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held at year-round constant temperatures of both 0°C as well as at 22°C in the lab. At a constant 30°C they no longer hibernated, although they still maintained their annual cycle of feeding and fattening that is normally associated with hibernation. These dramatic results showed that the squirrels consult an internal timer, and in analogy with the previously known circadian or daily rhythms, Pangelley and Fisher coined the word “circannual” (circa = approximately, annum = year). Such circannual calendars have since been demonstrated in the timing of bird migration and in other subterranean rodent hibernators. They play a role not only in preparation for deep hibernation but also in arousal from it. The groundhog or woodchuck is an especially well known example.

The groundhog is a large ground squirrel that according to the legend popularized by or of the old Pennsylvania Dutch settlers has an amazingly precise internal calendar. At least one of them, Punxsutawney Phil, emerges punctually from his burrow every year on the second day of February (in Pennsylvania) (at the precise time the news media start to roll TV cameras) to check if he can see his shadow to decide whether or not to go down and sleep for another two weeks.

A groundhog’s survival does indeed depend on accurate scheduling: The squirrel must synchronize its life with the availability of veggies. If possible, it feeds on lettuce, carrots, peas, beans, and other freshly picked produce. Its natural food, grass and weeds, will do only if it can’t get into a garden. In either case, food is available for only about a third of the year. Furthermore, greens don’t store well in the constant moisture under snow and in underground burrows. They’d quickly become a moldy mess. (A relative of rabbits, the pica that lives in mountain areas where there is more wind and dryness has hit on a solution. It gathers greens and dries them to make hay, which is stored and later eaten throughout the winter.) The groundhog has a different solution. It converts the summer greens not to hay, but thanks to a prodigious and adequately preprogrammed appetite, to thick rolls of body fat.

Obesity has its advantages, such as when the animal can be safely inactive in its den. For the rest of the time obesity makes the animal a considerably more attractive meal to predators, all the while compromising its speed and agility. To minimize its duration of obesity, the groundhog must maximize the speed and extent of becoming obese. To be successful in this endeavor, it delays fattening until near the end of the summer. So, it must not only know what to eat, it must also consult a calendar as to when to start eating as if life depended on it. Thus, as in the golden-mantled ground squirrel, a circannual clock is vital for its winter survival.

FOR PROBABLY THE MOST remarkable story of hibernation and winter survival of any mammal, I turn now to the arctic ground squirrel (Spermophilus parryii). This tawny-gold and gray ground squirrel with small white spots is larger than a chipmunk and smaller than a woodchuck. It is the northernmost mammalian hibernator across the Canadian and Siberian tundra. For eight months of the year this squirrel curls up into a ball close to the ice of the permafrost, and maintains a body temperature at or below the freezing point of water. Brian M. Barnes and colleagues at the University of Alaska at Fairbanks have for many years tried to decipher how these animals survive. They have studied them in the field at the Toolik Lake station at the foothills of the Brooks Range, and in enclosures and in the lab at Fairbanks.

Like other ground squirrels, this species digs hibernation burrows and builds underground nests. However, because of the permafrost of their environment, the squirrels cannot dig deep enough to escape the subzero temperatures of winter. Instead, in late summer and autumn, when the temperatures are merely freezing, they dig down into the soil. The temperature surrounding them declines through fall and winter and continuous records of body temperature