A long, and rare, winter's nap: The groundhog is one of only a few animals in our area that hibernates

I've been talking about raptors to school groups for decades.

At one point, I ask the students what the prey animals - i.e. mice, rabbits, chipmunks - are doing in spring. Inevitably, a student will say "coming out of hibernation."

That just isn't true for most of our familiar resident mammals. Let's take a closer look at one local mammal that does hibernate and is celebrated at this time of year, the groundhog, aka woodchuck.

Hibernation defined ... sort of

While some mammals are serious sleepers during the winter months, that is not the same as hibernation. True hibernation, according to many experts, is a prolonged state of suspended animation when a mammal reduces its pulse and breathing rates to barely discernible levels.

This sluggish metabolism also lowers the animal's body temperature from perhaps 90 degrees Fahrenheit to 39 degrees.

That said, some authorities use a looser definition which I think accounts for the fact that not everyone agrees on which animals hibernate and which don't, but we'll get to that later.

How groundhogs prepare to hibernate

Differing definitions aside, most everyone agrees that our groundhog (Marmota monax) is a true hibernator. Its body temperature drops from 98 F to 38 F, while its heart rate decreases from 80 beats per minute to just four or five beats per minute.

In addition, a woodchuck's respiratory rate slows from 25 to 30 breaths per minute to one breath every five minutes.

Groundhogs start to accumulate excess fat in August and September for utilization while they hibernate. Adults begin hibernating in October, while yearlings and juveniles wait until November. Hibernation usually ends in mid-February to early March.

Although woodchucks can lose up to one-third of their body mass while hibernating, not all of their stored fat will be used during this time. In fact, some of that fat helps the now-active animals get by in early spring when food is still scarce.

Whether it is spring or fall, woodchucks are always gnawing on plants. So, like other rodents, most notably the non-hibernating beaver, they have incisors that are continuously growing.

If you are constantly chewing it certainly is handy to have constantly growing teeth unless, of course, you are hibernating for months at a time. Not to worry, Mother Nature has allowed for this.

The woodchuck's teeth stop growing during hibernation.

Some processes, such as digestion, continue during hibernation. This means that every few weeks the groundhog needs to take a bathroom break, and no, this is not how Groundhog Day got started.

You see, woodchucks have indoor plumbing. To be more precise, they have a dedicated toilet chamber in their burrow. Speaking of which, woodchucks plug the entrances to their burrows just before hibernation begins.

Other true hibernators

Most of the other few true hibernators such as bats, jumping mice, and ground squirrels are rarities in our area. One of them, the Franklin's ground squirrel (Spermophilus franklinii), is listed as a threatened species in Illinois.

This species was named after arctic explorer Sir John Franklin, which brings up an interesting point. The farther north you go, the fewer hibernators you'll find. At first, this seems counterintuitive. After all, shouldn't there be more hibernators where the winters are more severe?

It is thought that the increased risk of freezing in your burrow and the inability to store enough energy to endure the very long winters makes hibernation a risky survival strategy for denizens of northern latitudes.

The confusing case of chipmunks

Which brings us to the confusing case of chipmunks (Tamias striatus). According to Joyce Hofmann, author of "Field Manual of Illinois Mammals," chipmunks do hibernate. However, Donald Hoffmeister, author of "Mammals of Illinois," makes a convincing case that chipmunks are not true hibernators. He writes:

"Unlike hibernators, eastern chipmunks do not put on fat during the fall, do not reduce their metabolic rate for long periods of time, and are active at various if not all times during the winter.

"A state of inactivity in chipmunks is best described as torpor, not hibernation ..."

The trouble seems to lie with the word torpor. Hofmann defines hibernation as "a prolonged period of torpor during the winter." So, by her definition, chipmunks do hibernate. I prefer the more detailed definition of hibernation offered at the outset of this article.

Not to belabor the point, but the definitions of torpor don't all agree either. I would define torpor as being in a dormant state.

You are what you sleep

For those of you who care for lawns or gardens, the mention of chipmunks and woodchucks might prompt a grumble or two. But if you can be objective for a second, think about what these two critters are often eating.

I expect you'll agree that groundhogs prefer herbs and grasses while chipmunks gather seeds and nuts. Now, which of these foods would best survive winter storage?

Is it any wonder that the woodchuck stores its greens as fat on its back while the chipmunk maintains a food cache in its burrow?

So then, the chipmunk is able to raid the fridge when it gets hungry, which would be difficult to do in a state of suspended animation.

Whatever sleep it is

In his poem After Apple-Picking, which concludes this story, Robert Frost is not too worried about the differing definitions of torpor and hibernation. He is, however, musing on the nature of sleep.

This makes me wonder what woodchucks muse upon as they prepare to end their monthslong hibernation.

"One can see what will trouble

This sleep of mine, whatever sleep it is.

Were he not gone,

The woodchuck could say whether it's like his

Long sleep, as I describe its coming on,

Or just some human sleep."

• Mark Spreyer is the executive director of the Stillman Nature Center in Barrington. Email him at stillmangho@gmail.com.