Dietary Adaptations

Carnivorous Vertebrates vs. Herbivorous Vertebrates

In various parts of Unit 6, differences between carnivores and herbivores are discussed. For example, you are asked to contrast the intestines of tadpoles and frogs, and also to note some of the differences found among the skulls of various mammals. In general, since meat is easy to digest, carnivores have short intestines; in contrast, plant material is much more difficult to digest and so herbivores tend to have long intestines which prolong the digestive process. Furthermore, capturing and swallowing animals requires teeth that are different from those which are most useful in nipping off and chewing vegetation.

Differences in intestine length and in teeth are not surprising, at least in hindsight. Less obvious are other morphological specializations which can distinguish carnivores and herbivores. One is the gall bladder. Large amounts of bile are only required if large quantities of meat are to be eaten at one time. (What is the bile used for?) Animals which rarely or never consume meat do not actually need gall bladders; the liver can directly supply enough bile as rapidly as it is needed. Thus, the Norway rat and the pigeon, both of which are primarily herbivorous, do not have gall bladders. On the other hand, a blind pouch of some sort in the digestive tract is often useful in the digestion of plant materials. Such a pouch can house symbiotic procaryotes and protists which participate in the digestive process. Furthermore, the pouch can serve as an area where food is retained for prolonged digestion. Thus, herbivores, but not carnivores, tend to have well-developed caeca. Most caeca, as in the rat or a human, are at the junction of the large and small intestines. Sometimes, as in most fish, the caecum is located at the junction of the stomach and small intestine.

In the chart below, various characteristics are listed for three organisms: cats, humans, and rats. On the basis of this information, can you guess what kind of diet humans are biologically adapted for? Be sure to note these characteristics of the rat when you perform your dissection of the white rat.

CELLULOSE DIGESTION IN RUMINANTS

Cellulose, the most abundant organic compound on earth, is also one of the most difficult for animals to digest. Because of its abundance and the store of energy-rich glucose molecules it contains, many animals have evolved special adaptations for digesting it.

Like starch and glycogen, cellulose is a polymer of glucose molecules. It contains linked b-glucose molecules, while starch and glycogen are composed of a-glucose groups (a).

Two forms of glucose. Notice the position in each molecule of the hydroxyl group (OH) attached to the first carbon atom (marked by *).

The difference in the position of the hydroxyl group (OH) attached to the first carbon atom accounts for the difference between these two forms of glucose. While the disparity does not seem great, the linkages that result are very unlike, both structurally and functionally. Most animals manufacture special enzymes that can hydrolyze the bonds between the a-glucose units in starch and glycogen, but few are capable of hydrolyzing the b-glucose bonds in cellulose.

The inedible cell wall surrounding most plant cells is composed largely of cellulose. How do the animals that feed on these plants manage to hydrolyze the bonds in cellulose and thus liberate the glucose molecules for later use as a source of energy? One group, the ruminants (such as cattle, sheep, and deer) solve the problem by harboring in their digestive tract an army of microorganisms (bacteria and Protozoa) that are capable of digesting cellulose. These organisms produce the cellulose-digesting enzymes their hosts lack and use them to break the linkages between adjacent molecules of b-glucose. Ruminants are so named because they possess four stomachlike chambers (b), which contain the microbes necessary for the digestion of cellulose. Swallowed food enters the first two chambers, where the microbes begin digesting it. The larger particles are periodically regurgitated for further chewing - i.e., the animal "chews its cud." The food is again swallowed and exposed to the action of the microbes, and the cycle is repeated until all large particles have been broken down. Once this has been accomplished the food moves on to other parts of the digestive tract, and from there the glucose molecules can be transported to other parts of the body for storage or energy-related tasks.

Humans lack both the cellulose-digesting enzymes and the microbes of the ruminants. They benefit from cellulose by feeding it to such ruminants as cows and then consuming their milk and meat.

The digestive system of a ruminant. A cow with the various chambers in approximately their correct locations. The large chambers at the anterior end of the digestive tract contain huge numbers of cellulose-digesting microorganisms.

DENTITION

Mammals' dentitions vary within one basic mammalian pattern. Mammalian tooth patterns are interesting because no other vertebrate has different kinds of teeth within one individual.

In the front of the mouth are incisors, teeth that function in nipping or chiseling. On each side of the incisors may be found the canines, useful in biting and piercing prey. Behind the canines are the cheek teeth: the premolars, that do some grinding, and the molars, that do most of the grinding and chewing.

Mammals' diets vary. Some eat mostly plants and are called herbivores. Goats, cattle, deer, and rabbits are examples of herbivores. Other mammals eat mainly flesh; these are the carnivores. Cats, weasels, and wolves are carnivores. Omnivores eat both plants and animals. Bears are typical omnivores.

INCISORS

Examine the incisors (front teeth) of the various skulls on display in the Study Center. Generally, mammals must do some nipping and thus most species have incisors. The armadillo is one of the few North American mammals lacking incisors. The tusks of elephants are long upper incisors. Look closely at the goat's skull on display. This herbivore has a special adaptation that is typical of ruminants. The lower jaw has incisors, but the upper jaw lacks them. A developed, bony plate in the front of the upper jaw functions with the lower incisors in securing stems and hay. Rodent skulls such as the woodchuck have incisors that gnaw. These incisors wear down with use so it should be no surprise that these incisors grow continuously. The anterior teeth of carnivores such as the cat are specialized for cutting and shearing flesh.

CANINES

Now look at each skull's canine teeth. These teeth show great variation. Herbivores have little use for the flesh-piercing functions of the canine teeth. Hares lack canine teeth entirely. Deer (ruminant herbivores) have only reduced lower canines and no upper canines.
Carnivores, especially cats, have highly developed, elongated canine teeth. The saber-toothed tiger's "saber teeth" were canines. Have you ever noticed how closely a cat's upper and lower canines mesh as the jaw closes? This arrangement is very effective in prey capture and piercing. Just because a skull has long canines does not mean that the animal was a carnivore. Some species' canine teeth are useful in defense or mating display. And omnivores, such as bears, may have well-developed canines in addition to their broad posterior teeth which they use for crushing.

PREMOLARS AND MOLARS

Premolars and molars function in grinding. These teeth are very important to herbivores. Ruminants' molars are especially developed since they must chew and, after regurgitation, re-chew the tough plant materials that they eat. This is demonstrated on the goat skull. In carnivores, the small amount of chewing and grinding makes elaborate molars unnecessary. Insectivores are small mammals such as shrews and moles. They typically eat mainly worms, insects, and small vertebrates though some also eat eggs, roots, or fruit. Most insectivores have many small sharp teeth for piercing and holding onto worms and insects. This is not unlike the many sharp teeth of marine fish-eating mammals such as the dolphin. Consider normal human dentition. To what type of diet do humans seem suited?

REDUCED DENTITION

Mammals that feed on termites and ants often have few teeth. This includes mammals such as the anteaters and the armadillo. The echidna has no teeth left at all and its lower jaw is reduced to a single splinter.

BASED ON ITS TEETH, DO YOU THINK THE RAT IS A HERBIVORE OR A CARNIVORE?