucose from carbohydrate precursors via hepatic glycolysis.
Other differences between strict and facultative carnivores are illustrated by the inability of the cat, as compared with the dog, to meet its needs for vitamin A from provitamin A carotenoids, for niacin from trypto-phan, or for arachidonic acid from linoleic acid. These needs all are met by adequate concentrations of these nutrients in the tissues of whole animals. Whether there are comparable metabolic adaptations related to food choices of obligate piscivores, crustacivores, myrmeci-vores, insectivores, planktonivores, or sanguivores is presently unknown.
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