The Big Freeze: How Reptiles and Amphibians Survive Sub-Zero Temperatures
Reptiles and amphibians are thought to be synonymous with harsh deserts and tropical rainforests. Yet, across the world, weather is unpredictable. In many ecosystems, temperatures can plummet below freezing with very little warning. To the reptiles and amphibians that have inhabited these regions for millions of years, even the extremes are just another day in the fight for survival.
A moor frog (Rana arvalis) in Spring
Against all odds
Reptiles and amphibians are ectotherms, meaning they do not regulate their own constant internal temperatures and must rely on external sources, usually the sun. This means they are at the mercy of the environment around them.
Some, however, have developed adaptations that allow them to survive seasonal temperature lows below freezing point that would kill other species. A process known as supercooling prevents cell damage that would be otherwise fatal. The formation of ice crystals causes the damage, but by redirecting the use of solutes in the fluids of their bodies, species such as the wood frog (Lithobates sylvaticus), spring peeper (Pseudacris crucifer), gray tree frog (Hyla versicolor) and Western chorus frog (Pseudacris triseriata) can effectively prevent the cells from freezing and survive the process. This not only allows them to survive harsh winters but they are also able to breed at a more opportune time, earlier in the season.
Solutes used in this way are known as cryoprotectants because they prevent the alignment of water molecules to form ice crystals. Usually, the cryoprotectant utilised in amphibians is glucose or glycogen. Cryoprotectants do not prevent freezing entirely but decrease the size of the crystals formed, and importantly, slow the process down significantly which allows fast deployment of cryoprotectants throughout the body to minimise damage, almost like ‘antifreeze’. Cryoprotectants are produced in the liver and exported within minutes of freezing.
Wood frogs can end up with approximately 65% of their extracellular body fluids converted to ice. During this time, respiration, blood flow, and even the heartbeat stop. The body's cells must rely on alternative fuels to remain viable. Studies so far have identified six genes that are activated during the freezing process that regulate biochemistry.
A frozen Wood Frog (Rana sylvatica) Credit: J.M Storey
The Siberian salamander Salamandrella keyserlingii is another species that can stay frozen indefinitely, then thaw and walk away as if nothing happened. Folklore states that some of these salamanders may have frozen while mammoths still walked the earth, but scientists doubt these claims, as exciting as it would be if they were true. What is true, however, is that they have been found to survive temperature extremes of -50°C. The species has mastered its environment wherein the bottom layers of the soil are frozen in permafrost. They spawn in small ephemeral summer ponds and have no competitors being second closest to the Arctic Circle of any amphibian, with the Siberian frog (Rana sibirica) being closest geographically.
While the details of the processes are not as widely known as that of the wood frog, Siberian salamanders can prepare for freezing by also altering their body fluids and producing cryoprotectants, protecting their cells when they hit freezing point. Younger individuals are more susceptible and take shelter within rotting trees and debris where temperatures only reach about -15; many are killed when moving between ponds if a sudden frost appears. Adults are more resilient and will utilise nearby water sources to escape sudden freezing. Supercooling freeze-tolerant species are not invincible – they must have time to produce their cryoprotectants and not freeze below their supercooling point. They also seek insulated refugia to stave off the freeze as much as they can.
A Siberian salamander (Salamandrella keyserlingii)
A cold snap
It’s not just amphibians that take extreme measures to prepare themselves for frosts during winter. The American alligator (Alligator mississipiensis) is renowned for its tactical preparation for frosts. Although alligators will typically overwinter in burrows, they have been observed submerged in ice-covered lakes and ponds across their North American range. When the surface of the water freezes over, the upper half of the animal is trapped in ice while the snout is poking above the surface allowing the alligator to breathe. Termed ‘icing’, this is essentially an extreme form of brumation – the same process that popular captive species may undergo if they sense a seasonal drop in temperatures. Due to the extremely slow metabolic rate of the alligator in this state, breathing is barely required. It has been reported that alligators in this state can survive for over 8 hours without taking a breath.
Interestingly, not all American alligators do this. It is thought that those from the coldest regions of their range had naturally survived the cold spell and thus passed the technique onto their offspring. Studies of this method began in the 70s and were first published in 1982 when John M Hagan et al, recorded it in the South American Journal of Herpetology. Since then, researchers have recorded other populations of alligators struggle to break the ice with their snouts suggesting it is a highly localised adaptation.
Of course, other animals in Florida are not so adaptable. During cold snaps in the winter months, the invasive green iguanas (Iguana iguana) are often reported to fall out of trees as their metabolism slows, due to the extreme weather. As they have not adapted to cold spells naturally, they pose a genuine risk to people driving, as they appear to fall from the sky.
Some sea turtles can suffer disorientation because of extreme cold spells too. In February 2021, a severely cold storm dropped water temperatures on the East coast of the USA prompting a ‘cold-stunning event’. The event was the biggest of its kind, beating the 2010 record of 4613 affected turtles in Florida waters. In 2021, 12,155 turtles were reportedly stunned and disorientated off the Texas coast. Of those, 5,300 required immediate intervention and were rescued by Sea Turtle Inc. Although data suggests only around 35% of the affected turtles survived, the human intervention drastically supported the juvenile green sea turtle (Chelonia mydas) population in that part of the world.
Cloacal respiration
Although there are a lot of highly specialised adaptations in the natural world, few are quite as weird as cloacal respiration. Freshwater turtles from temperate areas of the world, such as the painted river turtle (Chrysemys spp.) will survive for up to 5 months submerged underwater, in extremely cold temperatures by breathing through (pretty much) their bum.
Surprisingly, many reptiles and amphibians use the broader method of ‘cutaneous respiration’ to ensure they get enough oxygen into their body while their metabolism is slowed. Being cold-blooded, reptiles' and amphibians health is largely dictated by their surrounding temperatures. The cooler the climate, the slower the metabolism and the less active the animal becomes. As such, many species have developed a way in which sufficient oxygen from the surrounding air or water, can be absorbed into the body through blood-vessel-rich areas of the skin.
This method of ‘breathing’ is extremely important for sea snakes, where blood is moved away from the lungs and closer to the skin to help them survive long dives. Amphibians such as the hellbender (Chryptobranchus alleganiensis) use this method to survive underwater, where cutaneous respiration makes up around 90% of its total oxygen intake.
Turtles do this slightly differently. They will use two enlarged cavities known as cloacal bursae to suck water into their cloaca. This area has lots of blood vessels and well-ventilated skin, to allow the turtle to absorb plenty of oxygen. The turtle will then use those muscles to dispel the water, now filled with carbon dioxide, back into the pond or lake. For some species, such as the Fitzroy river turtle (Rheodytes leukops) this is their primary source of oxygen, making up almost 70% of their total intake. For other species, this method is used as a survival tactic, particularly in extremely cold climates.
Painted river turtles, common snapping turtles (Chelydra serpentina), and many other species use this method of receiving oxygen, to varying degrees, to deal with extreme climates. These species can stay submerged in extremely cold water for over 100 days. In fact, at the top of their range in Ontario, Canada, river turtles and snapping turtles can spend almost half of their lives in some form of brumation. During this time, the cold water slows their metabolism down tremendously meaning less oxygen is required and thus, the lungs become redundant, and the cloaca does all the work. It is thought these turtles can survive temperature drops as low as 2°C for 11 days.
Below the surface
The viviparous or ‘common’ lizard (zootoca vivpara) has the widest distribution of any reptile on the planet. Stretching from Ireland to Japan and as far north as Scandinavia into the Arctic circle, this species is subject to extreme cold weather. Just how they survive these extremes is still a bit of a mystery. Like many Lacertids from temperate climates, common lizards will burrow into a hibernaculum once temperatures begin to drop.
Naturally, at cooler temperatures, the lizards are more likely to bury deeper into the soil to hibernate. Reports vary, suggesting the lizards will bury themselves anywhere between 10cm underneath the soil, right up to 40cm deep. This also varied across altitudes, with locales from higher altitudes burying themselves deeper to avoid extreme weather. Researchers also discovered that the lizard could tolerate, on average, a minimum of -2C to -4C without mortality. This is the parameter at which the lizards’ internal liquids would be prone to freezing. Of course, extended periods at this temperature would seriously diminish the animal’s survival rate.
We know that common lizards avoid the cold, rather than employing anti-freezing superpowers like the wood frog, but how do they manage this in the coldest winters of Siberia? Researchers in Russia discovered that different locales of the species are much hardier than others, especially when compared to their European cousins. Bermann et al, write:
“It should be emphasized that the minimum temperature endured by common lizards from the Western Siberian populations (-10 C) is 2.5 times as low as that of conspecific lizards from Europe; so far, this is the record level of cold hardiness for adult reptiles.”
“It may seem that the observed level of cold hardiness would not be sufficient for the common lizard to survive under the extreme winter conditions of Yakutia. However, the minimum soil temperatures in the warmest habitats of southwestern Yakutia and probably the entire Southern Yakutia are close to those in the southeast of Western Siberia (-4.5 to -7.0 C) since lower air temperatures in Yakutia are compensated for by the soil-heating effect of the non-freezing groundwaters. Such thermal anomalies are quite abundant in the south of Yakutia, which determines the broad distribution of the common lizard in this region. The observed level of cold hardiness (-10 C) is sufficient for the lizards to survive in these territories. Thus, considerable cold hardiness of the common lizard and favourable hydrogeological conditions of some territories (the presence of talik zones) determine the wide distribution of the species in the extremely cold regions of Siberia.”
The common lizard’s wide distribution over such a huge percentage of the earth is possibly due to them finding these ecological niches. With areas closer to big lakes generally providing warmer conditions beneath the soil, they inhabit further and further into areas that would otherwise be considered extreme. It is the geological significance below the surface that allows common lizards to survive such cold climates.
Heat in captivity
Reptiles and amphibians are built to survive. Hundreds of millions of years of evolution have created some of the most unusual adaptations on the planet. Whether this is to survive extreme heat or extreme cold, the relationship these ectotherms have with temperature develops from the second the egg begins incubation.
In captivity, providing temperature gradients has been considered the norm for decades now. Over the years heat mats, spot bulbs, projectors and various other products have come and gone, adapted, and developed and today we have a much better understanding of how to heat our reptiles. With many keepers successfully keeping a wealth of species outdoors and even more tropical species in greenhouses, we are beginning to understand the biological requirements and hardiness of captive reptiles.
Providing even more drastic gradients than ever before is starting to become normalised within the hobby and there is plenty of research to suggest this is the way we should be moving forward. Even bearded dragons (Pogona vitticeps) will regularly face temperatures below 10°C, much cooler than most households. This is not to say we should be encouraging extremities, as shielding our animals from the perils of nature is considered the main plus of captive care. We must also recognise that our captive-bred animals are reasonably far removed from their wild counterparts. Without further research into the tolerances of our captive-bred bearded dragons, avoiding these natural cycles is probably encouraged. However, we are now equipped to provide a full light spectrum to mimic heat from the sun using products on our shelves. Considering how we can advance husbandry to allow our reptiles and amphibians to have choice over a much greater range of temperatures is certainly something for keepers to consider.