Dog Taste – primitive in origin

Taste – primitive in origin
The gustatory, or taste sense has been less studied than the olfactory acuity of dogs and is present in the neonatal puppy and suspected to be present prior to birth. According to Thorne, nearly all the knowledge is “…based on neurophysiological studies” derived from studying the dog’s facial nerve and lacks support from any behavioral studies (Thorne, 1995, Ch 7).
Physiology of the tongue
The dog’s tongue is part of the digestive system and used to test the palatability of food and to stimulate the salivary, gastric and pancreatic secretions necessary for digestion. The dog’s tongue, as in humans consists of dependent groups of cells called taste buds that are responsible for sampling concentrations of small molecules and relaying the information to the brain.
According to Thorne, the tongue is connected to several nerves responsible for relaying information back to the brain and is described as follows.
The facial nerve is only one of the neural paths involved in taste perception. The taste buds of the anterior two-thirds
of the tongue are innervated by the chorda tympani branch of the facial nerve those of the posterior third of the tongue
are innervated by the lingual branch of the glossopharyngeal nerve, and those of the pharynx and larynx are innervated
by the cranial laryngeal branch of the vagus nerve. The chorda tympani nerve in the dog is associated with the taste buds
from the fungiform papillae located on the anterior two-thirds of the tongue (Olmsted, 1922). The lingual branch of the glossopharyngeal nerve is associated with taste buds from the vallate papillae, but no studies describe the role of the cranial laryngeal nerve. In addition, there are the free nerve endings innervated by the trigeminal nerve.
The results of the neurophysiological studies of the facial nerve have identified at least four neural groups of taste buds, according to Thorne. Each of these neural groups has a “…resting firing rate” and can be “…either increased (excited) or decreased (inhibited) by chemical stimulation.” According to Thorne, each neural group has the capacity to perceive both types of stimulation essentially providing “…eight potential taste categories of which six have been identified.” According to Thorne, the Boudreau studies are responsible for this identification (Thorne, Ch 7, 1995).
The first group containing the most receptors is identified as “Group A” and responds to sugars, including artificial sweeteners, fructose, sucrose and most sweet tasting amino acids. This same group inhibited by some amino acids such as quinine and L-tryptophan considered bitter tasting, similar to humans’ experience of sweet and bitter taste. As a diet adaptation for the canine, sweet tastes indicate acceptable food groups and sources of energy.
According to Lindsay, taste studies in dogs are similar to the same pattern of taste receptors in humans, indicating that “…salty, sugary, and sour tastes are localized toward the front two-thirds of the tongue, while gustatory responses to bitter tastes are located toward the rear third of the tongue.” However, even though specific tastes are stronger in these specific areas, these same tastes are detected over the entire surface of the tongue (Lindsay, 2000).
The second most receptors are “Group B” known as the “acid units”. This group has a low receptivity rate and responds to chemicals such as distilled water, inorganic acids and some amino acids including sulphur compounds such as L-taurine and L-cysteine and inosine monophosphate a severe inhibitor.
The third “Group C” is specific to carnivores and consists of nucleotides, characteristic of meats and the fourth “Group D” receptors could be characterized as the “furanol receptors” and can be described as “fruity-sweet” (Thorne, Ch. 7, 1995).
It seems the dogs’ ability to taste salt is debatable according to Lindsay who says, “…several studies have demonstrated that dogs have a clear gustatory response to salt” and in opposition to this he says “…Boudreau (1989) found that dogs totally lack salt-specific taste receptors.” Lindsay says, Boudreau “…noted that the ability to taste salt is common among mammals, especially herbivores, who need to find it in order to supplement a salt-deficient vegetarian diet” and since the carnivore diet lacks this need it is unnecessary to possess these taste receptors (Lindsay, 2000).
According to Lindsay, “Boudreau has speculated about the evolutionary function of the dog’s ability to taste furaneol in terms of its omnivorous eating habits.” He quotes Boudreau saying “…the presence of this taste system and its absence is readily detectable in the natural eating behavior of canines and felids” stating “in a natural environment canines will supplement their small animal diet with fruit of the season, unlike felids” (Lindsay, 2000).
The animal’s ability to differentiate tastes provides a biological advantage in determining needed nutrients and avoiding poisons. Dogs seem to have strong avoidance to bitter tastes associated with decay and nutritive value, providing “survival value” since bitter tastes are associated with poisonous items.
Additionally, taste receptors are replaced approximately every ten days, similar to olfactory receptor cells. Habituation and adaptation affects taste similar to the sensitivity of smells and may be responsible for a dog’s preference of a novel diet.
Taste preferences
According to Lindsay, “…flavor and taste preference depends on a composite of olfactory and gustatory factors, as well as past experience and learning.” Studies conducted by “Garcia and colleagues (1966) found that intense and lasting taste aversions can be readily established toward a novel food item if its ingestion is followed by the induction of nausea.” This may be effective even after delays of more than one hour.
Preference to food may be complicated due to genetic preparedness for recognition of acceptable food, previous experience, taste and novelty. Studies indicate raising groups of puppies on different diets had profound influence on later taste preferences, concluding those dogs lacking exposure to certain food groups developed clear preferences only to those early familiar foods. However, according to studies conducted by Mugford, he found “two primary factors” that influenced food preference. The first was palatability, associated with moisture and previous exposure, indicating a preference for novelty. He also determined that novelty without palatability produced short-term preference and novelty and palatability produced a long-term preference. In addition, Mugford found puppies reared on a restricted diet preferred novelty over familiar food. Results from studies by both Kuo, who removed his puppies at birth and Mugford, who removed his puppies after weaning, determined that young dogs tend to develop lasting food preferences to familiar items prior to weaning and after weaning; they tend to be more flexible. A possible link influencing taste preferences may be “fetal taste experiences” suggesting the fetus may consume amniotic fluids that may play a role in the development of taste preferences; this possibility has largely been overlooked. Other studies using rats suggest both prenatal and postnatal influences combine to affect taste preferences and those taste preferences are established by “taste cues” provided by the mother’s milk prior to weaning (Lindsay, 2000).
Still other studies concluded dogs prefer cooked meat rather than raw and prefer their meat in descending order as beef, pork, lamb, chicken, horsemeat and finally sweetened foods. These same studies determined dogs with an impaired sense of smell still prefer sweetened foods and meat rather than dry food without having the preference of individual meats of dogs unimpaired.
In spite of dogs preference to novelty they can be maintained on bland diets. However, owners attempting to please their dogs mistakenly succumb to finicky dogs providing diets detrimental to their physiological needs possibly leading to behavior problems. According to Lindsay, dogs provided with daily novelty become increasingly more difficult and harder to please. Lindsay says a recent survey suggests obese owner’s have profound effect on obesity in their dogs, often interpreting their needs as requests for food. Dogs tend to succumb to eating what is presented after a few days of not eating and dogs that have lost interest in certain food items can be encouraged by placing some food in their mouth stimulating their taste receptors.
Taste aversion
Taste aversion, described by Lindsay as an “example of associative learning that does not fit neatly into the classical conditioning paradigm.” Taste aversion usually results when an animal eats something followed by a “nausea-producing illness” shortly thereafter. According to studies performed on rats, a lasting taste aversion may even occur if nausea is induced several hours later and through one time learning. The inconsistency is the lack of “repeated contiguous pairings of the CS and US” normally associated with classical conditioning. According to Lindsay, this may be explained by “special learning sensitivities connected with taste and nausea” helping animals to differentiate between safe food items. This makes sense that animals learn quickly to avoid harmful food items. An animal puts itself at risk during illness; therefore, it makes sense to avoid this kind of danger (Lindsay, 2000).
The use of taste aversion has been helpful in controlling predation by coyotes and coprophagia in dogs. However, the treatment of feces with specific compounds creating taste aversion is not consistent in treating dogs according to a study by Hart and Hart (1985), says Lindsay.
The procedure requires direct treatment of feces or owner induced vomiting using chemical compounds, known to induce vomiting. It is suggested an appropriate emetic is necessary and commonly used chemicals such as “…ipecac are inappropriate for establishing such learning.” The most common compound used is “lithium chloride”, suggested by Lindsay. However, this treatment requires supervision and monitoring by a knowledgeable veterinarian in these procedures to avoid any undesirable side effects or risks to the animal (Lindsay, 2000).