February 2022 Natural Mystery Answered

Most of our Natural Mysteries call for identifying a species. Our February mystery asked "What is This Sign?" Congratulations to Mark Erikson, Bill Kass and Kirsten Welge for correctly interpreting this sign and identifying the behavior that caused it. Further congratulations to Mark and Bill who also identified the species that most likely did this.

This sign was left by squirrels, most likely gray squirrels, stripping bark for nesting material.

At first glance, the shredded bark may resemble a deer antler rub. There are several clues that this is not deer sign. Mark notes that “a deer rub would be smoother.” This has a shaggy appearance, with many delicate strands of shredded bark still hanging from the tree. We are also some months after the rut, and this sign appears quite fresh.

The incisor marks are another clue, and also suggest some specific behaviors. As Kirsten explains:

“There are 3 mm wide double gouges present on the smooth which appear to be incisor marks from a small rodent. With incisor marks on trees, we might consider the animal was feeding on cambium layer of trees, tapping the tree for sap, or harvesting dead bark in strips for nesting material.”

There are a few features here that can help us distinguish feeding sign from bark harvesting. To begin with, tapping for sap happens later in the season when daytime temperatures are consistently above freezing, and involves making small slits in otherwise intact bark. Both tapping and cambium feeding only happen on living branches. As Kirsten noted, rodents harvest bark from dead branches, when the bark easily pulled away from the wood. The way the bark is sloughing off wood here suggests that the branch is dead. Finally, feeding on inner bark leaves incisor marks across the entire surface as rodents scrape the live cambium off of the heartwood. Here we see only a few small, scatters tooth marks.

As Bill explains:

“Given the ragged appearance, I am guessing that the intent of this animal was not to “chew” the tree but rather to remove bark.”

He goes on to discuss what species may have done this:

“The incisor marks are incredibly small. With that said, a strict tooth size measurement and comparison with reference books can not be clearly dependable because the depth of the teeth may not have captured the true width of these incisors. This makes species is hard to determine.”

It is indeed. Incisor marks may offer us a lower limit on size, and given the size of these we can rule out a mouse. That leaves our squirrels as possible culprits. There are three species of squirrels we might find near St. Paul campus: red, gray and flying. Mark notes that this behavior is common in eastern grays. Bill points out that gray squirrels are the most abundant species in the area, and that the large amount or bark removed here suggests a larger animal. I agree. It is also likely that this is the sign of multiple animals, and perhaps multiple species. A branch like this one that easily yields good nesting material is a valuable resource which will likely be found and used by many animals in an area.

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