The sense of taste is confined to the tongue, the palate, and the epiglottis, and is therefore sensitive only to substances brought into the mouth. Our sensation of flavor is a combination of the taste and the odor of a food, perceived by the nose, and it is reasonable to suppose that cats also perceive something akin to flavor. The majority of the taste buds in cats are circular structures about 1 1/4 inches in diameter and grouped on fungiform papillae on the upper surface of the tongue, and also in four to six large cup-shaped (vallate) papillae at the back of the tongue. The shape of the taste buds is different from those of other animals, although the functional significance of this, if any, is unknown.

Almost all of our knowledge of the sense of taste in cats stems from neurophysiological work at vet college, which, unlike for the senses of hearing, vision and olfaction, is poorly supported by behavioral studies of thresholds and discriminations. There are four cranial nerves, the facial, glossopharyngeal, vagus and trigeminal nerves, which probably carry information about taste sensations from the tongue and the palate. The facial nerve innervates the taste buds on the front two-thirds of the tongue only, and is therefore unlikely to convey information representative of the whole tongue. However, detailed studies of the spectrum of compounds to which these taste buds respond, in both dog and cat, have indicated that the specificities of the facial taste system can be rationalized in terms of the evolutionary origins.

In cats, the most abundant taste buds are those that respond to many amino acids, although a few with hydrophobic side-chains L-tryptophan, L-isoleucine, L-arginine, and L-phenylalanine, inhibit the spontaneous discharges of the neurons; of these, only L-tryptophan is inhibitory for the dog. These compounds taste bitter to man, and cats may experience something similar. Also inhibitory to these neurons are monophosphate nucleotides, which may be partly responsible for the cat's dislike of carrion (dead tissue.)

The greatest difference between dogs and cats is the complete lack of response to any sugar in the cat, and this is supported by behavioral evidence. The second most abundant group of taste receptors are the acid units, which are stimulated by similar compounds in both species, including phosphoric acids, carboxylic acids, nucleotide triphosphates, histidine, histidine dipeptides, and protonated imidazoles. A few amino-acids, including the sulphur compounds L-taurine and L-cysteine, also trigger substantial positive responses, particularly in the dog.

The other units are less well characterized; they all discharge in response to nucleotide di- and triphosphates, in both dog and cat, but sub-groups can be identified, based on sensitivity to other groups of compounds. One such group is stimulated by a diversity of substances, including quinine, alkaloids, and tannic, malic, and phytic acids.

Units which respond to nucleotides are characteristically found in carnivorous animals; analogous receptors are known from puffer fish, and some blood-sucking arthropods. The acid units exhibit a different chemical profile of response to those of omnivores, and the loss of the sweet response to sugars in the cat may be an extreme adaptation to meat-eating.
Presumably, both the amino-acid and nucleotide units are used to distinguish between meats of different nutritional quality. The more omnivorous dog has retained its ability to detect sugars and other sweet-tasting compounds that may indicate plant materials with a high digestible energy content, such as fruits.

The greatest contrast between the cat and dog on the one hand, and the majority of mammals on the other, is their apparent lack of salt-specific taste buds, which account for over half the neurons in the facial nerve of the rat and the goat. Since sodium is essential for renal and nervous function, detection of the salt content of food is evidently a high priority for most herbivores and omnivores, but the food of carnivores is inevitably salt balanced as a direct result of that priority. However, it is quite possible that salt-responsive units remain undetected in the glossopharyngeal nerve, which has not yet been adequately characterized in either the dog or the cat.

*Cats are unable to detect sugar, but are sensitive to sweet-tasting amino acids.
*Cats lack salt-specific taste buds.