Amphibians and reptiles are well-known for being ectothermic (cold-blooded). This means that they are unable to internally regulate their body temperature, and instead they rely on their external environment to do so. Consequently, when we think about species like snakes and lizards, we tend to associate them with hot climates, often picturing them basking in the sunshine. Although they do inhabit hot climates, amphibians and reptiles can be found all over the world except for Antarctica. The UK has 14 native species of amphibian and reptile, and a few hardy species can even be found in extreme cold climates such as in the Arctic where temperatures can drop to -45°C. So, how are these species adapted to survive in such extreme conditions?
One important strategy used by amphibians and reptiles is brumation, where they go into a state of dormancy during the cold winter months. They typically brumate in burrows or under log piles, but different species will use a variety of habitats, with some common frogs even brumating in the mud at the bottom of ponds. Although similar to hibernation, the key difference is that brumating animals will emerge for short periods to forage before returning to their state of dormancy, usually on warmer days. All of the native UK species brumate to avoid the coldest weather and conserve energy. Nevertheless, British weather can still be challenging year-round. Common frogs are particularly hardy, and have been found breeding at the highest altitude of any amphibian in the UK at 1,120m in the Scottish Cairngorm Mountains. Higher altitude habitats offer a much shorter window for breeding, so to cope with this, high altitude common frog populations in Scotland have higher growth rates and shorter larval periods compared to low altitude populations (Muir et al. 2014). This ensures metamorphosis is completed before temperatures drop again to maximise chances of survival. On the other hand, some common frog tadpoles will instead delay metamorphosis until the following spring and overwinter as a tadpole rather than a froglet; this may be beneficial in colder temperatures as they will be able to metamorphose at a larger size, improving their chances of survival (Walsh et al., 2008).
Another UK species which is particularly well-adapted to cold conditions is the leatherback turtle. It often comes as a surprise when people find out that the leatherback turtle is native to the UK, but they are active in our surrounding seas despite the low number of sightings (read more on this here). Leatherbacks have a larger range than other sea turtles and can survive in our cold waters by maintaining a deep body temperature that is 18°C higher than the surrounding water (Frair et al., 1972). They do this through a countercurrent heat exchange process in their flippers (Greer et al., 1973), a process much more commonly associated with mammals and birds than reptiles. Blood vessels in their flippers are closely packed together so that warm blood moving from the core to the extremities passes in close proximity to the cold blood returning from the extremities to the core. Heat hHeateat is transferred into the cold blood and returned to the core, avoiding the extremities where it would be lost to the external environment (Greer et al., 1973). This method of reducing heat loss at the extremities and circulating that heat back into the core of the body is critical for their survival in cold water.
A very different adaptation is a species’ reproductive strategy, of which there are two key options, oviparity (egg-laying) or viviparity (live-birth). Most reptiles are oviparous as this requires less investment per brood, enabling them to have multiple broods per year. However, reptiles in cold climates are actually more likely to be viviparous because internal development allows the mother to thermoregulate more efficiently, thus improving offspring survival in cold or unpredictable environmental conditions (Tinkle and Gibbons, 1977). Cold temperatures can also reduce the length of the breeding season, making the possibility of multiple broods per year unlikely, and thereby eliminating the benefits of oviparity (Tinkle and Gibbons, 1977). In Scotland, all of the native reptiles are viviparous with one exception; the grass snake. The grass snake is widespread in England and Wales but only found in the far south of Scotland. Could this suggest that being oviparous makes them less suited to the cooler Scottish climate? Interestingly however, there is a population of oviparous lizards, the sand lizard, living on the Scottish island of Coll. They are native to England and not Scotland, but were introduced to Coll in the 70s and are still there today. This may suggest that although viviparity can be beneficial in cold climates, it is not essential.
The number of amphibians and reptiles that thrive in the British weather is undoubtedly impressive, but even more remarkable is the array of hardy species which survive extreme cold-climates such as the Arctic. For any species this would be a challenge, and for most ectotherms freezing temperatures are lethal – the fluids within their cells freeze and form ice crystals which can then rupture (Storey and Storey, 1988). However, there are two fundamental strategies used by cold-climate ectotherms to combat this; freeze avoidance (supercooling) and freeze tolerance. Freeze avoidance is where cell fluids remain liquid despite reaching freezing temperatures, while freeze tolerance is where an individual is able to survive freezing to an extent by restricting freezing to extracellular areas (Storey and Storey, 1988). These processes typically rely on the production of a cyroprotectant, a substance which reduces the freezing point of water and can be used to prevent freezing overall (freeze avoidance) or in cells and key organs (freeze tolerance). Different substances can act as cryoprotectants in amphibians and reptiles, with some common ones including glycerol, glucose and taurine. Cyroprotectants are also used in medicine, for example when donor organs are preserved through cooling, cyroprotectants are often used to prevent cell freezing and rupture.
One species which uses these strategies is the wood frog, the only frog known to live in the Arctic Circle. They can tolerate being frozen at -3°C for two weeks, with up to 70% of their body water freezing (Costanzo et al., 1993). They use glucose as a cyroprotectant, which they produce in high quantities in key organs to prevent ice formation (Costanzo et al., 1993). This restricts freezing to less important parts of the body where it is less likely to cause damage. Red-sided garter snakes in Canada use a similar strategy and can survive at -2.5°C with up to 40% of their body water freezing, using taurine as their cryoprotectant (Churchill and Storey, 2011). Unlike wood frogs they have a much shorter period of tolerance, with only a 50% chance of survival after 10 hours of freezing (Churchill and Storey, 2011). This may suggest that this is not a strategy that is used regularly, but instead an adaptation to survive short periods of frost in the autumn or spring soon before/after brumation.
Although species like the wood frog and red-sided garter snake are incredibly hardy, there is one species which can out-do them all; the Siberian newt. Temperatures in Siberia can reach as low as -45°C and it seems impossible that much life could thrive here, yet the Siberian newt does. Incredibly, it can survive being frozen to -35°C for 45 days, or -50°C for 3 days (Berman et al., 2016). It is not entirely clear how this remarkable species survives such hostile temperatures, but it seems likely that they produce a cryoprotectant to protect their cells and key organs from freezing, although it is not known what this cyroprotectant might be.
Amphibians and reptiles are ectothermic, and consequently are often associated with hot climates. In reality, they can be found in a huge variety of habitats and climates and have an array of behavioural and physiological adaptations which enable them to live and thrive in even some of the coldest parts of the world. Adaptations such as brumation, viviparous reproduction, and efficient heat transfer systems are just a few of the things that can help amphibians and reptiles survive in cold or variable climates. Remarkably, in the coldest regions, species use even more extreme adaptations of freeze avoidance and freeze tolerance in order to stay alive. The ability of some species to tolerate being frozen at extreme temperatures for extended periods tells an incredible story of survival and adaptation, and reminds us never to underestimate the brilliance of amphibians and reptiles.
Written by Mirran Trimble
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