Wood Frog

Originally published in The Manitoban, January 26, 2011.

Hello and welcome to January, 2011. I was looking out the window earlier, enjoying the hoar frost, when it occurred to me that really, Winnipeg isn't so bad in winter. We give ourselves a lot of credit for how tough we are out here on the prairie, boldly heading out to party and bullshit in windchills that bring the mercury plummeting to -40°C but really, we don't know what tough is. We would not stand a chance in a Winnipeg winter without all our scarves, sweaters, Uggs, Emus, Fox Racing jackets, and opposable thumbs. If you wanna see some serious winter survival, look to those who must breathe through their skin, I say. Look to Rana sylvatica, the humble North American wood frog.

The wood frog is a small anuran, two to six centimetres in length, found throughout Canada and much of the mid-eastern United States. In fact, the wood frog is the only amphibian in North America that can live north of the Arctic Circle. It is that brownish little frog you have very likely seen before; the one wearing the cute little black mask.

During the spring breeding season, male wood frogs congregate in small ponds, working day and night to attract females. Their call, which sounds a bit like the 'quack' of a duck, is produced by inflating the lungs and expelling air at high pressure across the vocal chords. The neck pouch, or vocal sac, acts as a resonating chamber to amplify the frog's call. 

Tadpoles hatch in late spring and feed primarily on algae and plant materials, while the adult diet consists of invertebrates such as insects, worms, and snails. Adults have also been known to eat other small frogs on occasion. 

In order to deter would-be predators such as shrews and aquatic insects, adults and older tadpoles produce repulsive skin secretions. Adults also make a defensive call, more grotesquely referred to as a “mercy scream,” if they find themselves under attack by shrews; tiny but ruthless mammalian predators. 

This is all pretty great stuff, yet the coolest thing about the wood frog is arguably their capacity for freeze tolerance.

Freeze tolerance is the ability to survive the formation of ice crystals within the body. This is typically a bad state of affairs for living organisms as ice crystals can puncture delicate yet important tissues such as blood vessels and cell membranes, causing all the vital stuff inside to spill out—at which point, as you can imagine, chaos and death will ensue. 

There are all kinds of other negative side effects associated with ice crystal formation in the body. Forming ice will draw water out of cells, causing them to shrink and become damaged. If blood cells were to freeze, the delivery of oxygen and nutrients to vital organs would be interrupted, spelling out bad times for the future of said organs.

With all these crazy, deadly drawbacks, how can freeze tolerance be a good winter survival strategy, you ask? Why does the wood frog not simply borrow some fur from a fox, or some fat from a bear, you want to know? How the hell does the bloody frog just go and freeze until spring thaw like it ain't no thang?! I will tell you, of course.

Animals such as our hero the wood frog successfully engage in the practice of freeze tolerance by combining the ability to allow fluids outside of the cells (extracellular fluids) to freeze with mechanisms that prevent the fluids inside the cells (intracellular fluids) from freezing. 

The wood frog hibernates on land under leaf litter and produces special ice nucleating proteins that allow the frog to seed (start) ice crystal formation early; typically when the temperature reaches just below 0°C. By starting the freezing process at a higher temperature, the frog can freeze slowly, allowing time to make the metabolic changes necessary to survive the freeze. 

While frozen, the metabolic, breathing, and heart rates slow dramatically. The organs and tissues are well adapted to survive in spite of very low and slow rates of oxygen and nutrient delivery. In order to prevent the insides of the cells from freezing, wood frogs build up high concentrations of glucose sugar in their tissues. Water will be drawn out of the tissues and everything around the organs and tissues will freeze, while inside the cells and organs, the highly concentrated sugar solution will remain liquid, thus preventing freeze damage. In fact, wood frogs can tolerate blood sugar levels 100 times their normal amount, without ill effect.

There are many unknowns about the freeze-abilities of the wood frog, Rana sylvatica. The chemical signals which allow the frog to respond to its metabolic needs throughout the winter are not well understood, nor are the ways in which the frog can fine-tune the freezing process. For example, ice sometimes begin to form within the blood, an event that would be deadly, and yet the frog is able to make subtle changes in the concentrations of its bodily fluids that quickly break apart these unwanted ice crystals, leaving the remarkable wood frog frozen solid and ready to face another cold, winter day.