In all species, the mouth functions to bring food into the body, initiate physical mastication, and mix the food with saliva. Saliva is secreted in response to the sight and smell of food. It acts as a lubricant to facilitate both chewing and swallowing and also serves to solubilize the dietary components that stimulate the taste buds and impart flavor to food. In addition to its function in digestion, saliva is also important to the dog (and less so in cats) for evaporative cooling. Compared to many ruminant and herbivorous species that thoroughly masticate their food, dogs and cats often swallow large boluses of food with little or no chewing. However, there are also important differences between dogs and cats. Although domesticated dogs and cats have the same number of incisor and canine teeth (six incisors and two canines on both the top and bottom jaws), the dog’s mouth contains more premolars and molars than does the cat’s mouth. These teeth are associated with an increased capacity to chew and crush food, which is indicative of a diet containing a larger proportion of plant material. Thus the dentition of dogs is suggestive of a more omnivorous diet than is the dentition of cats, which is more typical of the pattern seen in most obligate carnivores. ¹ Although both dogs and cats are considered to be “meat-eaters,” the dog has evolved to consume a diet that is more omnivorous in nature than that of the cat.

Another important role of the mouth is its importance in taste perception (gustation). Taste refers to the sensation that arises from stimulation of the taste buds, spherical or ovoid clusters of papillae that are located on the surface of the tongue. Taste receptor cells are located at the tip of each taste bud and are classified into five general receptor types: sweet (sugars), sour (acids), salty, bitter (alkaloids, peptides), and umami (monosodium glutamate, disodium guanylate, “meaty” flavors). Dogs and cats possess taste systems that are consistent with the general pattern seen in other carnivorous species. Early studies classified the cat’s taste system into four general types of units (I, II, IIA, and IIB). ² A similar and closely corresponding set of units has been identified in the domestic dog. ³ In general, both dogs and cats are highly sensitive to the tastes of amino acids and to various types of organic acids and nucleotides. All of these are substances found in abundance in animal tissues. ⁴ Another similarity between dogs and cats is that neither species exhibits a strong preference for salt solutions. ⁵ Although there is evidence that salty flavors may enhance the attractiveness of some foods for dogs and that cats show a slight preference for salt at relatively high concentrations, these species do not demonstrate the “salt appetite” that is reported in other omnivores and herbivorous species.

Dogs and cats also have several interesting differences in taste receptors. One of the most dramatic is in their sensitivity to the taste of sweet. Dogs, but not cats, are sensitive to and show preferences for sweet foods. 6. ^{and 7.} (This is one of the reasons that theobromine toxicity, as a result of chocolate ingestion, is a significant risk to pet dogs but not to pet cats.) Recent studies have shown that one of two receptor genes known to encode for the sweet-taste receptors in taste buds is not expressed in cats. ⁸ This defect may be responsible for the lack of response to sweet flavors observed in the domestic cat and in several other felid species. In lieu of these receptors, cats possess a receptor that is highly sensitive to quinine, tannic acid, and alkaloids—flavors that are thought to be perceived as bitter. ⁹ In contrast, dogs are typically repelled by most bitter flavors. There is speculation that these differences between dog and cat taste systems reflect the cat’s evolutionary adherence to a more strictly carnivorous feeding pattern, leading to taste preferences for foods derived from only animal tissue, and a loss of the ability to perceive flavors that are more typically found in fruits and vegetables. In contrast, the dog evolved from a canid species that, although also a predator, was more omnivorous in nature. Further, the process of domestication selected for a more varied omnivorous diet in dogs, as scavenging gradually replaced cooperative hunting as a primary feeding behavior. ¹⁰ The impact that taste and other special sense perceptions such as odors and food textures have upon food selection and preferences in dogs and cats are discussed in detail in Chapter 19 (see pp. 191-193).

The stomach acts as a reservoir for the body, allowing food to be ingested as a meal rather than continuously throughout the day. The proximal section of the stomach is capable of expansion to allow storage of large meals, a function that is assumed to be of greater importance for dogs, who tend to eat large meals at a given time, than for cats, who prefer to eat multiple small meals per day (see Chapter 19, pp. 192-193). In addition to its storage function, the stomach also initiates the chemical digestion of protein (and possibly of fat, in the dog), mixes food with gastric secretions, and regulates the entry of food into the small intestine. The gastric glands, which are located in the mucosal lining of the corpus portion of the stomach, secrete mucus, hydrochloric acid (HCl), and the proteolytic enzyme pepsinogen. In dogs, gastric lipase is secreted throughout the stomach, but appears to be much less important for fat digestion than pancreatic lipase. ¹¹ For this reason, it is believed that the majority of fat digestion still takes place in the small intestine. Mucous secretions protect the gastric mucosa and also lubricate the ingested food. HCl is necessary to maintain a proper pH for the occurrence of enzymatic action. It functions to slightly alter the composition of ingested fat and protein in preparation for further action by digestive enzymes in the small intestine. Along with previously formed pepsin, HCl also converts pepsinogen to the enzyme pepsin. This enzyme initiates hydrolysis of protein molecules to smaller polypeptide units in the stomach. The activity of pepsin is highest in the acidic environment of the stomach and is reduced and eventually inactivated when food leaves the stomach and is exposed to the neutral pH of the small intestine. ¹²

Both neurological and hormonal stimuli are important for the secretion of HCl and mucus by the stomach. Neurological stimuli are produced in response to the anticipation of eating, the sight and smell of food, and the presence of food in the stomach. In addition, psychological stimuli such as fear, stress, and anxiety can affect gastric secretions and gastrointestinal functioning in animals. The hormone gastrin is released in response to the presence of food and distention of the stomach. It is produced by mucosal glands in the antrum portion of the stomach. Gastrin stimulates the secretion of HCl and mucus and also increases gastric motility. Another local hormone, enterogastrone, is produced by glands located in the duodenal mucosa. Enterogastrone is secreted in response to the presence of fat entering the duodenum and counteracts the activity of gastrin by inhibiting acid production and gastric motility.

Peristaltic movements of the stomach slowly mix the ingested food with gastric secretions, preparing it for entry into the small intestine. The mucosal cells located in the antral portion of the stomach secrete mucus that has a more alkaline pH and is low in digestive enzymes. Thorough mixing in this portion results in the production of a semifluid mass of food called chyme. Chyme must pass through the pyloric sphincter to enter the small intestine for further digestion. Like the cardiac sphincter, the pyloric sphincter is a ring of muscle that is usually in a constricted state. This ring relaxes in response to the strong peristaltic contractions that originate in the stomach and travel toward the intestine. While open, the sphincter allows small amounts of chyme to enter the duodenum. The pyloric sphincter controls the rate of passage of food from the stomach into the small intestine. The rate of gastric emptying is affected by a number of factors, including the osmotic pressure, particle size, and viscosity of the chyme, as well as the degree of gastric acidity and volume. In general, large meals have a slower rate of emptying than small meals, liquids leave the stomach faster than solids, and very high-fat meals may cause a decrease in stomach-emptying rate. Diets that contain soluble fiber as a fiber source cause a decreased rate of stomach emptying when compared with diets that contain insoluble dietary fiber (see Chapter 2, p. 14, Table 2-1). In addition, there is evidence in cats that dry cat foods leave the stomach at a slower rate than canned foods, except when very small meals are consumed. ¹³ The shape of the kibble in dry cat food can also influence the rate of gastric emptying, with triangular-shaped pieces leaving the stomach at a slower rate than round pieces. ¹⁴