Chipmunk Lives

It was eight degrees at dawn on February 6, and once again I was out on snowshoes looking for animal tracks. That’s when I spotted eastern chipmunk tracks emerging from a burrow hole beneath one log and moving over the snow to a hole underneath another log. Possibly it was a male checking out the females in his vicinity. Still, I was surprised he was out when it was so cold and snowy.

During mild winters with little or no snow on the ground, I have watched mating chases in February. Male chipmunks track down receptive females by sniffing along their paths, and several males chase a single female in what biologists call “mating bouts.” If she is not ready to mate, she leads them on a merry chase, diving into hollow stumps or under logs.

But last winter remained cold and snowy and yet, on another day, at zero degrees Fahrenheit, chipmunks were abroad. Of course, all chipmunks wake every three or four days to eat the food they have stored in several football-sized chambers in their underground burrows. Each chipmunk burrow has a single, two-inch diameter entrance hole in use at any one time, which leads to a complex of tunnels as long as 100 feet and as deep as three feet. It also contains a separate 24 by 15 by 10 inch nesting chamber, filled with chewed and crushed leaves. Although a female chipmunk uses this for her young, both males and females use it during hibernation.

Scientists have proven that chipmunks are true hibernators by surgically implanting temperature-sensitive radio transmitters in them. When chipmunks are in a torpid state, their normal 60+ breaths a minute decline to less than 20 breaths and their usual body temperature of between 96 to 106 degrees F. falls to between 42 degrees to 45 degrees F. Unlike woodchucks, which live on fat they have accumulated before hibernation, chipmunks depend on their food stores to keep them fed over the winter.

Studies have shown that chipmunks adjust the depth and length of their torpor based on the size and composition of their food reserves, and since there was an abundant acorn mast here in the fall of 2014, despite the cold and snow, the chipmunks may have had enough food reserves to emerge at their usual time. And, in fact, males, which emerge at least two weeks ahead of females, may eat more than females during hibernation so that they are in top shape for early spring mating. Females, on the other hand, may maintain deeper torpor and save their food hoard for pregnancy and nursing in the spring when fresh food is scant.

In early March, the chipmunks stepped up their game. Even though we still had 18 to 28 inches of snow on the ground, along the Far Field Road I found a myriad of chipmunk tracks emerging from beneath logs and moving 100 feet or more before doubling back. Chipmunks rarely venture beyond 150 feet of their burrow entrance except when males are in search of females. Then they may briefly leave their home ranges of between 1075 square feet to 2.5 acres.

After a few minutes, from beneath a pile of fallen trees above on the road bank, I heard and then saw two chipmunks. Five days later, wherever I snowshoed, I found chipmunk tracks and holes through the ice and snow. In the woods near our deer exclosure, three chipmunks chased over the snow. Mating bouts were in full swing.

Since a single female is only receptive for about six-and-a-half hours, each one is beset by several suitors and mates numerous times. But once the eight-day mating season is over, the normally antisocial chipmunks return to their own burrows. Far more than one burrow is found on an acre in most forests because each chipmunk’s home range overlaps its neighbor’s by as much as 75% so there can be as many as 10 to 30 chipmunks an acre. And most of those chipmunks are related because young chipmunks usually set up home ranges close to their mothers and siblings.

Thirty-one days after mating, a female chipmunk has three to five young. She cares for them during their 44 days in the burrow nesting cavity before they emerge above ground. Then, except for giving the young access to her burrow or another she has dug for them, she ignores them. Instead, the siblings learn to forage, box, play, and aggressively fight and chase each other as preparation for their future solitary lives in which they are constantly defending their burrows and the territories around them.

They also practice their high-pitched, birdlike “chip, chip,” which may be uttered as many as 130 times a minute for at least 10 minutes. Their “chuck, chuck” may occur after an aerial predator crosses the area, because Lang Elliott, who studied chipmunks in the Adirondacks, noticed that when one chipmunk spotted a predator, it would start “chuck, chucking” and other chipmunks would respond in a warning chorus that spread across the forest. In any case, they whistle when fleeing a predator and during mating chases.

Although chipmunks have a second breeding period in July, the spring breeding period is more successful. Only some females breed during both the spring and summer and a previous year female may breed for the first time in July. Most have life spans of over a year, in time for one breeding, and few live more than two or three years, but there are those who have survived as many as 13 years in the wild.

I often wonder if that is the case of the chipmunk that eats our fallen birdseed in late fall and appears to have a burrow near the base of a veranda column. We sit on our veranda during warm autumn days as the chipmunk dashes past us, cheeks bulging with food it has gathered from our overgrown front yard, and once I put up our birdfeeders on the back porch in early November, that chipmunk crams its cheek pouches with oil sunflower seeds as late as December. Such oil-rich food keeps it well-fed even when the mast crop fails.

Still, we may be seeing a succession of chipmunks because burrows and good foraging areas are at a premium and young chipmunks are always looking for their own homes, so one will quickly occupy an empty burrow. This is especially true in autumn when the chipmunk population is at its highest point. Then the forest is filled with young chipmunks searching for burrow systems and establishing home ranges while older chipmunks are defending their territories and burrows from the youngsters. In addition, all of them are gathering food for winter.

In Pennsylvania, the favorite wild winter storage foods are beechnuts, maple seeds, acorns and hickory nuts. Throughout the rest of the year, chipmunks also eat insects, including cicadas, earthworms, fungi, especially the underground fungus Endogone, small snakes, birds and their eggs, snails, flower bulbs, frogs, unfolding new leaves, roots, seeds, salamanders, wild and cultivated fruits such as strawberries, blackberries, and raspberries, and young mice.

They, in turn, are eaten by large snakes, hawks, crows, foxes, bobcats, and domestic dogs and cats. But their most successful predators are weasels, which can enter their burrows and chase them down.

The eastern chipmunk’s scientific name, Tamias striatus, means “striped steward,” a tribute to their brownish black and white stripes and their food hoarding. “Chipmunk” comes from the Chippewa name for red squirrel “chetamon,” meaning “head first,” which refers to the way it descends trees. The Pennsylvania Dutch called it “fensermaus” or “fence-mouse,” because this wild creature also lives comfortably in human-modified environments.

The Delaware Indians of southeast Pennsylvania knew January as the “ground squirrel month” when chipmunks emerged from their burrows. However, the naturalist William Bartram from Philadelphia, back in the early nineteenth century, noted that “on February 17 the ground squirrel came out of his winter quarters frisking about in the warm sun,” proving that not much has changed in the lives of these fascinating creatures.