By Misi Stine, Environmental Educator and member of the MN Herpetological Society
Wood Frog
Dave Huth from Allegany County, NY, USA, CC BY 2.0 <creativecommons.org/licenses/by/2.0>, via Wikimedia Commons
Minnesota has fourteen species of frogs and toads that employ three primary overwintering strategies that help them survive our typical winters. The first is to dig down below the frost line, the depth varies from year to year based on the temperatures and amount of snow. The depth that allows them to be below the frost line can range from 6 inches to 3 feet deep. Second, some species spend their long winters brumating (similar to hibernating) on the bottom of lakes, rivers, and ponds. They are not sleeping but are less active during this time. At times, you may see them swimming around if you look through the ice.
The third strategy, which may seem a little unusual, is the strategy that stands out. Species like the spring peeper, boreal chorus frog, wood frog, and both species of treefrog (Eastern gray and Cope’s gray) will crawl under the leaf litter and freeze for the winter. The process that allows them to freeze for the winter starts in the fall when temperatures start to drop. These frogs then start to amass glycerol. This protects the cells from rupturing when the frogs partially freeze. In freezing temperatures, ice crystals form in cells when they freeze and rupture, leading to frostbite in human and other species.
When the temperatures are cold enough the liver converts the glycerol into glucose and circulates the glucose to all the frog’s organs so that ice crystals cannot form and damage the organs, essentially, they produce their own antifreeze. Their heart stops and their breathing ceases and they appear to be dead.
However, ice does collect in the cavity of the frog’s body, around the organs and between muscles. In 1982, research led by Bill Schmid at the University of Minnesota uncovered that up to 65% of their total body will freeze during the winter. The frogs can become as cold as 19 degrees Fahrenheit and can remain at that temperature for weeks. Once the temperature begins to warm, their heart starts to beat again, their respiratory function returns to normal, as their bodies thaw out and resume normal activity.
Which brings me to what is happening during this unprecedented winter weather. There have been reports every month this winter of reptiles and amphibians out and about when they would typically be brumating. Frogs that spend their winter frozen are having a particularly challenging time because they they are not staying inactive, which means they burn more energy without food available to maintain fat stores. This reduces their chance of survival while they wait until the insects become available in the spring. It is hard to say what the result of this unusual winter will on these species. If you see reptiles and amphibians out and about this winter, please do your best to not disturb them, further depleting their energy stores, if they are in the path of danger gently move them and cover them with leaves. Document your finding in citizen science apps such as, HerpMapper (https://www.herpmapper.org ) or iNaturalist (https://www.inaturalist.org/) the data that is contributed will help science learn more about these unusual trends in weather in a changing climate.
Photos by Misi Stine
Bibliography
Biological Miracle. (2022, October 6). Retrieved from National Park Service: https://www.nps.gov/gaar/learn/nature/wood-frog-page-2.htm
Emmer, R. (1997, November 24). How Do Frogs Survive Winter? Why Don't they Freeze to Death? Retrieved from Scientific American: https://www.scientificamerican.com/article/how-do-frogs-survive-wint/
Frogs and Toads. (n.d.). Retrieved from Minnesota Department of Natural Resources: https://www.dnr.state.mn.us/reptiles_amphibians/frogs_toads/index.html
Moriarty, J. (202, March 02). How Do Forgs Survive Winter? Retrieved from Three Rivers Park Distirct: https://www.threeriversparks.org/blog/how-do-frogs-survive-winter