Hibernation

Even though many mammals of this region disappear from sight for several months of winter they don’t hibernate any more than people who remain in their homes or, better still, travel to the South where the air is warm, birding is wonderful, sailing is great, golfing is good and the food delectable.

Take a Chipmunk for example. Those in our yard usually retire by early November when continuous freezing occurs. The first “chippie” here appears around the middle of March as fleet-footed and pert as the day he thumbed his nose at winter and vanished underground.

A curious, Thirteen-lined Ground Squirrel often called Gopher.

Their so-called hibernation can be quite variable. These well-liked mammals go into a fairly normal slumber, very likely on top of their well-supplied food cache in their underground chamber.

Thirteen-lined Ground Squirrels, commonly referred to as Gophers, are true hibernators. They may spend as much as seven months or more in their subterranean winter quarters. Studies indicate that their winter body temperature drops to only a few degrees above freezing. Imagine an annual temperature range from about 36 degrees F in winter to 105 degrees F in summer. Their heartbeat falls from between 200 and 400 per minute in summer to about five per minute in winter.

The study of hibernation has fascinated people for centuries. Why should an Arctic Fox remain an active hunter even at a phenomenally low air temperature of minus 60 degrees F, while a very plump well-fed Woodchuck living along the Niagara Escarpment in Door County may retire to its winter den by late September in the midst of plentiful green food?

Actually very few warm-blooded animals enter a true state of hibernation. Complicated body changes will occur involving the amount of insulin in the bloodstream, functions of body muscles and brain, and the elimination of body wastes. Cold weather and a shortage of suitable food surely must be two important factors known to trigger hibernation. Internal seasonal rhythms, like built-in biological time clocks, have also been found to influence animals into entering that unusual condition of dormancy when bodily functions and activities are reduced considerably. It is one of nature’s ways of permitting survival during times when food is difficult to obtain.

North and Central America lay claim to about 3,700 of the world’s 17,000 to 18,000 mammals. Eastern Wisconsin lists about 39 wild mammals. Twenty-six of these 39 remain active throughout the year. They include the White-tailed Deer, Coyote, Cottontail, all squirrels, Deer Mouse and Shrew. Others in this active group are the Beaver and Muskrat whose activities are confined to that zone beneath the snow and ice. A Muskrat can swim as much as 180 feet under water while remaining submerged for up to 15 minutes.

Six species go into a normal slumber and generally remain out of sight for most of the winter. They are the Chipmunk, Badger, Skunk, Black Bear, Opossum and Raccoon. The Raccoon, like the Black Bear, accumulates plenty of fat and then goes into a sleeping period. They will occasionally venture out during warm winter days.

A fat Woodchuck on our lawn.

The remaining six are true hibernators, the Little Brown Bat, Big Brown Bat, Red Bat, Woodchuck, Gopher and Hudsonian Meadow Jumping Mouse. Even though all are considered to be warm-blooded creatures they are essentially cold-blooded as well, capable of turning down the setting of their thermoregulators.

One Woodchuck, that proverbial star of Groundhog Day, was studied during the coldest weather of its long winter sleep. The researchers discovered that its heart was beating once every five minutes! What a perfect example of conservation of energy. Punxsutawney Phil of Gobbler’s Knob in Pennsylvania has been right 39 percent of the time, surely not accurate enough to bet the farm on! Obviously this Woodchuck star of February is more about style and ecotourism than science.

The bats’ normal breathing rate of 200 breaths per minute will be reduced in winter to around one breath every five minutes. A normal summer temperature of 104 degrees F will now hover near 42 degrees F. It is only this hibernating temperature that will prolong the lifespan of a bat up to 15 years compared to the high-strung non-hibernating shrew that will be lucky to reach one and one-half years of age.

Some deep hibernators nearly wake up several times during the winter, as though practicing for the real thing. The heartbeat of, for example, a gopher, increases from four to around 300 or more per minute over a period of a half hour. This requires considerable energy, and the tiny animal may not have enough energy left for the real thing in May. It is felt that many wintering creatures, be they cold-blooded or warm-blooded, never make it through the winter in their state of suspended animation.

Don’t forget about the hundreds of other animals that hibernate such as snails, slugs, and many insects. One insect that proves to be as amazing as it is perplexing is the ordinary House Fly. Thousands must spend the winter between the walls of some buildings. It was while Charlotte and I were living in the Rangelight Residence at the Ridges in Baileys Harbor during the 1970s that scores would squeeze to daylight through the tiniest cracks. We swore that 99 percent would find their way into the house rather than outdoors from where they had come.

A young Chipmunk.

Studies have shown that when invertebrates, such as flies and butterflies, do “freeze” only abut one-half of their body fluids freeze. As long as their vital inner body fluids remain unfrozen the insects will survive in a dormant state. Three butterflies of this region that frequently hibernate in Door County are the Mourning Cloak, Painted Lady and Milbert’s Tortoise-shell. One spring, according to my journal, we sighted our first Painted Lady while we photographed and studied Skunk Cabbages in Southern Door on April 3.

It has been said that there is a world of difference between the first day of spring and the first spring day. Even though we thoroughly enjoy most aspects of winter, we long for our own private and official first spring day. It is when we’ve put the snow shovels away, placed the rain gauge back in position on the upper deck railing and seen our first chipmunk. It is then that spring has sprung!