If you believe the dog food commercials on television and in magazines, dogs may seem very sensitive to taste — but you may be surprised at the reality.

Taste is a very old sense in evolutionary terms. It evolved from direct interactions of the first living things with the giant bowl of chemical soup in which they were immersed. The substances that were suspended or dissolved in water were important to the survival of these primitive living things. Some substances provided food, some gave warning, and some could cause damage or even kill.

As animals evolved, the taste system became more specialized and sophisticated. Sensations of pleasure and disgust provided by taste serve a survival function. A reasonable rule of thumb, at least for natural substances, is that bad tastes are a signal that the animal has encountered something that is harmful, indigestible, or poisonous, while good tastes signal useful, digestible substances.

Because it is important for survival, it is not surprising to find that taste is one of the earliest senses to begin functioning in dogs. Young puppies seem to have only their sense of touch, taste, and smell working at birth but the taste sense still requires a few weeks to completely mature and sharpen.

As in the case of humans, the dog's sense of taste depends upon special receptors called "tastebuds." These are found in small bumps on the top surface of the tongue called "papillae." There are some tastebuds in other places as well, such as the soft part of the roof of the mouth (the "palate") and the back part of the mouth where the throat begins (the "epiglottis" and the "pharynx").

An animal's taste sensitivity depends upon the number and type of tastebuds that it has, much the same way that sensitivity for smell depends upon the number of receptors. Humans win the sensitivity contest for taste, with around 9000 tastebuds as compared with only 1700 for the dog, but dogs have considerably more tastebuds than cats, which average only about 470.

Specific tastebuds appear to be tuned to specific chemical groups and produce recognizable tastes. Traditionally, when talking about human tastes, we have identified four basic taste sensations. These correspond to the tastes that we call sweet, salty, sour, and bitter. Early research did show that the taste receptors of dogs responded to the same kind of chemicals that trigger human taste sensations.

There was one clear difference, however, and that has to do with the taste of salt. Humans, and many other mammals, have a strong taste response to salt. We seek it out and like it on our food. Pretzels, potato chips, and popcorn, for example, are snack foods that are usually liberally dosed with salt. Salt is needed to balance our and there is not much of it to be found in vegetables and grains.

Dogs, however, are primarily carnivores and in the wild, most of their food is meat. Because of the high sodium content in meat, the wild ancestors of dogs already had a sufficient amount of salt in their diet and did not develop our highly tuned salt receptors and strong craving for salt.

Dogs are not exclusively carnivorous but are usually classified as omnivores, meaning that they eat not only meat but plant material as well. Nonetheless, in the wild, more than 80 percent of a canine's diet will be meat. For this reason, in addition to sensors for sweet, salt, sour, and bitter, dogs also have some specific taste receptors that are tuned for meats, fats, and meat-related chemicals. Dogs will tend to seek out, and clearly prefer the taste of things that contain meat or flavours extracted from meat.

The sweet tastebuds in dogs respond to a chemical called furaneol. This chemical is found in many fruits and in tomatoes. Cats are virtually "taste blind" for this substance. It appears that dogs do like this flavour, and it probably evolved because in a natural environment, dogs frequently supplement their diet of small animals with whatever fruits happen to be available.

The tastebuds for the basic flavours are not distributed equally across the tongue. Sweet is best tasted at the front and side portion of the tongue. The sour and salty tastebuds are also on the sides but further back, with the salt responding area being rather small. The rear portion of the tongue is most sensitive to bitter tastes. Sensitivity to meaty tastes is scattered over the top of the tongue, but mostly found in the front two thirds. However, all areas of the tongue can respond to all of the taste stimuli if they are strong enough, it is just that the areas that I mentioned are noticeably more sensitive.

Because of dogs' dislike of bitter tastes, various sprays and gels have been designed to keep dogs from chewing on furniture or other objects. These compounds often contain such bitter substances as alum or various substances derived from hot peppers. Coating items with such bitter-tasting material will eventually keep most dogs from chewing on them, but the key word is eventually. Part of the problem is that the tastebuds that sense bitterness are located on the rearmost third of the tongue. This means that a quick lick or a fast gulp will not register the bitter taste. Only prolonged chewing will let the bitterness work its way back to where it can be tasted.

Dogs also have tastebuds that are tuned for water, which is something they share with cats and other carnivores, but is not found in humans. This taste sense is found at the tip of the dog's tongue, which the part of the tongue that he curls to lap water. This area responds to water at all times but when the dog has eaten salty or sugary foods, the sensitivity to the taste of water increases. The guess is that this ability to taste water evolved as a way for the body to keep internal fluids in balance after the animal has eaten things that will either result in more urine being passed, or will require more water to adequately process. This is useful since dogs are carnivores, and as we already mentioned, there is a high salt content in meat. It certainly appears that when these special water tastebuds are active, dogs seem to get an extra pleasure out of drinking water, and will drink copious amounts of it.

Stanley Coren is the author of many books including: The Left-hander Syndrome.

Copyright SC Psychological Enterprises Ltd. May not be reprinted or reposted without permission.