Our recent snowstorm will make it a bit more challenging for male Woodchucks intent on mating, for they must work their way up through a foot or more of snow upon wakening. In March and April they come out of hibernation having lost 20 – 40 percent of their weight over the winter. Even so, sex is the driving force, not food, which is fortunate, as there is little for them to eat this early in the spring. Males dig their way out of their burrows and head straight for the burrows of females. After mating, the female goes back to sleep for several weeks and the male returns to his burrow and does the same. Snow makes these tunnels much more obvious and thus easier to find, as dirt is scattered around their entrances. Equally obvious are the muddy trails males leave when in search of females.
As fun as it would be to see a Woodchuck on Groundhog Day, it’s not likely to occur in New England, even taking into account climate change, at least not yet. Rather than migrate or remain active and adapt to winter conditions, Woodchucks lower their metabolism and hibernate through the winter. Their heart rate, body temperature and breathing rate are decreased significantly in order to conserve energy.
One assumes that once a hibernating animal’s metabolism is lowered, it remains lowered for the duration of hibernation. However, it turns out that this is not the case. All of the species of hibernators that have been studied have woken up periodically throughout the winter and warmed themselves up. During these two-three day (on average) arousals, the Woodchuck’s body temperature (roughly 38°F. during hibernation) rises to 98.6°F., its normal temperature during the summer. During these arousal times Woodchucks do not eat. Rather, they rely on deposits of stored body fat, which results in their losing about 40 percent of their body mass by the time green plants are available in the spring. Woodchucks’ bouts of hibernation are initially short, then they lengthen to an average of eight days, and then shorten again as the season progresses.
Arousal consumes a lot of energy (a single arousal may consume as much energy as ten days of hibernation) so it must have a crucial function. Theories regarding this function include restoration of depleted nutrients in the blood, invigoration of immune system, elimination of toxic substances, dealing with potassium loss and facilitation of sperm production in males. As of today, however, the reason for this arousal phenomenon has not been determined.
Rodents have two pairs of incisors that oppose each other; two in the front of the upper jaw and two in the front of the lower jaw. These four teeth never stop growing, and thus have to be constantly filed down. (Squirrel incisors grow about one-half inch a month.) While nipping herbaceous plants and cutting into bark help do this, it isn’t enough to significantly wear down the teeth. When mice, beavers, woodchucks and other rodents nip or gnaw food, they move their jaws in such a way that these two pairs of teeth grind against each other. Most rodents have hard orange enamel on the outer side of their incisors (woodchucks are our only rodent with white enamel), and softer dentine in the back. When the incisors grind against each other, the dentine wears away faster than the enamel, creating a sharp, chisel-shaped edge to the incisors. This, more than anything else, keeps the growth of rodent incisors under control, just as a nail file keeps our nails from getting too long.
If a rodent breaks an incisor, there is nothing to wear down the opposing incisor. The tooth opposite the missing incisor will continue to grow unchecked in a circle until it causes the death of the rodent, either by piercing its skull or by preventing the animal from being able to eat. In the pictured woodchuck skull, the lower left incisor has broken off, allowing the upper left incisor to grow through the woodchuck’s palate and into its brain.
Woodchucks are one of the few species of mammals that enter into true hibernation. When the temperatures dip into the 40’s, usually in October or November in the Northeast, most woodchucks leave their summer burrows and head for the woods, where they dig a tunnel that ends in a chamber that is well below the frost line (and therefore above freezing). Here they curl up in a ball and live off of the 30% additional body weight they put on in the fall. In order to survive until March or April, a woodchuck’s metabolism slows way down. Its heartbeat goes from 100 beats a minute to five, and its body temperature goes from 96 degrees F. down to to 47 degrees F.
As is true for Black Bears, if climate conditions are mild and food (such as remnant corn in fields) is available, Woodchucks have been known to remain active year round in parts of their range. However, they typically hibernate during the winter in underground burrows, living off the fat (equaling about a third of their body weight) they accumulate in late summer and fall. In Pennsylvania, where Groundhog Day is first referenced in North America, male Groundhogs, or Woodchucks, emerge from their burrows at the end of January or beginning of February. In northern New England, however, we don’t usually see signs of this largest member of the eastern squirrel family, Sciuridae, until the end of February or beginning of March, when males actively start searching for mates. Currently our Woodchucks are still curled up in a ball hibernating beneath the ground, with their heart rate reduced from 100 beats a minute to 15, and their body temperature down from about 96°F. to 47°F.