Incisive Communication

Communication is an essential part of life for all animals. For squirrels and their kin spring is a time of intensive communication, as they select nest sites and prepare to birth and raise young. And the leafless canopy of early spring affords good visibility, so it’s a great time to spot the messages left by these animals.

The strong, sharp incisors possessed by all rodents–two in the upper jaw and two in the lower–are perfect tools for inscribing messages. In the photo below bright gouges show where a squirrel bit into the sides of a narrow cleft in a tree. The cleft was at eye-level, so the reddish marks of the animal’s incisors were easy to see. After making the bites the animal may have rubbed its cheeks on the bark to leave a scent message. Squirrels have a well developed sense of smell, so the saliva left in the bites and the scent from the cheek rubbing may have been even more meaningful than the visual marks.

Squirrels also create “stripes” on tree trunks. In the next photo gray squirrels–probably more than one–made many bites in an elongated zone between four and six feet off the ground along the trunk of a white oak. If you look carefully you’ll see that the individual bites vary in color, from bright reddish (the most recent) through grayish red to dull gray (the oldest). The varying age of the bites indicates that this stripe has been worked in the same way over several years. The individuals making the bites probably also did some cheek rubbing, so the bark would have been perfumed with an abundance of scents.

Marks like the ones pictured above are usually found in the general vicinity of nesting or feeding areas, but sometimes the bites seem to indicate a claim to a particular nest site. I’m not talking about summer nests, the leafy dreys seen high in the branches of large trees. In areas with cold winters, both winter and birthing nests are located in safe, weather-proof sites like hollow trees or underground cavities, and the supply of good sites may be limited. The hole shown in the photo below was located about 20 feet up in a large tree. There was probably a perfect nest cavity inside–the opening looked well used, and the varying intensity of the bitten areas suggested that the site had been used for at least several years. By marking the opening, the resident squirrel was able to establish ownership of its chosen refuge.

Squirrels are not the only creatures associated with tree holes. The openings shown below were made by pileated woodpeckers. From a distance the bright margin of a woodpecker hole may look like the chewed edges of a squirrel hole, and the size and shape may be about right. But it’s easy to tell that these holes are not squirrel holes. The first clue is their rough, splintery margins. Another clue is the number of holes–in this case there were five similar openings distributed along the trunk. These holes were made for feeding, and they’re distributed up and down the trunk because the insects the woodpecker was seeking–probably carpenter ants–had colonized much of the tree. Woodpecker holes are often more irregular in shape, and when that’s the case it’s easy to tell that they weren’t chewed on by squirrels.

Whether a mark was made by a red or a gray squirrel is often hard to determine–both are known to make stripes along trunks, and both probably mark the entries to nest cavities. Flying squirrels may mark in similar ways, but I haven’t been able to find any mention of that in the tracking literature and I don’t have any examples to share. If anyone knows the location of a definite flying squirrel nest hole, I’d love to hear about it.

Chipmunks tend to use underground refuges rather than tree cavities, and I haven’t seen anything in the literature about chipmunk marking. But a few weeks ago I came across the hole in the photo below, located a little above my head in a small tree. It puzzled me at first because the chews looked rougher and more irregular than the typical squirrel chew. But the mystery was solved when I stood on my tip toes to get a closer look. Out of the hole came the frantic chittering of a chipmunk. The animal sounded so upset that I left quickly, but I thanked it for showing me what a chipmunk can do with its teeth.