James Marino
The wood frog (Latin name Rana sylvatica) is a species of frog that lives in a wide range across North America. Scientists are interested in the wood frog because of what it does in winter: they freeze almost entirely solid. “Almost” is key to the frog’s survival. If too much of the frog’s body freezes, it will die. Living things are made of cells, and cells contain a lot of water. When water freezes, it expands in size. If this happens in a cell, the cell dies. When a lot of cells die of cold, whole patches of skin or entire organs can be permanently damaged, which can be lethal.
The range of the wood frog, shown in blue
A wood frog
The wood frog has devised a way to escape this chilly fate. In the fall when it gets cold, the frog’s liver produces chemicals which slowly build up in the body, and they then burrow into shallow soil. When winter comes, these chemicals limit how much ice can form in the frog’s body, and they protect it against any damage that might happen in the freezing and thawing process. The frogs go through several natural freeze-thaw cycles throughout the winter because of normal variations in temperature. Each time a frog thaws or freezes, this chemical protection process must happen all over again. It takes a lot of energy out of the frog to do this repeatedly, but, usually, there aren’t that many freeze-thaw cycles in one winter. The frogs have enough energy to survive, and they thaw out for good once spring returns.
However, climate change is making winter a more unpredictable time. With warmer winters becoming more common in many North American habitats, there are more times each winter when the ground has no snow on it. This means the ground often does not stay frozen—so neither do the wood frogs. A group of researchers from both the University of Wisconsin and the University of Melbourne wanted to find out how this would impact the frozen amphibians. They developed a model to help them make predictions about the impacts on the frogs, using data on weather, climate, and wood frog biology. These predictions forecasted up until the end of the 21st century.
Based on the model, they discovered that wood frogs in different regions would be impacted differently. In the northeast United States, it was predicted that warmer winters would mean the frogs would not freeze as often, and therefore would go through fewer freeze-thaw cycles. This prediction stayed the same until the end of the century, and it means that wood frogs in the Northeast will likely have to spend less energy to survive winter in the future, something which may impact them positively. However, the model also highlighted that warmer temperatures may mean that wood frog tadpoles have less time to develop before their spring pools evaporate in hotter temperatures brought on by climate change.
In the northern Midwest (such as Wisconsin and Minnesota) the researchers’ model predicted that parts of the regional climate, including lake-effect snow, would increase the number of freeze-thaw cycles each winter. The implication here is that the frogs would have to spend more energy to survive. But in the model, this was only the prediction until mid-century. Then, from mid-century until the end of the century, the model predicted that increasingly shorter and warmer winters would mean it would no longer get cold enough to freeze in the first place—the same as the prediction for the Northeast. Again, this would mean the frogs have to endure fewer freeze-thaw cycles in winter—which could be good for them—but it also means they may struggle to survive during other times of year in other life stages.
This study presents no immediate answers, but very valuable insight. If the wood frog can spend less energy during the winter, perhaps the species will have more surviving adult members year-round. However, warmer springs and summers may harm tadpoles and froglets. Studies such as this are important because they allow humans to consider the possible futures a species may face. As our world changes alongside our climate, and many species and ecosystems are put in danger, it is vital to understand them, so we may better protect them.
Source:
Fitzpatrick , M. J., Porter, W. P., Pauli, J. N., Kearney, M. R., Notaro, M., & Zuckerberg, B. (2020). Future Winters present a complex energetic landscape of decreased costs and reduced risk for a freeze-tolerant amphibian, the wood frog (lithobates sylvaticus). Global change biology. https://pubmed.ncbi.nlm.nih.gov/32871618/
Image source:
Wikipedia Contributors. (2024, March 14). Wood frog. Wikipedia. https://en.wikipedia.org/wiki/Wood_frog