By Bob Engel
Marlboro College Professor Emeritus of Biology and Environmental Science
The average high temperature in Albany, New York has risen from 31°F in January to 46°F on March 20th. Average lows are in the mid-twenties.
Here in Marlboro, a place of never-ending winter, we’ve had almost a week of scattered lows that were at or just below zero. Some of our recent highs were below Albany’s normal lows. On March 14 we were buffeted by a nor’easter with high winds and maybe 18 inches of snow.
March is a tough time for animals. The easy food is long gone and almost nobody is carrying much body fat. Some are actually digesting their muscle tissue; the quest for energy becomes unrelenting. But what if you’re inactive or hibernating? How do you deal with temperatures at or well below freezing? How do you prevent the rupture of cell membranes in tissues that get below zero? It’s just like a beer bottle left outside. When water freezes it occupies 10% more volume than it did as a liquid. Pop goes the bottle cap and out comes a plug of beer. Cell membranes could also rupture. Bummer.
So that’s our question for today. How much freezing, if any, can a dormant or hibernating animal tolerate? And how do these critters prevent becoming partly or mostly frozen?
Let’s look at the obvious, first. All resident birds keep their body temperatures well above freezing, usually just a bit above 100°F. A few, like the Black-capped Chickadee, drop their temps at night to conserve energy (like we do with our thermostats). But then they go right back up to “hot” for the next harried day of foraging.
Just a few of our mammals are true hibernators (e.g., a few bats, woodchucks, and a few obscure rodents like jumping mice), meaning that they drop their body temperatures to 34°F up to 40°F, and are virtually inert. A pile of sawdust placed on a woodchuck stays right there until mid-spring. But hibernators do not freeze and they will elevate their body temps a bit if they happened to have not chosen a good spot to spend winter. Intermittently active types like chipmunks do not hibernate in this sense. Bears stay inactive but hardly drop their body temperatures. This is not an option for small hibernators—they’d run out of stored energy. To be sure bears are ready for anything even vaguely digestible when they emerge.
That leaves the “cold-blooded” types like herptiles and arthropods, and this is where is gets interesting. Their body temperatures drop to whatever their ambient temperature is. But freezing may not occur even if the ambient temperature is well below freezing.
I used to worry about some frogs like spring peepers and wood frogs. The story was that they snuggled down into the leaf litter (thereby getting some insulation) and then “hoped” for snow that provided more and prevented freezing. But what about cold periods without snow? Should we assume that some freezing will cause tissue damage and elevated winter mortality rates for these creatures? Well, no.
Several lab studies have shown that a variety of frogs can tolerate total freezing in a few larger biological compartments, but not all. In some species, the duration of the freeze is important; a close relative of the peeper, for example, can tolerate freezing for about a week. But what about a longer period? Then the trick moves to preventing freezing in more critical body compartments like those that contain important organs.
Almost all of the “cold blooded” critters produce some kind of antifreeze. It’s just like what’s in your car’s radiator; freezing is precluded by preventing ice formation or ice crystal growth. The first antifreeze identified was glycerol, a pretty simple three-carbon molecule that prevents ice formation by bonding to water molecules. It’s common in polar fish and a lot of other things. We have since identified sugars like glucose (in the wood frog) and a variety of proteins that bond to several sites of ice crystals, preventing further ice growth. These proteins are found in everything from insects to reptiles. Proteins are expensive to make (glucose is robbed from the liver), but they prevent even more expensive outcomes like death.
There is one more biochemical trick: the antifreeze proteins and sugars also exert an osmotic pull on water surrounding organs like the heart and digestive glands. The resulting “dried” organs are much less likely to freeze.
Even with these molecular solutions, reptiles and amphibians start to peel away as it gets colder with latitude. Southern Canada is about the best they can do (woods frogs and a few other “true” frogs, and the common garter snake are exceptions). But insects go much further north. The champion is a gall producer in Arctic willows (appropriately called the willow gall fly). As a larva, it can survive minus 60 °C (-76°F)! It’s body fluids are half water and half glycerol! It stole all that carbon from the willows, who took it from the air. It’s range includes Alaska; I don’t know if it goes all the way to the end of land in Greenland. The Arctic willow does that, which is as far north as any woody plant goes. Tough bugs; tough plants.