The frugivores (gorillas, chimpanzees and other primates) have intestinal tracts twelve times the length of the body, clawless hands and alkaline urine and saliva. Their diet is mostly vegetarian, occasionally supplemented with carrion, insects, etc.

Flesh-eating animals lap water with their tongue, whereas vegetarian animals imbibe liquids by a suction process. Humans are classified as primates and are thus frugivores possessing a set of completely herbivorous teeth. Proponents of the theory that humans should be classified as omnivores note that human beings do, in fact, possess a modified form of canine teeth. However, these so-called "canine teeth" are much more prominent in animals that traditionally never eat flesh, such as apes, camels, and the male musk deer.

The shape, length and hardness of these so-called "canine teeth" can hardly be compared to those of true carnivorous animals. A principle factor in determining the hardness of teeth is the phosphate of magnesia content. Human teeth usually contain 1.5 percent phosphate of magnesia, whereas the teeth of carnivores are composed of nearly 5 percent phosphate of magnesia. It is for this reason they are able to break through the bones of their prey, and reach the nutritious marrow.

Zoologist Desmond Morris writes in his 1967 book, The Naked Ape: "It could be argued that, since our primate ancestors had to make do without a major meat component in their diets we should be able to do the same. We were driven to become flesh eaters only by environmental circumstances, and now that we have the environment under control, with elaborately cultivated crops at our disposal, we might be expected to return to our ancient feeding patterns."

Dr. Gordon Latto notes carnivorous and omnivorous animals can only move their jaws up and down, and that omnivores "have a blunt tooth, a sharp tooth, a blunt tooth, a sharp tooth -- showing that they were destined to deal both with flesh foods from the animal kingdom and foods from the vegetable kingdom...

"Carnivorous mammals and omnivorous mammals cannot perspire except at the extremity of the limbs and the tip of the nose; man perspires all over the body. Finally, our instincts; the carnivorous mammal (which first of all has claws and canine teeth) is capable of tearing flesh asunder, whereas man only partakes of flesh foods after they have been camouflaged by cooking and by condiments.

"Man instinctively is not carnivorous," explains Dr. Latto. "...he takes the flesh food after somebody else has killed it, and after it has been cooked and camouflaged with certain condiments. Whereas to pick an apple off a tree or eat some grain or a carrot is a natural thing to do; people enjoy doing it; they don't feel disturbed by it. But to see these animals being slaughtered does affect people; it offends them. Even the toughest of people are affected by the sights in the slaughterhouse."

R.H. Weldon writes in No Animal Food:

"The gorge of a cat, for instance, will rise at the smell of a mouse or a piece of raw flesh, but not at the aroma of fruit. If a man can take delight in pouncing upon a bird, tear its still living body apart with his teeth, sucking the warm blood, one might infer that Nature had provided him with a carnivorous instinct, but the very thought of doing such a thing makes him shudder. On the other hand, a bunch of luscious grapes makes his mouth water, and even in the absence of hunger, he will eat fruit to gratify taste."

Wiiliam S. Collens and Gerald B. Dobkens note:

"Examination of the dental structure of modern man reveals that he possesses all the features of a strictly herbivorous animal. While designed to subsist on vegetarian foods, he has perverted his dietary habits to accept food of the carnivore. It is postulated that man cannot handle carnivorous foods like the carnivore. Herein may lie the basis for the high incidence of arteriosclerotic disease."

Keith Akers in A Vegetarian Sourcebook (1983), responds to the argument that killing animals for food is natural:

"The main problem with this argument is that it does not justify the practice of meat-eating or animal husbandry as we know it today; it justifies hunting. The distinction between hunting and animal husbandry probably seems rather fine to the man in the street, or even to your typical rule-utilitarian moral philosopher. The distinction, however, is obvious to an ecologist. If one defends killing on the grounds that it occurs in nature, then one is defending the practice as it occurs in nature.

"When one species of animal preys on another in nature, it only preys on a very small proportion of the total species population. Obviously, the predator species relies on its prey for its continued survival. Therefore, to wipe the prey species out through overhunting would be fatal. In practice, members of such predator species rely on such strategies as territoriality to restrict overhunting and to insure the continued existence of its food supply.

"Moreover, only the weakest members of the prey species are the predator's victims: the feeble, the sick, the lame, or the young accidentally separated from the fold. The life of the typical zebra is usually placid, even in lion country; this kind of violence is the exception in nature, not the rule.

"As it exists in the wild, hunting is the preying upon isolated members of an animal herd. Animal husbandry is the nearly complete annihilation of an animal herd. In nature, this kind of slaughter does not exist...

"Why are hunters, not butchers, most frequently taken to task by the larger community for their killing of animals? Hunters usually react to such criticism by replying that if hunting is wrong, then meat-hunting must be wrong as well. The hunter is certainly right on one point--the larger community is hypocritical to object to hunting when it consumes the flesh of domesticated animals. If any form of meat-eating is justified, it would be meat from a hunted animal."