Home / Blogs / Surviving Winter: The Amphibian Way
February 6, 2024
Wood frog (photo credit: PA Fish and Boat Commission).
Winter is a time of seeking warmth, adding layer upon layer before hurriedly shuffling to our cars to crank the heat to full blast and fire up the heated seats. Ohhh the heated seats; whoever thought of those was an absolute genius. As humans, we have evolved so that our key to surviving the winter in modern times is to go outside as little as possible and stay by the fire, if you’re lucky like me, or turn the thermostat to 75 and snuggle up under your favorite plush blanket. Every year (well, except last winter, so most winters) reminds us how harsh the cold can be, including this year when we had a few days of single-digit and negative real feel days. From inside my cozy abode on one of those frigid days, I was thinking how comical it would be to see a toad hopping along on top of the snow which landed me on the question I’m sure all of you are thinking too: how do amphibians survive winter? Well, I’m glad we’re asking the same question because boy, do I have some answers for you!
For those who don’t know, amphibians include our slimy friends: frogs, toads, salamanders, and newts – each of which are ectotherms (cold-blooded) and typically have aquatic, gill breathing larval stages usually followed by a terrestrial, lung breathing adult stage. Ectotherms cannot internally regulate their body temperatures like mammals can and instead regulate their body temperatures based on external factors such as the environment they are in. Right! Now I bet you’re now really thinking how in the world do amphibians survive the winter? Amphibians being coldblooded makes the question that much more intriguing.
If you want the short, dull, unenthusiastic answer it’s brumation.
If you want the short but enthusiastic answer it’s BRUMATIONNNN!
The longer answer is a bit more fascinating. Brumation is a semi-dormant, sluggish state in cold weather in which the heart rate and metabolic rate of the individual slows drastically to conserve energy. The individual can increase its activity periodically to find food. Brumation is to ectotherms (cold-blooded) species what hibernation is to endotherms (warm-blooded) species; however, hibernation is more severe (slows metabolic rate more, relies on stored fat, and is minimally active).
Brumation locations can look different based on the species. Aquatic species (American bullfrog, leopard frog and pickerel frog, etc) settle into brumation underneath frozen water. Terrestrial species (American toad, spotted salamander and dusky salamander, etc.) burrow into the ground below the frost line, find deep nooks in rocks, use rodent’s burrows, or crawl under other organic materials to create solitary or communal hibernacula.
To explain brumation a bit further I’ll use a notable species – the American bullfrog (Rana catesbeiana). American bullfrogs during the warmer months are typically found around larger, more permanent bodies of water: lakes, ponds, and marshes. The same is true during the colder months as bullfrogs overwinter in the depths of these bodies of water. They will swim to the bottom of the body of water where the temperature stays warmer than the water at the surface due to the dynamics of the water (another fascinating topic). Once at the floor of the lake or pond, the bullfrog will settle into brumation. Unlike turtles, they don’t burrow fully into the mud because the bullfrog, similar to other aquatic frogs, needs its skin exposed to obtain oxygen from the water. Even though brumation is a state of energy conservation, throughout the winter on the days that provide warmer conditions the individual will periodically, in a state of lethargy, swim around feeding.
American Bullfrog (Rana catesbeiana) (Photo credit: PA Fish and Boat Commission).
Wood frogs (Lithobates sylvaticus) undergo brumation when freezing temperatures arrive, but they add a twist. During winter they shelter under leaf litter and find small cracks in rocks or other nooks in the forest. This poses an issue for them because all of these locations are above the frost line making the wood frog susceptible to freezing. The wood frog is up for the challenge. How, you ask? They freeze! You read that right! They are able to freeze and then “come back” to life in the spring. This is because the wood frog has adapted a way to make a natural antifreeze. Cool! I know! Utilizing a glucose solution the wood frog’s body produces when it senses the seasons changing, it is able to support its cells from becoming frostbitten. Individuals can survive multiple months down to 3 degrees fahrenheit with up to 60% of the water in their bodies frozen. Unsurprisingly, they are the only frog species known to be found North of the Arctic Circle.
Wood frog (Lithobates sylvaticus) in central Pennsylvania during the spring emergence (Photo credit: Tim DeWalt).
If you find the wood frog impressive you’ll find the Siberian Newt (Salamandrella keyserlingii) extremely impressive (found in Siberia and not the Chesapeake Bay watershed, but worth mentioning). The Siberian Newt survives freezing temperatures with a similar “antifreeze” idea the wood frog utilizes, but it can endure the much more astounding temperatures of – 58 degrees Fahrenheit. WOW! The Siberian Newt might top our local species of amphibians in terms of surviving the cold, but we still have dozens of stunning amphibians that call the Chesapeake Bay watershed home alongside us.
We might be in the dead of winter, but spring is up around the bend and that means the spring emergence! The spring emergence is when amphibians, starting with the spotted salamander and wood frog, emerge from their hibernacula to mate. This magical event happens during the first few warm and rainy nights of spring (mid-late March) when temperatures are warm enough for amphibious creatures to make the journey to ephemeral wetlands where possibly hundreds of other individuals of the same species will be. If you happen to find yourself near a vernal pool or wetland during one of these nights grab a flashlight, raincoat, and some waterproof boots and see if you can experience the wondrous sights and sounds. Now, since I’ve gotten you excited about amphibians, in the coming few months keep an eye, and ear, out for our slimy friends to welcome them back from brumating this winter!
The spotted salamander (Ambystoma maculatum). One of the first amphibians to emerge during the spring emergence (Photo credit: PA Fish and Boat Commision).
Forests Projects Field Specialist
Forests for the Bay