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Phenology Report, July 19 2022

Wood frog
Wood frog

Our phenology coordinator Sarah Mitchell is off this week, capturing dragonflies, listening to loons, and sleeping in her hammock. In lieu of the usual segment summary, she has left us an article about how amphibians manage to survive the cold Minnesota winters. Never fear, though- you can listen to the phenology report segment using the 'play' button at the top of the article!

This article originally appeared in Cedar Creek's Fall 2021 newsletter. It has been slightly modified for this purpose.
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Check in on our amphibians!

As Minnesotans well know, fall is a time to prepare for winter. Water-based organisms are particularly vulnerable as the weather turns: water expands when frozen. If this happens within a cell, the cell can ‘pop’ and internal cell structures are damaged beyond repair. This is the process that produces frostbite in humans, and we’re not the only ones vulnerable! To avoid this painful fate, trees shed their leaves, bears go into their dens, and snowshoe hares grow a thick coat of insulating fur. Many of these strategies and adaptations are things we can relate to as humans. However, one group of Minnesotans takes some unusual approaches: our amphibians have some novel and exciting tricks to escape the frost!

As temperatures drop and daylight wanes, Minnesota’s amphibians are also preparing for winter. There are 20 species of amphibians found in Minnesota (14 species of frogs and toads and 6 species of salamanders), and there are a variety of adaptations to the onset of winter! One relatable tactic is to avoid freezing temperatures. For instance, American toads, blue-spotted salamanders, and tiger salamanders have a relatively simple solution; they dig down in leaf litter and loose soil until they are below the frost line, and wait out the winter without danger of freezing. Others, like the green frog and the leopard frog, find a permanent body of water deep enough to avoid freezing solid, and spend the winter there- however, since they absorb oxygen through their skin, they do not burrow into the anoxic (oxygen-poor) muck. They can be found just above the bottom of the lake throughout the winter, sometimes even slowly swimming in the chilly water!

Other frogs have a more innovative solution. Wood frogs and tree frogs (including the boreal chorus frogs, spring peepers, Cope’s gray tree frogs, and gray tree frogs) wedge themselves into cavities and crevices, hide under leaf litter, or squeeze under tree bark. While these spots function as good hiding places, they are not protected from freezing temperatures. These frogs have adapted by pushing water out of their vital organs and replacing it with glucose (sugar) and urea, which act as a natural antifreeze. Thus, the cells in these vital organs are far less likely to freeze, even in very cold temperatures. Outside those vital organs, ice forms in the body cavity and extracellular (outside the cell) areas, though the size of the ice crystals is limited by another antifreeze compound. During this time, the frog’s heart does not beat, the lungs do not breathe- they are, for all intents and purposes, completely dead (colloquially, a frogsicle). However, with the advent of warm weather, their bodies thaw, and their organs resume their usual functions within just a few hours. These frogs are able to survive up to 65% of their body fluids freezing without suffering lasting harm!

There are a few lessons we can learn from these amazing creatures. From a philosophical standpoint, we learn that to survive hard times, it pays to fill ourselves with sweetness. If you prefer a more practical lens, researchers are studying this process in order to extend the viable lifetime of donated organs. Currently, donated organs are only viable for about four hours after removal. If scientists can learn to mimic the frogs’ methods, doctors could freeze organs without damaging the tissues, greatly increasing our ability to get life-saving donations to recipients across the globe. For me, knowing the extremes our local amphibians go through during the winter makes their springtime choruses all the more enjoyable!

As a mail carrier in rural Grand Rapids, Minn., for 35 years, John Latimer put his own stamp on a career that delivered more than letters. Indeed, while driving the hundred-mile round-trip daily route, he passed the time by observing and recording seasonal changes in nature, learning everything he could about the area’s weather, plants and animals, and becoming the go-to guy who could answer customers’ questions about what they were seeing in the environment.
Heidi Holtan is KAXE's Director of Content and Public Affairs where she manages producers and is the local host of Morning Edition from NPR. Heidi is a regional correspondent for WDSE/WRPT's Duluth Public Television’s Almanac North.
Charlie Mitchell (she/they) joined KAXE in February of 2022. Charlie creates the Season Watch Newsletter, produces the Phenology Talkbacks show, coordinates the Phenology in the Classroom program, and writes nature-related stories for KAXE's website. Essentailly, Charlie is John Latimer's faithful sidekick and makes sure all of KAXE's nature/phenology programs find a second life online and in podcast form.


With a background in ecology and evolutionary biology, Charlie enjoys learning a little bit about everything, whether it's plants, mushrooms, or the star-nosed mole. (Fun fact: Moles store fat in their tails, so they don't outgrow their tunnels every time conditions are good.)