Living animal tissues need a variety of chemicals to stay in good condition. Among them are sodium and chlorine. Both play vital roles in the delicate balance of body fluids and both are needed to carry electrical nerve signals. We get these two chemicals from the salt we use to season our food. Most wild animals get their salt by licking certain stones. The farmer serves salt to his domestic animals in the form of salt licks, or salt bearing rocks.
Most books about animals forget to mention the importance of salt. Maybe the observers don't notice this, but the deer and other wild animals must know where to find salt or they sicken and die. Usually they return again and again to lick a slab of halite or rock salt. They take only a taste, just as we take only a small pinch of salt with our food. But the entire ecology of a region may depend upon. whether or not the wild animals can find a little salt.
In most regions there are enough natural salt licks lying around to meet the need, though sometimes the deer walk miles to find them. But domestic cattle are limited to their home pasture. If there were salt licks around, in time the cattle would use them all up. A good farmer knows this and takes no chances. He places a few chunks of rock salt or some other salt bearing rock in the pasture. Usually he half buries them so that the cattle can lick them more comfortably. They take as much as is good for them, no more and no less.
Animals have a deep hunger for salt because it regulates a number of very vital body processes. Ordinary salt, alias sodium chloride, is a chemical compound of sodium and chlorine. These elements are dissolved and separated inside the body and each is used to perform its special duties.
Chlorine is a very active halogen chemical found in all plants except the conifers. In animals, its atoms are in the form of charged ions. These ions must be present in the delicate chemical mixture present in the body fluids. They also help to build the important mucus materials that line the internal organs.
Sodium plays an even more vital role in the body fluids. If the amount is not just right, it will shrivel the tissues, or swamp the body fluids through the cell walls where they do not belong. Both sodium and chlorine are needed in precise proportions to maintain the electrical balance of the body's biochemistry.
Sometimes the deer and other vegetarians can get salt from plants that grow near old salt marshes. Often a porcupine chews the handle of an ax or some other man made tool. Observers suspect that he is eager to get the salt left there by a man's perspiring hand.