| Herbivorous mammals face several challenges when feeding: plant matter
is tough to digest, relatively low in fat and protein, and the majority of
nutrients are located within tough cell walls. Mammals lack the enzymes
needed to break down these cell wall compounds, which include cellulose and
lignin, and thus can only access a small portion of the nutrients in vegetation
by themselves.
Many microorganisms, on the other hand, can synthesize cellulytic enzymes.
A symbiotic relationship exists between almost all herbivorous mammals
and microbes: mammals provide food and living space for the microbes, and
receive the byproducts of fermentation (including fatty acids and
sugars) in return. However, microbial fermentation is a slow process,
requiring time and space in order to occur. In the majority of herbivorous
mammals (including the Perissodactyla)
the large intestine is expanded to house microorganisms (a special sac called
a caecum is frequently present). The downside to such hind-gut fermentation
is that many byproducts of fermentation cannot be absorbed so far down the
alimentary canal. Ruminants are able to take advantage of these byproducts
because fermentation occurs BEFORE digestion occurs, with microbes being
given space in their capacious stomach.
The stomach of all ruminants has four chambers: the first chamber (the rumen)
forms a huge fermentation vat, containing billions of microbes (bacteria,
protozoans, and fungi) which anaerobically break down the ingested plant
material before it is enzymatically digested by the ruminant. The second
and third chambers, the reticulum and omasum, act as filters, allowing liquids
(including microorganisms) and small, digestible food particles to pass through
to the fourth chamber, the true (enzymatic) stomach known as the abomasum.
Larger particles are selectively filtered out and retained in
the rumen to be broken down further, allowing ruminants to extract maximal
nutrition from their diet.
To complement the fermentation process, ruminants periodically regurgitate
and rechew their food. Aptly called rumination (or "chewing the cud"
), this mechanical digestion physically breaks down the larger food particles
to increase the surface area for microbial action. By chewing their
food twice, ruminants avoid having to chew their food thoroughly while foraging
(often a vulnerable activity) - proper mastication can be performed while
resting, with the head raised and alert for danger.
As in hind-gut fermentation, microbes in the rumen produce sugars and short-chain
fatty acids during the digestion of cellulose - these are the principal source
of energy for the ruminant. However, the same microbes also synthesize
vitamins, detoxify plant poisons, and recycle nitrogenous compounds into
proteins - all actions which help the microbes survive. Unfortunately
for the microbes, as fluids are selectively filtered out of the fermentation
chambers, the microorganisms are also carried to the stomach to be digested:
ruminants actually derive the majority of their nutrients from microbes,
not directly from plant material! While this is a highly successful
strategy, neither foregut fermentation nor rumination are defining characters
of this suborder: similar strategies have evolved independently in camels,
kangaroos, and colobine primates, among others. |
