First, some definitions: hibernation, colloquially-speaking, is the long, uninterrupted sleep that animals undertake to avoid the winter. More correctly, hibernation is one kind of dormancy along a continuum—kinds of deep rest that muddle across periods of time and kinds of organisms, including: torpor, estivation, brumation and diapause. But let’s focus on hibernation2. Two important things happen in hibernation that set it apart from the others I’ve just listed: it involves a lowered body temperature and a lowered heart rate. In turn, this means a lowered metabolism.
Now here in Southern Ontario, we have few animals that enter true hibernation. Bears (and in our case we’re talking Black Bears or Ursus americanus) do not enter true hibernation. But Groundhogs (Marmota monax, pictured below), Eastern Chimpunks (Tamias striatus) and Little Brown Bats (Myotis lucifugus) do.
Now our enjoyment of, as a species of naked apes, the winter season may lead us to believe that other animals enter hibernation to avoid the same cold weather that (some of us) suffer though. The truth is somewhere else—these hibernators have depressed their metabolism so they can avoid having to find food. So, rather than an inability to deal with cold weather, it is the lack of food that has driven the evolution of entering a physiologically dormant state when food disappears.
We’ve (the naked apes in Souther Ontario) have been enjoying a warm period this January and its supposed to get as warm as 6 degrees Celsius on Sunday. Before we tackle the larger question of climate change, what might a warm period like we’ve been having do for those animals hibernating right now?
I’m turning to the great book Winter World: The Ingenuity Of Animal Survival by Bernd Heinrich to help me answer the question and I’m going to focus on the ground squirrels (the Chipmunk and the Groundhog) just to simplify things. Chipmunks and Groundhogs, though they both hibernate, show different strategies in preparation. Simplified: Chipmunks collect food and Groundhogs get fat.
In fact, Heinrich writes that a chipmunk’s “availability of stored food affects whether is remains active or enters [hibernation]” (p.99) meaning that in years with lots of available food, Chipmunks can be active the entire winter and don’t need to suppress their metabolism. This was the subject of an entire doctoral dissertation, so I can’t possibly give a nuanced answer, but being active in the cold seems to be associated more with lots of stored food rather than warmer temperatures. So, for Chipmunks, it will be some combination of the two factors (food & cold) that will predict the depth and length of their hibernation. In fact a Southern species, the California Chipmunk (Tamias obscurus) does not hibernate at all.
Groundhogs are different. They follow something called a circannual calendar—named, to continue our connection to Southern Ontario, by two researcher from the University of Toronto—which means they prepare for, enter and exit hibernation on an internal clock, independent of external conditions.
So, what do these intermittent warm periods do? For Groundhogs, nothing, as they’re busily following the ticking of their circannual clock, waiting for it to rouse them sometime in mid to late February. Like most things in the natural world, there is an exception to Groundhog’s circannual clock: in the U of T experiments, when the air temperature remained above 30 degrees Celsius, the Groundhogs did not enter hibernation. Chipmunks, on the other hand, are easy to rouse, even when in hibernation. Does this mean a longer period of warm weather would wake them up? Perhaps. Likely, those that are active are the Chipmunks with access to food in their cache, so being active, and the higher metabolism, isn’t as serious.
What would a warming climate do? This is where the guess work comes in. Over a long period of time, I would guess that the Eastern Chipmunk would come to resemble its southern cousin and not hibernate at all. For the Groundhog, it would seem that the climate would need to warm a great deal—above 30 degrees Celsius in the wintertime—for them not to hibernate. That’s not an immediate concern right now, and if ever Southern Ontario’s winter temperature ever got to 30 due to global climate change, I think the lack of hibernation would be the least of a Groundhog’s worries.
In the short term, it’s a bit more interesting to guess. Different populations of Groundhogs emerge at different times. This is evident in the fact that Groundhog day falls on February 2nd even though our Southern Ontario Groundhogs emerge later in February. February 2nd is the time of year the southern Groundhogs, specifically those in Punxsutawney, Pennsylvania, generally emerge. This shows some plasticity in the length of that circannual calendar. I would suspect with a warming climate that the clock would begin to change—Southern Ontario groundhogs entering their den later in the year and emerging earlier. Would these climatic changes occur faster than that the circannual calendar could change? I don’t know. For Chipmunks, it would be similar to the long-term changes. Eastern Chipmunks, I suspect, in combination with access to food, will be more active.
What all of this points to though, and what I haven’t really addressed in relation to global climate change, is that access to food continues to be the most important factor associated with hibernators. The question that then needs to be asked: how with global climate change, change the plant communities of Southern Ontario and what impact could that have on the mammals that currently live here?
Perhaps I’ll take a stab at that next time…
- As aside, here’s why Twitter is awesome—before Twitter, how or where else do you get to have an informal chat with the executive director of Ontario Nature and the founder of environmentally-focused career site? [↩]
- If you want more in-depth, scientific information, I suggest this open learning source [↩]