November 2022 Natural Mystery Answered

We received a wide range of guesses for our November Natural Mystery. Many animals leave prominent sign on trees, including beaver, deer, woodpeckers, bears, porcupine, and some squirrels (including marmots), and some of these sign can appear similar--especially at first glance. This particular sign is distinctive enough to be identified to the genus, once we know what to look for. Congratulations to Mike Holtz, Kirsten Welge, and one anonymous tracker for correctly interpreting this sign and identified the species responsible for most of the marks we see here.

This is marking behavior by gray squirrels (Sciurus carolinensis). Fox squirrels (Sciurus niger) also engage in this behavior. As Mike Holz notes, this sign is sometimes called a “message board” or “vertical stripe.” Mike continues:

“Gray squirrels will gnaw and rub their cheek glands as a form of communication, sort of a community message board. Squirrels will often check out the existing marks before adding their own. These are often on the protected side of one of the larger trees in the area.”

Kirsten Welge goes into some detail in how she interpreted this sign, and offers some great tips for getting familiar with this common sign:

“Portions of the light gray outer bark have been removed, leaving burnt-orange patches of live bark showing through. These orange patches shows rough-edged gashes, aligned horizontally to near-vertically. The broad swatch of patches runs vertically along the trunk. On further inspection, many similar marks are visible on the bark, but do not show the high color contrast of the freshly removed bark.

One approach to interpreting sign comes to us from Cybertracker Evaluator & Senior Tracker David Moskowitz: Consider the tools you would use as a human to make that sign. Then, consider which animals have those tools, and have the motive to create such a sign.

In this case, I'm seeing small chisel marks a few millimeters wide, repeatedly applied with some force. Rodent incisors are a prime candidate for this work, and eastern gray squirrels (Sciurus carolinensis) are known for making these large visual and scent-marks on the lee side of large trees and underside of large branches – chewing on the bark, then rubbing their cheeks along it, to communicate with others of their species. Elbroch's Mammal Tracks & Sing (2nd ed) gives an example of this clear “vertical stripe” on several different tree species (p 269), and notes that both eastern gray and fox squirrels will engage in this behavior – males much more than females, and fox squirrels more frequently than eastern grays. Elbroch notes that these stripes are seen on many different tree species, but the majority fall within 13-70” off the ground – about right for the height of this picture.

Casey McFarland referred to this sign during one Minnesota evaluation as a gray squirrel “bulletin board”. Taking in the mix of fresh and older marks layered in the bark here, it's clear these markings are completed over years. As gray squirrel average life span in about 6 years, we may be seeing the work of generations of gray squirrels on this tree.

Personal Commentary: I regularly see eastern grays marking the large silver maple in my Saint Paul yard, under a large low-hanging limb and up the lee side of the trunk. This is one of my favorite common signs to look for in neighborhoods and parks – look for the large trees, and check the lee side for vertical striping. If you want to get your nose involved, try smelling the unmarked bark, then the marked areas, to see if you can pick up a hint of deposited squirrel scent. When I've tried this with the stripe on my silver maple, there's a faint warm, musky smell on recently disturbed bark.”

As several people noted, the marks are on a large cottonwood tree (Populus deltoides). Mature cottonwoods are a popular location for these community kiosks.

So how do we know these marks were left by gray squirrels rather than fox squirrels? Basically, it's a numbers game. Although fox squirrels are sometimes seen in Ft. Snelling State Park, gray squirrels are much, much more numerous. While it is possible that a fox squirrel added to this "message board" at some point, it is safe for us to presume that most of the marks on this tree were left by gray squirrels. Similarly, Red Squirrels (Tamiasciurus hudsonicus) may occasionally "post" to these message boards, but so far as I know they do not create "vertical stripes." Where we see this sign, we can be confident that there is a local population of large tree squirrels.

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