Now is the time to start looking for thawing “frogsicles”. Some animals migrate to warmer environs during the Colorado winter and others burrow deep underground or in protected tree cavities to sleep until spring. One—a single species—freezes and waits out the winter in suspended animation. Wood frogs are found throughout North America, north through Canada, at all elevations up to tree line. They winter on land, hiding under leaf litter, where they actually freeze and “defrost” with their surroundings. They “thaw” with the snow melt and breed in the temporary ponds and ditches formed by melt water.
It’s not science fiction—it’s been happening for eons. The wood frog belongs to a small group of animals that are freeze tolerant. As the temperature drops below freezing each winter, two-thirds of the water in its body crystallizes into ice. Sound uncomfortable? For you and most other living things, this would be fatal.
Ice in body tissues can do a lot of harm. Ice crystals can puncture small blood vessels, and squeeze, deform, and shatter the cells that make up every organ in the body. Cells that aren’t broken are too dehydrated to function. Frozen blood cannot deliver oxygen and nutrients to organs. So what tricks of physiology allow these frogs to survive multiple freeze/thaw events during winter?
As soon as water in its surroundings turns to ice, the wood frog’s skin freezes. The frog becomes hard and crunchy. Special proteins in their blood, called nucleating proteins, cause the water surrounding the cells and in the blood to freeze first. Ice penetrates though all of the fluid compartments of the animal and within just a few hours a mass of ice fills the abdominal cavity encasing all the internal organs. Large flat ice crystals run between the layers of skin and muscle, and the eyes turn white because the lens freezes. This ice starts to suck the water out of the frog's cells. Luckily, during this same time the frog's liver makes large amounts of glucose—a type of sugar—and packs it into the frogs’ cells. The concentrated sugar solution helps prevent additional water from being pulled from cells, which can destroy them. Inside the cells there's thick sugary syrup, while outside the cells all the water is frozen. Breathing, heartbeat, and muscle movements all stop and the frozen "frogsicle" enters a state of suspended animation.
When temperatures warm and the ice melts, the frogs thaw. Water slowly flows back into the cells, the heart begins to pump, and blood starts flowing again. Clotting proteins rise in the blood so that any bleeding from broken cells can be quickly halted. As the frog revives, it starts to gulp, then breathe, and then the frog continues on its happy-go-lucky way.
Wood frogs’ choruses can often be heard over a month before Colorado’s other aquatic frogs, such as leopard frogs, break free from their ice-locked lakes and rivers to begin their own breeding. The wood frog’s breeding call sounds like a duck chuckling—a soft, ongoing quacking.