The wood frog is one of the hardiest amphibians in Canada, ranging farther north than any other frog on the continent and turning up even beyond the Arctic Circle. It is a small, brown creature, marked by a dark patch across the eyes that gives it the look of a masked bandit. For much of the year it lives quietly among the leaf litter of northern forests, breeding each spring in the shallow meltwater pools that dot the woods. Unlike many of the creatures it shares those woods with, it makes no attempt to slip below the frost line when winter closes in.

Instead, the wood frog survives the season by doing something almost no other backboned animal can manage: it lets much of the water in its body freeze solid. As the cold takes hold, the frog's liver pours a flood of sugar into its blood, and this sugar works as a natural antifreeze, shielding the cells while ice forms in the spaces around them. The heart slows and then stops, breathing ceases, and for weeks the frog lies stiff and lifeless beneath the snow. When the thaw comes, it warms, its heart starts again, and within hours it hops off as though nothing had happened.