Although the dog and cat have about equal status as companion animals in our society, it is important to recognize that they belong to two separate species. This truth is evidenced by well-defined physiological, behavioral, and dietary differences. In the following chapters, differences between the cat’s and the dog’s requirements for a number of nutrients are discussed in detail. These differences include the cat’s unique energy and glucose metabolism, higher protein requirement, a requirement for dietary taurine, sensitivity to a deficiency of the amino acid arginine, inability to convert beta-carotene to active vitamin A, and inability to convert the amino acid tryptophan to niacin.

An examination of the phylogeny and evolutionary relationship of the domestic dog and cat offers some clues to their inherent dietary dissimilarities. Although both species are of the class Mammalia and the order Carnivora, the dog belongs to the modern-day Canoidea superfamily, and the cat belongs to the Feloidea superfamily. ¹ Included with the dog in the Canoidea superfamily are several families with very diverse dietary habits. For example, the Ursid (bear) and the Procyonid (raccoon) families are both omnivorous, but species of the Ailurid (panda) family are strictly herbivorous. The only carnivorous species included with dogs are the Mustelids (weasels). The Feloidea superfamily, on the other hand, includes three families: the Viverrids (genet), the Hyaenids (hyena), and the Felids (cat) (Figure 8-1). All of the species in these families, including the cat, have evolved as strict carnivores. Therefore, the evolutionary history of the dog suggests a predilection for a diet that is more omnivorous in nature while the history of the cat indicates that this species has consumed a purely carnivorous diet throughout its evolutionary development.