On The Trails: Vertebrates eating bark

This beaver has almost finished peeling a small branch. (Photo by Jos Bakker)

By Mary F. Willson

Trail-walkers here often see signs of bark-eating. For example, porcupines leave their "art-work" at the bases of hemlock trees, with patches of bark removed, exposing the wood beneath it.

Beavers leave cut-off stumps with a partly peeled trunk nearby, and they peel branches, especially of cottonwoods, and put them in dams or just discard them. Voles gnaw the bark from the bases of young trees, especially in winter, often girdling and killing the tree. Snowshoe hares nibble off twigs mostly in winter, leaving tooth marks on the cut end, and gnaw bark particularly from deciduous trees. Moose browse the upper twigs of shrubs in winter, leaving a flat-topped crown; they also strip bark from tree trunks with their lower incisors, leaving vertical scars (unlike the rodents and hares, they don’t have upper incisors).

The bark on the outside of tree trunks is composed of dead cells loaded with several difficult-to-digest chemicals. Underneath that layer is the inner bark, composed of phloem cells that conduct energy-rich carbohydrates (sugars) from the leaves, where photosynthesis occurs, to the rest of the plant.

The next layer is cambium, which makes the phloem cells on the exterior side as well as xylem cells on the interior side, where they accumulate as sapwood. Very young xylem cells, along with the phloem and cambium, are the attraction for bark eaters. Mature, dead xylem cells conduct water and nutrients from the roots to the rest of the plant. Old, defunct xylem cells comprise the heartwood in the middle of the tree. 

Plant cells, unlike animal cells, have cell walls composed of cellulose, hemicellulose, and lignin. Cellulose is made of many glucose molecules in a chain; many chains together make fibers that are bonded together by hemicellulose, making a matrix. Lignin gives the matrix some rigidity. Those materials are not directly digestible by animals; they made digestible by microorganisms that live in animal guts.

A dramatic example of "porcupine art" resulting from chewing bark. (Photo by Deana Barajas)

Bark is really a popular food for many mammals. The first treatment for all the consumers is done by the teeth, chopping and mashing the bark, then mixing it with saliva in the mouth, making a soup that is passed into the digestive tract. After that, mammals have evolved several ways of dealing with it. Basic to all of them is the use of symbiotic microorganisms to breakdown cellulose and the recycling of ingested material by various means. The anatomical arrangements for doing this are varied, as discussed in the next section, for four species.

Moose and deer eat twigs and sometimes strip bark from branches for winter food. The soup of saliva and mashed barks gets to the rumen or first stomach compartment where it is fermented by bacteria. The new mixture (a "cud") can be burped back up to the mouth for further mashing and mixing with saliva, and then re-swallowed. If necessary, that can be repeated several times before the well-mixed product is passed on to the next stomach compartment. There the material is sorted, and big particles are returned to the first compartment for more work while small ones are passed on. In the third compartment liquids and electrolytes begin to be absorbed, and the rest of the material goes to the fourth compartment, where enzymes and acids break down proteins and fats, in addition to digesting the bacteria that worked in the first compartment. From there, it all goes to intestines for nutrient absorption and discarding of wastes. A complicated system!

The next species are called ‘hind-gut’ fermenters (in contrast to the foregut ruminants), accomplishing cellulose digestion with the help of a special sac called a cecum, housing symbiotic microorganisms. They have relatively long intestines for the extended extraction and absorption of nutrients, as food passes gradually along.

Porcupines start digestion in the stomach, where protein is extracted. The rest of the material goes to the long small intestine for enzyme action and absorption of nutrients. Remaining material goes to the cecum at the beginning of the large intestine, where microbes ferment the cellulose into absorbable products. Thence to the large intestine for nutrient absorption and eliminating wastes. Beavers have a similar system, with very long intestines. They add another process called coprophagy (dung-eating): the first material to pass through the gut is only partly digested and it is excreted as soft pellets. The beavers re-ingest those pellets for additional processing and extraction of nutrients. After a second processing, harder pellets are excreted and discarded.

Cross-section of a tree trunk, showing the layers of bark. (Diagram by KMHocker)

Voles have a special chamber where the esophagus enters the stomach; this fore-stomach houses microorganisms that begin to ferment cellulose. The partially digested material goes to the stomach and a cecum for further microbial action. Thence to the relatively long intestines, for more fermentation. Voles can selectively retain large particles for further processing and practice coprophagy too. Furthermore, they are able to increase the size of their digestive organs when dealing with very high-fiber diets. Snowshoe hares have a long intestine and a cecum with symbiotic bacteria and practice coprophagy.

Bottom line: eating bark successfully requires symbiotic microorganisms and commonly involves recycling of material by cud-chewing, coprophagy, or movement up and down within the digestive tract. The anatomical arrangements for doing this differ among the bark-eaters.

• Mary F. Willson is a retired professor of ecology. "On The Trails" appears periodically in the Juneau Independent.