How do graminivores digest cellulose?

Graminivores (grass-eaters) face a major challenge: cellulose, the tough structural carbohydrate in grass, is indigestible by most animals. No mammal produces the enzyme cellulase, which is required to break down cellulose.

The solution is symbiotic digestion. Graminivores host vast colonies of symbiotic bacteria and protozoa in their digestive tracts. These microbes produce cellulase, which ferments the cellulose, breaking it down into volatile fatty acids (VFAs). The animal then absorbs these VFAs as its primary energy source.

This process occurs in two main ways:

1. Foregut Fermentation (Ruminants)

This is the system used by cattle, sheep, wildebeest, and kangaroos. The stomach is highly modified into multiple chambers (e.g., the rumen, reticulum, omasum, and abomasum in a cow).

• Grass is swallowed and enters the rumen, a massive fermentation vat.
• Microbes in the rumen begin breaking down the cellulose.
• The animal "chews its cud" (rumination) by regurgitating the partially digested food, chewing it again to break down fibers, and re-swallowing it.
• This process is very efficient, as it extracts nutrients before the food reaches the true stomach (abomasum) and intestines.

2. Hindgut Fermentation

This system is used by horses, rabbits, capybaras, and geese. The fermentation occurs *after* the stomach, in an enlarged cecum or a large colon.

• Grass is chewed, swallowed, and digested normally in the stomach and small intestine first.
• The tough, fibrous cellulose then passes into the large cecum, where the symbiotic microbes ferment it.
• This is generally less efficient than foregut fermentation, as the primary absorption site (the small intestine) has already been passed.
• Some hindgut fermenters, like rabbits, overcome this by coprophagy: they excrete and re-ingest special fecal pellets (cecotropes) to give the food a second pass through the digestive system.