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Comparison Between the Digestive Tracts of a Carnivore, a Herbivore and Man
Part 2: The gut of a carnivore — the Dog
The dog is a pure carnivore. As all carnivores' digestive systems are the simplest, being essentially a long piece of pipe with a single bulge near the beginning, we will consider the dog first.
The first thing to note about the digestive system of all carnivores is that they are remarkably similar and they all function in exactly the same way. Although they will be of different lengths, because carnivorous animals come in different sizes, the overall length of carnivores' digestive tracts are rather short: about six times the length of the animal's body. Let us traverse the digestive tract from one end to the other to discover what each part does.
Mouth . The dog's jaw contains incisors, canines and molar teeth in both jaws, and the molars are ridged. The jaw moves up and down. This fact, together with the ridging of the molars indicates that they are used for tearing or crushing. The salivary glands serve merely to lubricate, and do not have an important digestive function. Food is rarely chewed into small portions, but 'wolfed' down whole.
Stomach . The dog's stomach, the only bulge in the digestive 'pipe', is small, holding about four pints. Its small size gives a good estimation of the amount of food the animal can consume at any one time. The stomach serves two purposes. Firstly it is a reservoir. Although relatively small, this is all that is needed, as the food of a carnivore, wholly of meat and fat, is nutrient dense, allowing one small meal to suffice for many hours. The second function of the stomach is to subject the food to concentrated solution of hydrochloric acid, which dissolves and liquefies it. Only food that is dissolved can be digested. Different foods dissolve at different rates and leave the stomach at different rates. The ones that cannot be digested - raw vegetable matter, cellulose and bone - pass right through the animal unchanged, those that are too big to pass into the small intestine are vomited. The dog's stomach, if filled with its normal food of meat and fat will empty in about three hours. The stomach then rests until the next meal is eaten. So far very little digestion has taken place and, in the carnivore, the stomach is not an essential organ.
The small intestine . The small intestine, approximately twenty feet in length in a dog, is vitally important. Without it, no digestion could take place and the animal could not survive. The dissolved food, called 'chyme' at this stage, leaves the stomach in a series of spurts, controlled by a valve, the pylorus, and enters the small intestine. It is in the small intestine that food is digested and enters the bloodstream. After a few inches, two ducts connect from the pancreas and the liver to the small intestine. These two organs supply and deliver the enzymes needed to break down the fats and proteins into their component fatty acids and amino acids. Only in this form can they pass through the gut wall into the bloodstream. These enzymes are vitally important to the carnivore. Those from the pancreas immediately start to break down the chyme into its basic components and continue to do this throughout the chyme's passage along the small intestine.
The chyme is a watery mixture but fat will not mix with water so it requires some special handling. This is where bile comes in. Bile is manufactured in the liver and stored in the gall bladder until such time as it is needed. When fat is detected in the small intestine, this triggers the release of the stored bile, which enters the intestine through the bile duct. Bile acts just like a detergent in that it emulsifies the fat to make it soluble in water. This action makes fat susceptible to digestion by the digestive enzymes.
In the carnivore there are large amounts of fat in diet on occasion and, as bile is so important, its waste is not allowed. The liver makes bile continuously, the excess being diverted to the gall bladder to be saved and concentrated until it is needed (for the next meal). When a hormone in the upper gut signals that fat is again present in the gut, the stored bile is forcibly ejected to perform its function.
Digestion of food in a carnivore is performed by enzymes produced by glands in the animal's own body and all the absorption of nutrients in that food is through the wall of the small intestine. This is an important consideration when we compare it later to the digestion of a herbivore.
The digestion of protein and fat, with little or no carbohydrate, in the carnivore's gut is remarkably efficient. Experiments which have measured the amounts of various nutrients eaten and compared these with the amounts passed in the animal's excreta have shown that a healthy animal loses no more than four percent of its fat intake and only a trace of the protein.
As there is no enzyme in the carnivore capable of digesting cellulose, the material that the cell walls of all plants are composed, little or no digestion of carbohydrates can take place.
The caecum . The small intestine doesn't join the large intestine in a straight line, but at a right angle. At this point is a small appendage, two or three inches in length, called the caecum. While this has no functional use in a carnivore, it should be noted because it is one of the major differences between a carnivore and a herbivore.
By the time the chyme has passed through the animal's small intestine, the process of digestion and absorption of the nutrients in the food is complete. The large intestine, or colon , has just one function to perform. It would be wasteful to allow water to escape and so the colon extracts the water and compacts the rest of the waste material from what is left of the chyme into a small compact mass, where it is stored in the rectum until it is finally expelled through the anus. The colon in a carnivore is not essential, merely a convenience.
The gut flora. Practically the whole of the gastrointestinal tract of a carnivore is sterile. The hydrochloric acid in the stomach ensures that most bacteria and other micro-organisms in swallowed food are killed. Those that escape the stomach are rarely able to survive the digestive processes - they are, after all, made of protein. The colon is the exception. This, where no further digestive processes occur, does tend to harbour a variety of organisms which form certain vitamins such as pyridoxine, vitamin B-12, biotin, vitamin K and folic acid but, as these are not absorbed through the wall of the colon, they are of little account. These micro-organisms thrive in an alkaline environment and are of the putrefactive type.
The length of the gastrointestinal tract of a carnivore. The gut of any animal is usually measured after death when its muscles are relaxed. This gives a quite wrong impression. While that of the animal we have been discussing measures over thirty feet when the dead dog is dissected, this is not its normal length when the animal is alive. It has been found by passing a rubber tube through a living dog, which has a similar gut length when dissected, that the front end appears at its anus when little more that ten feet has entered the mouth. From measurements such as these it is generally reckoned that the total length of a carnivore's gut is probably about five to six times the length of the animal's body.
Reference
1. Walter Voegtlin, The Stone Age Diet, Vantage Press, Inc, New York, NY, 1976
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