The condition referred to by physiologists as "salt hunger" is not ordinarily experienced by man, except as a preference for some degree of salt flavor in the diet. But this cannot be the factor that makes some herbivorous animals travel great distances to "salt licks," because the salt is not actually mixed with their food. It is well known that depletion of the sodium chloride content of the blood and the tissues below a certain limit leads to serious disturbances in organ activity, but we do not know their specific effect on consciousness. Possibly it is a feeling of general discomfort and weakness, rather than any specific quality of sensation referred to any one part of the body, although marked reduction of the NaCl in the blood leads to absence of the hydrochloric acid in the gastric juice and, in consequence, digestive disorders.
The experiments of Forster on dogs and pigeons, and those of Lunin on mice showed that animals succumb sooner on a salt-free diet than if they are given no food at all. The symptoms developed by these animals are: weakness and cachexia, nervous hyper-excitability, tremors, and gastero-intestinal disorders (indigestion, vomiting, refusal of food). It is still an open question whether any animal, man included, actually needs sodium chloride in addition to the salts present in all natural foodstuffs. Smith, the veterinary physiologist, maintains that "both carnivora and herbivora obtain in their natural diet a sufficiency of salts, although there is a general impression that the wild herbivora long for sodium. It is quite certain that under conditions of domestication horses can be kept in perfect health without receiving any sodium chloride other than that contained in the food, and the amount of this in vegetable substances is very small." Nevertheless, it is a fact that herbivorous animals, wild as well as domesticated, ingest sodium chloride when available, while this is not done by the out-and-out carnivorous species. History seems to show that it is the agricultural races or tribes that have developed the need or at least the greatest desire for salt. According to Bunge, among the races living exclusively or mainly on animal food, salt is either unknown, unused, or actually disliked, while some of the agricultural tribes in Central Africa cherish salt as ardently as our children desire sweets. It is even believed by some anthropologists that this need or desire for salt or animal food is a factor in cannibalism.
Mungo Park, the traveler, describes his own feelings on long-continued salt want as follows: "I found the scarcity of salt actually painful. The continued subsistence on nothing but vegetable food produced finally a desire for salt so painful that it can hardly be described." We are not familiar with any prolonged experiments with salt-free diets on man. But persons on ordinary diets who have gone for a long time without sodium chloride complain in general of lack of appetite (owing to the unpalatability of the food), and of some vague or general bodily distress-a condition similar to that induced by monotonous and incomplete diets.
But even if we assume that the herbivora do experience a "salt hunger" in the sense of general bodily discomfort and weakness, or special distress referred to the digestive tract, how do the animals know that eating salt will relieve these sensations, and how do they know where to find the salt ? In all probability it is a matter of individual experiences and memory. We may assume that a certain amount of salt flavor is pleasant to these animals as a matter of inherited reflexes or because of its slight stimulating action. Hence they will eat or lick the salt wherever found, irrespective of actual need. Now, if actual salt need is expressed in a feeling of general bodily distress, and salt being available, the animal will sooner or later encounter the experience that eating salt eases the distress. As long as the calf feeds on the milk of its mother, no additional salts are required in its diet. But, of course, the calf follows the mother to the "salt licks," and through imitation or curiosity learns the taste of salt as well as the location of the "licks," both before and after he actually experiences salt hunger. Thus by aid of the parents or the herd, the individual experience is established.