Frozen Frogs: The Wood Frog's Unique Defense Against Winter Conditions

You might be surprised to learn that these freezing temperatures we’ve been having recently don’t bother wood frogs a bit.

Get ready to learn how to make frogsicles!

All you need is a wood frog (Lithobates sylvaticus), below freezing temperatures, and some leaf litter to house your frog. The first step is to make sure your wood frog has been well fed and has a healthy, fully-functional liver. Then, when temperatures reach below freezing, place your wood frog into the soil or mud and lightly cover it with some leaf litter. Let your frog sit for a couple of days and you’ll have a rock-solid frogsicle! No additional sugar needed, as the wood frog naturally produces a ton of glucose to prevent cellular damage from freezing temperatures.

**Disclaimer: Do not eat frogsicles! Frozen wood frogs are not intended for human consumption, rather as a method of defense against the winter’s cold temperatures.

Identifying wood frogs

The wood frog is a small species of frog that is distributed across Canada and the northeast United States, as well as down into the Appalachian Mountains. They are characterized by a thin white line on their upper lip, a black mask around their eyes, and a general red-brown coloration across their body. Preferred habitat includes moist forest woodlands, ephemeral wetlands, bogs, and swamps, though the wood frog prefers to spend most of its time on land rather than under water. While tadpoles, they are omnivorous and eat a variety of algae, vegetation, and other tadpoles of the same or different species. As adults, wood frogs are carnivorous and feed on invertebrates. 

Breeding & Brumation

Breeding migrations and locations are very important for the wood frog’s life history. Many wood frogs can travel long distances away, up to several hundred meters, from their breeding grounds, which are almost always woodland ephemeral wetlands and vernal pools. Adults will always return to the same breeding ground they went to their first time breeding, which is usually the breeding ground they were born in. A small percentage of juvenile frogs do travel to other breeding sites in order to facilitate dispersal. Wood frogs are also one of the first frog species to breed in the spring alongside the Spring Peeper (Pseudacris crucifer), which is thanks to their unique hibernation strategy. 

Most species of frogs undergo brumation, which is a period of slowed biological processes in response to cold weather and is different from the full dormancy period of hibernation. They settle at the bottom of water bodies and remain active, despite being considerably slowed down to conserve energy. This is designed so that they don’t fully freeze, which is very damaging to their cells and organs. Wood frogs take the exact opposite route for their winter survival. They let themselves freeze solid while remaining at the surface underneath the leaf litter. It sounds crazy, but it really works and it allows them to overwinter close to their breeding sites and thaw at the same time the ephemeral wetlands and vernal pools fill with snowmelt. This lets them breed much earlier than other frog species as the other frogs have to wait for the ice to melt from the surface of the water bodies to begin their migrations. 

Glucose is Key

So how in the world do these frogs survive being literally frozen solid? It’s all about glucose. Most people learn about glucose as the molecule that plants create during photosynthesis. But that’s only a small part of it, as glucose is one of the most important molecules for all living things. It is a simple sugar that provides the fuel for cellular respiration and  metabolism, which makes us all function. But most people don’t know that glucose is an incredible cryoprotectant, a substance that protects tissues like skin and organs from the damages of freezing. This allows the frog to be frozen solid, filled up to the brim with ice that permeates underneath their skin, in between their organs, and through their veins and not experience any damage. And don’t worry, that level of sugar isn’t going to be bad for the frogs. While humans aren’t able to withstand blood glucose levels over twice the norm, wood frogs can tolerate blood glucose levels up to 100 times their normal average. Now that’s a sweet frog!

Zombie Frogs?!

Wood frogs stay in this frozen state until they thaw, and they can be frozen for up to 8 whole months. They can’t move at all, not a single muscle. Even their heart stops beating while they’re frozen. These frogs are essentially dead during these frozen periods, the only life left occuring inside of the individual cells of the frog, which are surrounded by tons of glucose. When the temperatures warm up to the point that the ice in the frog thaws, the frog comes back to life. The heart starts beating, neurons fire back up, muscles are freed from their icy restraints, the organs start going about their normal functions. Once fully thawed, the once dead wood frog gets up and walks along its way to their breeding site, undamaged like nothing ever happened. 

Understanding more about these frogs is both ecologically important and important for humans. Conserving this species, which like all frogs is decreasing in population numbers every year, requires that we know as much as possible about their life histories, habitats, and survival mechanisms. For humans, knowing more about them could lead to significant breakthroughs in medical advancements. Most importantly, knowing how to freeze organs for storage and transport, which will make organ transplants much easier and more accessible. Also, by looking at the ways in which the wood frog tolerates such high blood glucose levels could be beneficial in finding ways to help individuals with diabetes manage their blood sugars. 

The wood frog is a marvel of evolution and adaptation, with one of the most unique biochemical defenses in the world. So next time you look out your window at the snow, think about the thousands of frogsicles sitting just below the surface, full of ice and glucose, awaiting their time to come back to life.