Some animal communication is just for the moment, gone as soon as it is created, and some is more permanent. Whether it’s a patch of earth pawed by a deer, a scat deposit carefully positioned by a fox, or a twist of grass left by an otter, messages left in physical media can convey information long after the author has left the area. Squirrels are especially adept at this type of messaging, and their medium of choice is something they are intimately acquainted with–wood. Tree trunks, branches, roots–all can serve as bulletin boards for intra-species communication. One of the best times to observe squirrel marks is early spring, after the snow is gone but before new leaves limit our view through the forest.
The photo below shows an opening into the trunk of a large red maple. Hollow trees provide critical winter shelter, and this one must have been prime real estate because the hole has been bitten around the edges by a squirrel. Gray, red, and flying squirrels (of both sexes) use their incisors to declare ownership of desirable nesting spaces. Theoretically the sizes of the gouges should tell us which species did the marking, but the hole was about thirty feet up, and it’s hard to measure tiny things like tooth marks when you’re that far away.
The creature claiming possession of the tree in the next photo is easier to determine. Gray squirrels, primarily males, make vertical marks called stripes to assert territorial claims. They seem to prefer rough-barked trees like the white oak pictured in the photo, and the stripes are generally found on large trunks between 2 and 6 feet above the ground. I’ve also seen gray squirrel stripes on red oaks, chestnut oaks, hickories, and tulip trees. After marking, a squirrel may rub its cheek on the bitten area to leave its scent. You can see from the varying degrees of weathering that this tree has been marked repeatedly over several years.
Red squirrels also have distinctive ways of creating messages, and one of the easiest to find is the branch marking associated with conifer middens. Middens are accumulations of discarded cone scales and cores found below habitual feeding perches. The photo below shows a midden at the base of a Norway spruce. Most conifers, with the exception of some pines, tend to retain lower branches for years after they have died, and these provide perfect feeding perches. The oversized cones (up to 8 inches long) produced by Norway spruces are prized by red squirrels, and the middens came become quite large.
If you examine the branches above a midden you’ll probably find bite marks like the ones shown in the next photo. The image shows a Norway spruce branch which extends horizontally about four feet up the trunk. The upper surface of the branch is adorned by numerous bite marks. You can see the midden (out of focus) on the ground below the branch.
Red squirrels also make marks at or near ground level. In the photo below you see a Norway spruce root which has crossed over and been lifted over the years by the swelling root crown of a neighboring tree. This tree was part of a plantation that dated from the 1960s, and the trees were close enough together that horizontally spreading roots often passed close to the bases of neighboring trees. This also happens in other conifers when they grow in crowded stands, and the small lateral roots have thinner bark than the trunk and the larger roots.
A closer look, shown in the next photo, shows that a red squirrel has bitten through the bark of the lateral root. The light colored gouges are recent marks and the whitish ones are older, probably made the previous year and covered with dried resin.
Norway spruce plantations were established throughout the east during the Depression and also later in the 20th century. With their large crops of oversized cones, stands of Norway spruce are preferred habitats for red squirrels and are great places to investigate red squirrel marking. Other conifers were also used for reforestation projects, and if they support resident red squirrels you’ll probably find evidence in the form of marking and middens. Both branch marking and root marking are the animals’ way of defending their underground larders of winter food.
Squirrels also use their incisors for purposes other than marking, such as debarking trees to get at the living cells of the cambium. The photo below shows a staghorn sumac that was fed on by a gray squirrel. I found this a few years ago in early March, and the color of the exposed wood indicated that it had been done not long before. Late winter and early spring can be a time of scarcity; stored food supplies may be exhausted and squirrels may be forced to turn to foods which are less nutritious or harder to access. I’ve occasionally found similar cambium feeding by squirrels on sugar maples.
Squirrels, both red and gray, also tap trees when the sap flows in spring. The animals choose vigorous trees, and bites are made in living, thin-barked branches by anchoring the upper incisors and drawing up the lower ones. This creates what Sue Morse calls a d0t-dash pattern. Two fresh bites on a sugar maple branch are shown below, and above them there’s an older bite. Interestingly, the sap is not consumed immediately, but is allowed to dry. Once the water has evaporated the squirrel returns to lick up the crystallized sugar.
When we find a mark made by a squirrel, we can infer something about the availability of food or the presence of a desirable nesting site, but for other squirrels there’s much more involved. The associated cheek rub or saliva deposit is unique to the individual and carries information about its sex, health status, and possibly other characteristics. Even though receiving these messages is beyond our abilities, I enjoy finding squirrel marks and imagining the messages they convey to their neighbors.