How to be an herbivore

…it’s harder than you might think!

Herbivores are vegetarians, although some snack on meat upon occasion (beavers eat some salmon, deer sometimes eat birds!). Herbivory typically refers to the eating of living plants: leaves, shoots, stems, roots, and flowers. Subsisting on plant materials is not easy, because so much of a plant consists of cellulose and sometimes lignin—the walls of plant cells are generally made of these indigestible materials. In comparison, subsisting on meat is relatively easy, because the cell walls of animals are readily broken down and digested. Although seeds are plant parts, seed-eaters are a special category, because they deal with highly nutritious, relatively easily digested material, made by the plant to nourish the enclosed embryo.

Plant-eating animals have evolved several principal ways of dealing with their difficult diet. Many kinds of insects have mouthparts that pierce cell walls, so the consumer can suck out the cell contents without ingesting all that cellulose. Other plant-eating insects chew up high volumes of plant parts and just excrete the unwanted material. A few can digest cellulose to some degree. (Insects that eat wood and other dead plant material usually depend on symbiotic protozoans or bacteria in their guts.)

Vertebrates face the same problem of dealing with cellulose and, being too large to suck out cell contents, they solve the problem in various other ways. Only a few have simple digestive tracts that pass food quickly without metabolizing much of the cellulose: pandas and some odd South American birds called plant-cutters chew up leaves and shoots in vast quantities (much more than expected for their body sizes); the plant cutters and perhaps the pandas depend on unusually high rates of enzyme activity to extract nutrients from their fibrous diet.

Other vertebrate herbivores generally rely on micro-organisms to help digest cellulose. Some house their digestive microbes mainly in the front part of the digestive tracts, where there are typically several chambers in the stomach or esophagus. The action of the microbes is called fermentation, and animals that have their symbiotic microbes in the front part of the digestive tract are called foregut fermenters.

Animals that are foregut fermenters assist their microbes by producing lots of saliva containing phosphorus and other chemicals that nourish the microbes. The microbes get digested too, so they have to reproduce rapidly to maintain a working population in the foregut. Although some fluids and small particles bypass the microbes and go right into the rear part of the gut, much of the ingested food is processed by the microbes before passing on down the gut. Some foregut fermenters regurgitate their food and chew it again (“ruminating”), before swallowing it once more and passing it into the rest of the digestive tract. Foregut fermenters often have some microbial fermentation in the rear part of the gut too.

Foregut fermenters include kangaroos, tree sloths, certain monkeys, camels, and the ruminants (deer, cattle, sheep). One really weird bird, the hoatzin of South American, is also a foregut fermenter, with such a large fermentation chamber it can hardly fly. (Parenthetically, but interestingly, baleen whales are foregut fermenters too, only they digest chitin from the shells of krill.)

Most vertebrate herbivores house their helpful microbes in the hindgut, in the large intestine itself or commonly in a special out-pocketing called a cecum, which is very large in some of these herbivores. The contents of plant cells are digested and absorbed in the small intestine, and the cell wall material is sent to the hindgut for fermentation. There the resident microbes extract or make various nutrient but don’t greatly increase digestion of fiber. But then what? How do these herbivores take advantage of their symbiotic microbes and the useful nutrients they produce in the hind gut?

Hindgut fermenters typically produce two kinds of feces, separating the gut material into two fractions, one well-digested but full of microbes and still nutritious and the other fibrous and nutrient poor. The nutrient-rich and microbe-rich feces can be distinguished by texture or smell and are ingested. Research has shown that coprophagy (eating feces) is truly essential to growth and good health in these animals.

Hindgut fermenters are prevalent among all groups of herbivorous vertebrates, including amphibians, reptiles, birds, mammals, and even some fishes, but all animals that depend on coprophagy are relatively small (less than about one hundred pounds).

Parenthetically: before you wrinkle your nose and say ‘yuck’ at the thought of coprophagy, think about this: Ambergris is a waxy material excreted by sperm whales, possibly to aid the gut passage of squid beaks, which are hard and sharp, and might rip up the gut membranes (but other squid-eating whales don’t make it…). This material reportedly smells fecal at first but eventually acquires a pleasant odor. It has been important for the perfume industry and certain medical uses and reportedly has sometimes been considered to be a delicacy for human palates. Eating sperm whale poop?????? Really! Now you may wrinkle your nose!

Even with special digestive adaptations, some plant-eaters maintain a relatively slow life style. Koalas, specializing on eucalyptus leaves, have low metabolic rates and sleep a lot. Porcupines and tree sloths have slow metabolisms and are typically quite slow-moving. Beavers sit out the northern winter in their lodges, eating little (except the young ones, which eat cached twigs, and keep growing) and staying warm. Manatees and dugongs have low metabolic rates and take days to slowly process their plant food. These observations beg the question: why aren’t all plant-eaters slow?!

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