When temperatures plummet below zero, animals have a few options. They can avoid the cold temperatures all together by finding a nice burrow or staying underwater where it hasn’t frozen, they can be lucky enough to be an endotherm (be able to produce their own body heat), they can die, or some can just let themselves freeze. This remarkable adaptation has been studied over the past 35 years using wood frogs (Rana sylvatica). Wood frogs are found all the way from the Arctic circle into the Appalachian Mountains. Throughout its range, wood frogs overwinter on the forest floor under a thin layer of forest debris (Costanzo, 2019).

Photos: Bethany Williams

Here you can compare what an unfrozen wood frog looks like compared to a frozen frog. Despite this frozen frog having no heartbeat whatsoever, it is alive and will emerge in the spring to mate.

So how do wood frogs do this? They must survive not only freezing, but a long period of not eating, dehydration, and long-term exposure to severe cold. For wood frogs in Alaska, hibernation can last up to 8 months. Therefore, wood frogs have an arsenal of adaptions that they use to survive overwinter (Costanzo, 2019).

To prepare for winter, wood frogs accumulate a large store of glycogen (the storage form of glucose). This stored glucose is an important source of energy during the long period of dormancy when wood frogs are not eating. For the freeze-tolerant wood frog, glucose is especially important as a cryoprotectant, a molecule that protects against freezing—more on those later. Unlike other amphibians which depend on their fat stores during dormancy, wood frogs convert all their fat to glycogen in preparation for winter. Additionally, wood frogs accrue urea as they prepare for winter, which would normally be excreted in urine. Urea helps by depressing the metabolism of the frog and is another important cryoprotectant (Costanzo, 2019).

Freezing injury occurs because as ice forms, water is lost to the ice crystals. This can lead to cell dehydration, membrane failure, and oxidative damage to the cell. Freezing for a long period of time can also deplete energy stores, lead to an accumulation of metabolic wastes that becomes toxic, and cause damage to structures when crystals form. This is where cryoprotectants come in. Once freezing begins, wood frogs mobilize their glucose stores and rapidly distribute them throughout their tissues. Urea, another important cryoprotectant, is already accumulated and distributed to the tissues prior to freezing. Another protective response to freezing is to redistribute tissue water from sensitive organs to places where damage from ice would be minimal like the body cavity. This also means that the concentration of urea and glucose in the cells is higher because there is less water. Both glucose and urea limit ice formation, minimize how much cells actually shrink, and can have other benefits such as regulating metabolism and protecting cell membranes (Costanzo, 2019).

Watch the video below to check out how all of this happens.

Video from PBS Nova ScienceNow

References

Costanzo JP (2019) Overwintering adaptations and extreme freeze tolerance in a subarctic population of the wood frog, Rana sylvatica. J Comp Physiol B 189: 1-15

Ritsko A (2005). Frozen frogs, PBS  Nova Science Now

Imagine you’re on a date—not just any date, the dreaded first date. It’s going reasonably well, but then the lights in the restaurant go out. You think maybe this isn’t a bad thing, candle-lit dinners are romantic right? The waiters are rushing around trying to get the candles lit, so in the mean time you have three choices. 1) Try harder to make yourself heard over the commotion. 2) Call the date a dud and go home. 3) Make a fool of yourself by spilling your wine all over the table, yourself, and your date, then go home.

Now let’s reimagine this scenario. Instead of a first date, there are two fish courting, and instead of the lights going out, a ton of mud has been dumped on their heads. This is the reality that many freshwater fish face. Turbidity, or the murkiness of the water, is increasing in many aquatic ecosystems due to high nutrient inputs increasing algal growth or greater inputs of soil. To visualize what fish courting might look like, check out the video of a male cichlid trying to woo a receptive female using a move appropriately termed a quiver. Fish in turbid waters can increase the time and energy they spend courting in hopes of attracting a mate, but this might not lead to increased reproduction for the male (the fish equivalent of making a fool of yourself?) (Candolin et al., 2007). In other words, it can be a waste of time. In colorful species like cichlids, turbid water leads to duller fish, fewer color varieties, and lower species diversity (Seehausen et al., 1997). Even when species do manage to choose a proper mate, turbidity can still hamper hatching success because the eggs become smothered and cannot obtain sufficient oxygen. Beyond reproduction, turbidity can also affect community structure, predator-prey dynamics, and cause infections by damaging gills (Gray et al., 2012a). In species that choose their mates based on visual cues, the inability to successfully choose a suitable mate could reduce population viability (Gray et al., 2012b).

References

Candolin U, Salesto T,  Evers M (2007) Changed environmental conditions weaken sexual selection in sticklebacks. J Evol Biol 20: 233-239

Gray SM, Chapman LJ,  Mandrak NE (2012a) Turbidity reduces hatching success in threatened spotted gar (lepisosteus oculatus). Environ Biol Fishes 94: 689-694

Gray SM, McDonnell LH, Cinquemani FG,  Chapman LJ (2012b) As clear as mud: Turbidity induces behavioral changes in the african cichlid pseudocrenilabrus multicolor. Curr Zool 58: 146-157

Seehausen O, van Alphen JJM,  Witte F (1997) Cichlid fish diversity threatened by eutrophication that curbs sexual selection. Science 277: 1808-1811