Bark Eaters

“Who’s eating my trees?” asked a participant on a hike last week. I’d been wondering the same thing! A few weeks ago I noticed creamy colored exposed wood on several small maple trees along my driveway.

“Why do animals even eat bark?” someone else asked. Good question. Most bark is composed of tough, dead, dry cells that are not very appetizing. Those cells are made of lignin, which makes wood rigid and is very hard for digestive systems to break down. White-rot fungi and a few bacteria are the only organisms who can consume lignin using specialized enzymes. Bark also contains tannins, which are bitter tasting chemicals that can inhibit digestion.

Unappetizing outer bark is how trees protect their slightly more appealing inner bark. In a tree trunk there are several layers of different cells, including xylem and phloem. Xylem, which is dead at maturity, carries water and minerals up from the soil. Old xylem becomes what we think of as wood. The living phloem carries sugars down from the leaves. They have a layer between them called the vascular cambium, which creates new xylem and phloem cells. Cambium is made of undifferentiated cells who can become anything - like stem cells. These three layers are considered the inner bark.

The living cells of the inner bark contain complex carbohydrates, sugars, and minerals. Right now, when the maple sap is running through the xylem, the inner bark is extra sweet! Even in the dead of winter, inner bark was a source of food for the animals who can access it…and digest it.

Porcupines are one likely culprit in the decortication (bark removal) of my trees. The bottoms of their feet are hairless and covered in a pebbly texture that improves their grip. Long, curved fronts claws also aid in tree climbing, along with bristles on the underside of their tail. To get at the most nutritious parts of a twig, porcupines will balance out toward the terminus of a branch and nip off its end using their self-sharpening incisors. Turning the stick around, they nibble off all the most tender twig tips and buds and then discard the rest. Sometimes you’ll see porcupine tooth marks on bigger branches, too, or even the trunk.

With long claws and several other adaptations, porcupines are able to eat seemingly unappetizing tree bark. Photo by Emily Stone.

Hemlocks are their favorite winter food. As spring progresses, porcupines nibble on a buffet of different trees and plants, making sure to eat each one at their point of peak nutrition. Even this careful food selection wouldn’t be enough without one more adaptation: porcupines have an extremely long large intestine filled with microorganisms who produce lots of enzymes. This extended digestion allows porcupines to extract more nutrients from their food.

Reading about porcupine digestion made me curious about their cousins, the beavers. Now, there’s no way that a beaver could have nibbled the bark on the twigs of trees still standing along my driveway, but these two big herbivorous rodents have quite a bit in common, and some important differences. An article in the Canadian Journal of Zoology suggests that beavers don’t chew their food quite as well as porcupines, but make up for it by having a small intestine that’s 70% longer! The porcupine has a longer colon, though, which allows them to absorb more water from their food. That makes sense, given their different habitats.

There’s one other big difference: beavers engage in coprophagy. Beavers will re-ingest their first round of poop so that they can have another go at extracting all possible nutrients.

Beavers share the trait of coprophagy with another bark eater: bunnies. Rabbits and hares have short digestive tracks, so they combine a good microbiome with coprophagy to enhance digestion. Both rabbits and hares eat their first round of soft, greenish cecal pellets, and then leave behind fecal pellets that look like M&Ms made of sawdust. This allows them to eat twigs and inner bark in the winter. Of course, they focus on the bark of small stems at the height of the snowdrifts…not in the tops of trees.

Voles also eat bark low to the ground. These little rodents like to hide in the subnivean zone under the snow and nibble on bark in relative warmth and safety. They can damage trees, even girdling and killing them. Voles have a specialized pouch called a cecum at the beginning of the large intestine that provides a place for food to be fermented. They may also use coprophagy to help absorb certain nutrients.

In comparison, deer have the most complicated gut for digesting bark and twigs. They are ruminants with four stomachs, like cows. Microorganisms in deer’s rumen break down tough materials, aided by them regurgitating and chewing their cud until it’s broken down enough to move on to the rest of the stomachs. Deer might strip bark off a young tree higher than a hare, but not high in the treetops like I’d observed.

So, who was the bark-eating culprit in the trees along my driveway? Judging by the tiny tooth marks, and my most commonly seen neighbors, they were gray squirrels. Squirrels have sharp teeth and excellent climbing skills just like porcupines, and can venture out onto smaller branches to nibble on the most tender bark. Squirrel tooth marks are less than 2 mm wide, while porcupines’ teeth are two to three times that big.

The tiny tooth marks high up on a sugar maple sapling are likely the work of a hungry gray squirrel. Photo by Emily Stone. 

Discarded bark strips litter a log below the decorticated tree.

Recent research suggests that special gut bacteria help gray squirrels extract calcium from tree bark. This adaptation might be what’s allowing gray squirrels to outcompete the native red squirrels in Great Britain, where gray squirrels were introduced. Our native red squirrels have been observed eating bark less frequently than grays, but they are smart enough to know that making a small incision in sugar maple bark this time of year releases another one of bark’s sweet secrets.

Emily’s award-winning second book, Natural Connections: Dreaming of an Elfin Skimmer, is available to purchase at www.cablemuseum.org/books and at your local independent bookstore, too.

For more than 50 years, the Cable Natural History Museum has served to connect you to the Northwoods. Our exhibit: “The Northwoods ROCKS!” is open through March 9. Our Winter/Spring Calendar of Events is ready for registration! Follow us on Facebook, Instagram, YouTube, and cablemuseum.org to see what we are up to.