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Trophic Level II - The Herbivores |
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Trophic Level II - The Herbivores |
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Ruminant Body Plan
Summary
Ruminants are even-toed ungulates. They have a four-chambered stomach that has been adapted to allow them to digest plant material ($$Jim-Insert a diagram of an antelope with a cut-away to show four stomachs $$). They are cud-chewers, which means that they regurgitate their food and chew it again. Ruminants are the most successful group of ungulates and make up 80% of the group.
Ruminant Evolution
Ruminants first evolved from primitive ungulates about 37 million years ago. They have several adaptations that allow them to exploit vegetation better than hindgut fermenters such as zebras and wild asses. Plant material, and in particular grass, is almost indigestible. Hindgut fermenters have evolved in one way to solve the problem (see Hind-gut fermenters) while ruminants have evolved in another. The most successful adaptations have been made by the ruminants.
The largest herds of mammals known are made up of members of the ruminant family. In North America during the 1800s, up to 40 million bison roamed the plains. Nearly as many pronghorns also lived there. The only hindgut fermenter was the horse and it was introduced to the environment. On the tundra today, there are herds of caribou numbering close to one million. In Asia, saiga antelopes existed in huge herds, while in Africa herds of springbok and wildebeest were found.
Today in the Serengeti-Mara, there are only between 200,000 and 250,000 zebra. Compare this with the populations of various ruminants and it is obvious which is the most successful. There are 1.5 million wildebeest and half a million Thomson's gazelles.
Secret to Success
The secret to the ruminant's success has to do with their ability to eat plants. Bacteria living in the ruminant's digestive system break down the almost indigestible plant cellulose and convert it into digestible carbohydrate. This process is called fermentation. It begins in the omasum (the first "stomach"). A wildebeest, for example, tears off the grass, chews it a little and then swallows it. Ruminants have no upper incisors like zebras and other hindgut fermenters. Instead, they have a thickly callused pad in the roof of their mouths. The grass or leaf is ripped or torn when it is caught between the pad and the lower incisors.
The unprocessed grass enters the omasum where bacteria begin to break it down. Here, the grass mixes with stomach juices to form a thick, stringy soup. When the wildebeest has eaten its fill, it will regurgitate some of this and re-chew it. The large molars and pre-molars of ruminants then grind the cud, further refining it. This gives them the characteristic side to side mouth movement most people associate with cows.
Next, the food passes into the second stomach, the rumen,
for further processing. 
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CDROM Index | Title Page | Contents | Index | Glossary | Bibliography | Appendices | LifeStories Online
Dave Taylor's African Safari - Book 3: Trophic
Level II - The Herbivores (Standard
Version)
Copyright © 1999 Dave Taylor & James Cash