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| Description: One way of looking at a vegetarian diet is as a search for rare essential nutrients, commonly hidden within a sea of carbon. For animals that eat whole tissues (as opposed to fluid feeders like aphids), an essential first step will be in the physical disintegration of the ingested food. This can be a bird crushing a seed in its beak, or repeated bouts of chewing by large ungulates. This process increases the surface area exposed to digestive enzymes and liberates nutrients from the confines of the plant cell walls. Animals that eat relatively hard plants will tend to have stronger mouthparts than those that eat softer plant tissues. Different herbivores possess different physiological mechanisms to detoxify the potentially harmful chemicals found within their food, once it reaches their guts. This could be as simple as having an alkaline pH to neutralize some compounds, to more elaborate adaptations such as mixed function oxidases (MFOs), which catalyze oxidizing reactions to detoxify a variety of compounds. The gut of many herbivores is also home to a diverse collection of microbial symbionts. These bacteria, fungi, and protists are able to digest cellulose and other complex plant compounds, converting them into new microbes. The host, in turn, digests the dead microbes, which can be the source of over 50% of the available N absorbed by ungulates. Many herbivores also make behavioural choices that allow them to eat better food, either through increased nutrient content or reduced toxicity.
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