Backyard Biology

Nature stories from my backyard and beyond

Nature Stories

Wooly Bear Winter

Thursday, December 12, 2013

Wooly bears. Wooly bears everywhere. Every 20 feet, a wooly bear crossing the path. I had never seen so many wooly bears on one hike. Each and every wooly bear off to seek a safe place to wait out the coming winter before making the big transformation in the spring - the transformation to adult moth. What kind of winter are they hoping for? Would they prefer a mild winter, one with a few warm days interspersed with some cold? Or are they hoping for a deep freeze lasting the whole winter? Or does the type of winter even matter to them? Of course, these wooly bears could not hope for any kind of winter because they had never experienced a winter before. So they will just have to take whatever Mother Nature sends their way and pray they are equipped to deal with it.

According to folk legend, wooly bears can predict the severity of the coming winter. The wider the brown band the milder the winter, and the narrower the band the more severe. According to scientists, there is no validity in this legend. In fact, some say they are better at predicting the past than the future. There is some evidence that the amount of brown is due to their age. The older the wooly bear, the more brown. Of course, in a very severe winter where the cold temperatures last later in the year, the wooly bears will awake from their winter sleep later in the spring. This means that the growing season for the two generations of wooly bears in the upcoming summer will be shorter. The second generation - the one that must prepare for the winter will be younger. These wooly bears should have a smaller brown region, thus denoting a long winter the year before. Of course, predicting the severity of the last winter is not as compelling a legend as predicting the winter to come.
Regardless of what the wooly bear tells us about the winter past or present, what kind of a winter is best for the wooly bear? In other words is a wooly bear more likely to live through a cold or mild winter? To understand this, we need to know how a wooly bear survives the cold.

The wooly bears we see crossing the path in the late fall are looking for a place to hibernate for the winter. They seek a sheltered spot under a log, a rock or the leaf litter. Here they are protected from the harshest of winter weather, but still must contend with below freezing temperatures. As the temperature drops, they begin to produce a chemical called glycerol. Glycerol is a sugar alcohol compound that we humans use as a sweetener and food thickener. In the case of the wooly bear, and many other insects, glycerol lowers the freezing point of the water in their body. Rather than freezing at 32 degrees F., the water remains liquid until it drops to about 17 to 21 degrees. Once it reaches this temperature, it freezes.
Unlike us humans and other warm-blooded animals, the wooly bear can survive this freezing. In fact, it is crucial to their winter survival. Wooly bears, and many other insects, have evolved strategies to allow the water in their bodies to turn to ice without damaging their cells. This is important, since wooly bears inhabit some cold climates, being found up into northern Canada. In fact, the scientific name of the wooly bear reflects this cold lifestyle. Pyrrharctia. Any animal with “arctia” in its name must be cold hardy.

So back to the original question. What kind of winter is the wooly bear hoping for? Would they prefer a winter of warm days interspersed with the cold, or a deep freeze? It turns out a good old-fashioned cold winter is better for the wooly bear. Studies have shown that their survival rate is better if they undergo a single prolonged freeze rather than a series of freezes and thaws. In one study, the wooly bears experienced a 30% mortality from alternate freeze and thaw compared to a 10% mortality from one continuous freeze.

As we here in New England experience warmer, milder winters due to climate change, there are many plants and animals that will find this change more to their liking. The wooly bear may not be one of them. The constant freeze and thaw of a mild winter may kill off more woolly bears. At some point, the wooly bears that we all enjoy watching crossing our path in the late fall may become a rare sight on a late fall hike due to the milder winters to come.