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Chapter 1 Most pet lovers are familiar with the idea that dogs and cats are carnivores. That is, both animals can and do derive valuable nutrition from the voluntary consumption of meat. In this regard, many mammals, including people, pigs, bears, raccoons, and myriad others have seemingly similar carnivorous tendencies. When meat is available, such animals will take advantage of the situation and eat it. There is a significant difference between cats and all of these other mammals, however. Dogs, people, pigs, bears, and raccoons, etc., are all omnivores that eat meat when it is available. Cats, big and small, are obligatory carnivores. The omnivore does not eat meat as a mandatory requirement for life; vegetable food sources can make up a very large part of their diet, and may even be properly balanced to provide all needed nutrients for health. For the cat, however, meat, and the nutrients found only in meat, are essential for survival.  The Cat Is Not a Small Dog Critical differences between dogs and cats, the most popular of all household pet animals, are clearly illustrated in the genetic, anatomic, and metabolic differences between the two. Scientists who have studied the dietary habits of carnivores, omnivores, and herbivores tell us that these “rungs” on the food-chain were established and reinforced during the evolutionary histories of each type of animal (see www.catinfo.org/zorans_article.pdf). The work of these experts suggests that the members of the superfamily Feloidea, including today’s cat, evolved rapidly in distant prehistoric times, but then stopped abruptly in that progression. Carnivorous animals belonging to other families of animals, including the Canoidea, to which the dog belongs, seem to have progressed beyond this point to meet changing evolutionary needs.
Good evidence for the cat’s relatively ancient nature can be found in the lower number of chromosomes in its genetic makeup, compared with a much larger number for the group that includes modern dogs. The cat’s cells carry thirty-eight chromosomes, while the dog’s cells carry seventy-eight. This does not mean that the cat lacks physical and genetic sophistication equal to the dog. It means that it made a perfect and permanent fit within its spot in the environment early on and experienced little additional pressure to change its genes.
Dogs and cats also have remarkably different, but highly specialized, anatomy. Dogs have forty-two permanent teeth, whereas cats have only thirty. Dogs have more molars than do cats, with a specialized shape for crushing, associated with their intake of plant material. In contrast, the shape of feline teeth is specialized for grasping and tearing flesh. By its structure, the cat’s jaw has far more restricted side-to-side and front-to-back mobility than does the dog’s, limiting its ability to grind a varied vegetation-containing diet as the dog can do. The cat’s eyes and ears are positioned forward on the head to provide exquisite acuity of vision and hearing when tracking prey, particularly at night. Retractable claws, seen on cats but not dogs, are another specialized feature of an animal that must chase, catch, and bring down all of its food in the form of wild prey.
The gastrointestinal tracts of the two species are also quite different. Those differences emphasize the differences in the natural diets of each. Science tells us that modifications in the basic structure of this important organ system from species to species are closely connected to diet. The cat’s stomach, caecum (appendix), and colon, segments of the gastrointestinal tract most associated with digestion of vegetable matter, are smaller than those segments in the dog. The length of the feline intestine in proportion to its body length is short compared with that of the dog, indicating that the cat’s evolutionary diet was highly digestible (protein and fat), whereas the dog consumes far more vegetable matter. The inner lining of the cat’s stomach has significantly greater surface area than does the same part of the dog’s stomach. Anatomists believe that increases in the relative size of this stomach area are an adaptation to the digestion of higher-meat, more calorie-dense diets. The caecum in the cat is very primitive, whereas it is much better developed in the dog. Once again, this portion of the gastrointestinal tract assists in the processing of fibrous, nonmeat dietary constituents.
Equally telling of the cat’s strictly carnivorous origins are its nutrient requirements, especially its requirements for protein. Research done on the 1970s and ’80s showed conclusively that protein requirements in kittens and cats far exceed those of puppies or dogs. The cat, unlike omnivores such as the dog, “burns” protein to make energy for its everyday use, under all circumstances. Most other animals burn large amounts of protein for energy only when protein is plentiful in the diet.
In contrast, the cat has an ongoing high requirement for protein to turn into energy, even when dietary protein intake is very limited. During starvation or excessive protein-restriction, the cat is forced to disassemble its body’s own constituent proteins (enzymes, antibodies, organ tissues, and so on) to produce fuel for energy to keep the cells alive and functioning. Thus, in the most fundamental way, the health and tissue integrity of the cat is dependent upon the continual intake of highly digestible protein, especially protein from meat.
Another of the cat’s claims to the top-predator spot in the food chain is the absolute requirement for an essential fatty acid, arachidonic acid, found only in meat. Also, cats must consume preformed vitamin A from animal-source foods because they are unable to make this essential vitamin from the beta carotene found in plants. The list of the specializations of the cat’s internal machinery that reflect its evolutionary adaptations to a life as an obligatory carnivore goes on and on.  Not All Livers Are Alike By far, the most fascinating characteristic of the cat compared to omnivores like the dog is the manner in which its liver functions. The cat’s very high protein and amino acid requirements arise from the constantly high activity of certain enzymes in the feline liver. These enzymes disassemble the amino acids in protein to make them available for production of energy in a process called gluconeogenesis. Essentially, the liver is the organ that is responsible for the high and constant burn rate of protein in the cat’s body. Omnivores such as the dog have a liver that is also capable of this function, but omnivores turn the rate of this function up or down depending on how much dietary protein is available. In contrast, the cat’s liver protein “burn rate” is set high at all times, even when dietary protein is scarce or entirely absent. Death from protein starvation can be very rapid in this species.
In the liver, protein amino acids are processed into glucose (sugar) and sent into the bloodstream to supply the body’s need for this energy nutrient. In a meat-eating species like the cat, accustomed to little dietary carbohydrate in its evolutionary environment, the liver will manufacture the great majority of the animal’s needed glucose, which is the primary energy supply for the animal’s brain. Because there is little glucose in a high-meat diet, this is an essential task for an obligatory carnivore. The liver of omnivores, including people and dogs, have multiple enzyme systems for handling dietary carbohydrate; the cat has only one such enzyme system, with limited capacity to deal with high carbohydrate consumption.
Such specializations make the cat fit its niche perfectly; indeed, the fittest animal in a niche will be the one with the fewest and simplest systems to meet its survival needs. The cat’s ancestors did not need the ability to turn their liver’s protein burn rate up and down. Similarly, they did not require significant carbohydrate-handling capabilities. The specialized glucose-from-protein systems that have been genetically retained by the modern cat are always active at a high rate, obligating felines to eat more protein than their omnivorous counterparts. Because of this, unfortunately, the cat will suffer far more harm than will omnivores in situations where protein is insufficient or absent. We will see how important this requirement is when we discuss many of the common diseases of our pet cats.  Out of Africa The present-day house cat (Felis domesticus) is generally thought to have descended thousands of years ago from a small wild cat (Felis lybica) native to the deserts of North Africa. Such a dry climate heritage would explain many distinct characteristics of this species. Cats are capable of surviving for long periods without water, and will naturally consume very little free water when they are feeding on canned cat food or fresh meat. Cats can produce urine that is highly concentrated compared to that of the dog and other animals that evolved in more water-rich environments. The cat’s natural tendency to produce urine with a great deal of metabolic waste in a highly concentrated form can be dangerous if a cat feeds on a diet that is low in water, because this desert animal has a naturally low thirst drive. The cat that is consuming dry cat food seldom drinks enough additional free water to balance the dry state of the food. This results in especially concentrated urine with attendant medical problems, including certain kinds of bladder disease. Dry food also contains ingredients that interfere with the natural acidity of the cat’s urine. Highly concentrated, alkaline urine from dry food consumption is associated with serious, even fatal urinary tract problems.  The Predator Lifestyle The cat’s ancient predator–behaviors are very much a part of its present-day life. Some wild cat spec...