Herbivorous Strategies

Among the professional fermentors, two distinct strategies have evolved. The chief difference between the two groups is in positioning of their fermentation vat relative to the stomach and small intestine:

• Cranial fermentors or ruminants have a large, multicompartmented section of the digestive tract between the esophagus and true stomach. These forestomachs house a very complex ecosystem that supports fermentation. Examples of ruminants are cattle, sheep and deer.
• Caudal fermentors, also known as cecal digestors, are similar to dogs and humans through the stomach and small intestine, but their large intestine, where fermentation occurs, is complex and exceptionally large. Examples of cecal digestors include horses and rabbits.

The process and outcome of fermentation is essentially identical in the rumen of a cow or the cecum of a horse. However, the positioning of the fermentation vat in relation to the small intestine has very important implications for animal's physiology and nutrition. These similarities and differences can be summarized as follows:

Function	Ruminants	Cecal Digestors
Ability to efficiently digest and extract energy from cellulose	Yes	Yes
Ability to utilize dietary hexose sources directly	No	Yes
Ability to utilize the protein from fermentative microbes	Yes	No

Recall that in all animals the small intestine is the only site in the digestive tract where simple sugars and amino acids can be absorbed. Ruminants can utilize dietary starch, but very little of it is absorbed as glucose. Rather, starch and other soluble carbohydrates are fermented to volatile fatty acids in the forestomachs. What little starch enters the small intestine is poorly digested in that organ due to a relative deficiency in amylase. In contrast, starch fed to a horse is digested to glucose by amylase and maltase in the small intestine, and that glucose is absorbed across the epithelium into blood.

Regarding protein, the bodies of microbes in the fermentation vat represent a large source of high quality protein. In ruminants, those microbes flow into the stomach and small intestine, where they are digested and absorbed as amino acids and small peptides. Because the fermentation vat of a horse is behind the small intestine, all their microbial protein is lost.

Both ruminants and cecal digestors are very successful. Each of these two strategies has what you might call strengths and weaknesses relative to the other, which predispose these groups of animals to distinctive diseases, as will be explored in subsequent sections.