<\/a>A hoary marmot nibbles a dandelion (Courtesy Photo \/ Jos Bakker)<\/p><\/div>\n\t\t\t\t
However, in general, plant tissues have lower levels of protein (usually less than twenty percent by dry weight, often much less). Plant-eating animals (herbivores) consume fresh green plant material and the wet weight varies with the water content, which is highly variable, depending on growing conditions, among other things. For comparison purposes, try fifty percent water, giving less than ten percent protein in plants. Plants and plant parts differ a lot in nitrogen content, availability, and digestibility; for instance, young needles of spruces have more than twice as much protein as old needles—but still a relatively low value. Pollen and seeds tend to have more protein than the green parts.<\/p>\n\t\t\t\t
Herbivores feed on green vegetation that often has relatively little nitrogen, so how do they obtain enough nitrogen? Some forage selectively; for example, the giant panda eats mostly bamboo but it favors young bamboo shoots, which have more nitrogen than older shoots. Another way is to eat large volumes of vegetation, which is most possible for large animals with large guts (elephants, horses, cows). Or an herbivore may pass small volumes of vegetation through the guts rapidly (panda); aphids and other sap-sucking insects ingest lots of fluid in order to get enough nitrogen, excreting the excess water and sugar as honeydew.<\/p>\n\t\t\t\t
Herbivores have a variety of specializations of the digestive tract. But fundamentally, all vertebrates (and invertebrates too) depend on vast populations of microbes to process food. Some plant-eaters do a lot of food-breakdown in the stomach, which is generally divided into compartments; examples are camels, kangaroos, hippos, sloths, and the ‘ruminants’ (deer, cows, sheep),which can burp up a meal from the stomach to be reprocessed by the mouth. The first stomach compartment (the rumen) holds populations of microbes that do the work of digestion. Other herbivorous mammals (horse, elephant, manatees, gorillas) have enlarged colons for microbial digestion. There are billions of these microbes, which have many digestive uses, but they have very short individual lives. When they die, their multitudinous bodies can be digested by still other microbes, which produce amino acids usable by the herbivore.<\/p>\n\t\t\t\t
Many herbivores have special physiological adaptations that allow them to recycle urea (a metabolic product of nitrogen digestion, which humans excrete in urine) back into the digestive tract where it is used by gut microbes. Herbivorous birds such as grouse and ptarmigan can reflux urine and digestive material back ‘upstream’ from the cloaca where it is collected into the large intestine to be reprocessed.<\/p>\n\t\t\t\t