How To Get Through Winter

HOW TO GET THROUGH WINTER 

November/December issue, 2006

It was well into winter about three years ago when I received an e-mail from a Tweed area resident about a frog she had found. She told of a wood frog, a name well founded in this case, as this specimen was well and truly, stiff as a board. There were ice patches on its back, and the eyes were glazed over. “I dared not touch it for fear it might shatter into a pile of dust!” she joked in her e-mail message.

However, inside there was doubtless life, as this little amphibian, one of the earliest to start calling in the spring, is one of Nature’s miracles. Because it chooses to spend the winter under some forest leaves or a log, rather than burrow down into the mud of a pond like other frogs, Nature has endowed this little frog, barely five centimetres in length, with the ability to survive the winter by freezing solid.

How does it manage to do this? By becoming sweet, and freezing into a frogsickle! When the freezing temperatures arrive in late November, the more sensitive eyeballs and extremities of the frog begin to freeze, triggering a message to its liver to convert glycogen to sugary glucose. Slowly, this glucose penetrates vital cells, preventing them from drying out. Scientists have discovered that wood frogs can easily tolerate blood sugar levels that are 100 times higher than normal, with none of the injuries that human diabetics suffer when their blood sugar rises by only a fraction of that amount.

During this period of suspended animation, up to 65 percent of the frog’s total body water is converted to ice. Muscle movements cease, everything becomes rock hard, and the heart stops beating. The frog appears as dead as the proverbial door nail. When the wood frog thaws, its heart starts beating again, and off he goes none the worse for the experience. We suppose it can do this repeatedly through the winter in response to winter thaws and returned freezes.

We learned in public school that most frogs and turtles burrow into the mud at the bottom of a pond to escape winter’s wrath. And indeed most do, but how do amphibians who depend on lungs during the warmer seasons, survive under water if they cannot breathe? Quite simply, they shut down their lungs, and obtain whatever oxygen they need during their low metabolic state through their skin. Hibernating frogs under water absorb oxygen and release carbon dioxide from the water through their skins. Even turtles are able to this through areas of the skin under their neck as well as their tail.

Amazing stuff, these natural processes. Even more amazing, however, are the insects. Such delicate creatures, and yet many of us don’t give them much thought. We know they disappear in the late fall, and return again in the spring, but what happens to them in the winter? Well, it depends on the species. We know the Monarch butterfly gets out of town by migrating to Mexico; and we have seen beekeepers cover their hives, leaving the bees inside, who heat up the house by remaining active, thus keeping the colony at a temperature much higher than the outdoor temperature.

Other insects handle the situation differently. They die. But not before thoughtfully packing their eggs with cryoprotectants such as glycerol or sorbitol, creating a syrupy solution, and further encasing them in an insulated capsule. No doubt you have come across the elongated cases containing praying mantis eggs, preserved in this manner but with the added protection of an interior which resembles brownish Styrofoam. We find them attached to plant stems in open fields. Other, like the black swallowtail butterfly, ensures the species’ existence by staying put in the pupa, while others winter over in the larval stage. The red admiral butterfly, for example, does this, or it may migrate as an adult, just to be on the safe side. The mourning cloak butterfly hunkers down behind a piece of loose bark somewhere and hopes for the best, wintering in this manner as an adult. The technique must work for they are among our earliest butterfly species to be seen in the spring as adults emerge from hibernation.

The most incredible though, is the goldenrod gall fly. We have all seen the bulbous swellings on goldenrod stems. Inside each of these is a gall fly larvae. Early in the year, the adult gall fly, barely the size of a pinhead, lays her egg on the stem of the plant. The egg hatches, and the larva burrows into the stem of the plant, the chewing and action of its saliva, thought to mimic plant hormones, resulting in the production of the galls which provide the larva with both food and protection. There it happily feeds and grows in this convenient pantry, passing through several larval stages, as it prepares to emerge in the spring. However, it has a very important chore to do first before it emerges as an adult.

To emerge, it must be able to chew its way out, and it cannot do this if it is an adult. To avoid being trapped in its own coffin, it needs to excavate a tunnel while it still has larval mouth chewing parts. The larva excavates a tunnel right to the outer edge of the gall, leaving only the plant epidermis remaining. Depending on large concentrations of glycerol as an intra-cellular anti-freeze, the larva allows most of its body tissues to freeze, but keeps the inside of its cells liquid. When spring arrives, the larva transforms into a pupa where the final transformation to winged adult takes place. Eventually, the adult fly emerges and crawls out to the end of the previously excavated exit tunnel. Here it anchors itself and pumps body fluids into a special portion of its head. This temporary protuberance bursts the outer skin of the gall and the fly pulls itself out, and flies away.

It has taken thousands of years for some animals to evolve successful ways of making it through the three lean months. While hibernation and bird migration is more clearly understood, there is a lot we still don’t know about how other species manage to survive the winter months in our area. The wood frog, and many insects though have proven that they have the skill down to a fine art.