Welcome to the first entry of Part 1 of our Animals in Extreme Environments series. Today we’re going to take a look at how terrestrial animals survive in some of the coldest environments on Earth. Evolution has had some pretty strong forces to contend with here and the results include some truly fascinating adaptions like the use of polyol “antifreeze” and the ability to hibernate;but that’s just the tip of the iceberg…
An Introduction to the Three Cold Biomes
Before we dive into the survival mechanisms employed by animals in extremely cold environments, I think it’s important to have a basic understanding of the three very cold biomes inhabited by these animals. While the exact geospatial boundaries between these three biomes aren’t always clean cut, there is certainly a degree of distinction.
The polar biome is usually defined as the area within the Arctic and Antarctic Circles, however in a bizarre geographical twist, classifying the polar biome that way actually excludes the northwestern-most tip of the Antarctic Peninsula which is still very, very cold and in my opinion should be classified as a polar habitat. That said, with regards to the Antarctic, let’s set the “polar zone boundary” at the outer boundary of the pack ice during the winter since I think we can all agree that everything within that boundary under such conditions is incontrovertibly “polar.”
In any event, the polar biome is essentially an ice sheet. Between the reduced solar radiation load, high albedo, low temperatures, and lack of soil, not much can find the polar zone very hospitable. Thus, for all intents and purposes, the far polar regions can be thought of as expansive, cold deserts. So what can survive in such desolate and inhospitable locations? Well, if you’re near the south pole, then there’s almost nothing. There is almost no vegetation to speak of in the Antarctic proper (though some vegetation does exist on the fringes) and likewise, there appears to be almost no terrestrial animal life. Sure, there are penguins and such, but they are really just seagoing visitors and can’t really be classified as terrestrial. The only true terrestrial species I was able to find that persist in the Antarctic proper were a few invertebrates; with the hardy little phytophagous mite Nanorchestes antarcticus claiming the title of “most southernly invertebrate”. Apparently hundreds of thousands of these mites persist in roving bands grazing on algae. If you’re curious about them, you can learn a bit more here on Macromite’s blog.
With regard to the northern polar biome (the Arctic), things are a tad more complex since there is a fair amount of tundra present within the Arctic Circle. This means that the fringe areas of the Arctic polar biome are capable of supporting a surprisingly decent amount of fauna, particularly invertebrates such as spiders, springtails, and enchytraeid oligochaete worms (aka Ice Worms, Mesenchytraeus solifugus) as well as larger vertebrate life such as the ever so lovable, but dreadfully dangerous, Polar Bear (Ursus maritimus) and the adorable Arctic fox (Alopex lagopus). But let’s not get distracted…we still have a lot more ground to cover as we move bit farther from the poles and into the tundra.
When I was doing coursework in animal environmental physiology, we defined the tundra biome as areas in which “the temperature remains below 0 degrees celsius for at least 7 months of the year”. Thus, in this environment, the deep ground remains so cold, that it is in a constant state of permafrost surmounted by a thin layer of active soil during the summer months. The vegetation here is thus limited primarily to species which can persist on a thin layer of soil and rocks, namely lichens and mosses, though some lowing-growing shrubs are capable of surviving, such as the dwarf willow (Salix herbacea).
Animal life is much more common in the tundra than in the aforementioned far-polar zones and includes such familiar fauna as the caribou, musk ox, snow sheep, arctic hare, arctic fox, goose, and the gyrfalcon (Falco rusticolus)(…ok, I admit you’ve probably never heard of that last one, but I threw it in there anyway because it’s definitely worth becoming familiar with.) But these are just the endothermic vertebrates. Ectothermic vertebrates are also capable of calling the tundra home, including the common adder (Vipera berus) and the viviparous lizard (Zootoca vivipara). Representing the planet’s invertebrate fauna are a collection of mites, springtails, nematodes, flies, and worms (among others.) Moving farther away from the poles, however, conditions get slightly more favorable and you enter into our third and final chilly biome: the Taiga (aka. the boreal forests).
The Taiga biome is absolutely expansive and wraps around the planet in a near continuous belt across North America, Asia, and Europe. The soil here (known as podzol for any soil buffs reading this) is nutrient poor and acidic and typically covered by a dense layer of decomposing conifer needles. The landscape is dominated by coniferous trees where possible and by mosses in locations where the substrate takes the form of peat bogs. In this habitat, much more animal life is capable of thriving including moose, marmots, voles, martens, grouse, wolves, tiger, lynx, and the wood-frog (Rana sylvatica). Invertebrate life appears to be similar to that of the tundra with nematodes, flies, earthworms, and weevils being commonplace. Regardless, the taiga is still an extreme environment requiring a range of adaptive traits and strategies.
Animal Adaptations and Strategies for Cold Living
General Patterns in Morphology
If you’ve ever heard of deep sea gigantism or island dwarfism, then you’re familiar with the fact that certain morphological patterns generally emerge in given environments. (And that makes perfect sense; if the same evolutionary pressures are being exerted on a group of organisms, then it makes sense that certain traits can evolve convergently.) If you’re aware of such morphological “rules”, then perhaps, then, you’ve also heard of Allen’s Rule. Allen’s Rule asserts that animals in colder climates tend to possess smaller extremities than their more temperate relatives. This is a reasonable assertion if you consider morphology from the standpoint of surface-to-volume ratios. The higher this ratio, the greater the rate of heat exchange between the environment and the interior of the animal. Since generating heat is energetically expensive and cold animals clearly need to conserve energy (because it’s hard to come by in very cold climates), they tend to possess stockier bodies with smaller, thicker extremities when compared to their temperate kin.
One of the best descriptions I’ve heard referencing Allen’s Rule involves comparing high-latitude terrestrial animals with footballs. The shape of a football is fantastic from a biological heat conservation standpoint so it makes sense that terrestrial animals in super cold climates would be more football-shaped than comparable temperate animals. By now you’re probably thinking about some of your favorite polar animals and beginning to see the pattern: arctic fox = fluffy football, lemming = fluffy football, wolverine = angry fluffy football. Soon you’ll realize that the very cold climates are full of variations of the “fluffy football” body plan and you now have a way to never forget Allen’s Rule.
This adaptation is probably the first thing that comes to mind when you imagine a terrestrial animal in a cold climate. In terrestrial animals, insulation typically takes the from of a coat of fine, densely packed hairs. Such coats can be so efficient, that the temperature of the skin can almost reach that of core temperatures (Willmer et. al 2009). That’s a remarkable feat considering that the air temperature in very cold terrestrial climates is routinely below freezing. Aside from a thick coat, the thought of blubber (a think layer of subcutaneous adipose tissue) comes to mind, but this is almost entirely unexpressed in terrestrial animals inhabiting cold climates for two main reasons. Firstly, blubber is very heavy and the metabolic expense of lugging all that fat overland is just too great to justify the expense (no to mention the risk of not being able to outrun a predator!). Secondly, blubber is such a good insulator that animals who have it tend to overheat very fast when moving across land. This is why you won’t see seals or polar bears moving quickly across land for any extended period of time. Their bodies would get waaaaaaaay too hot and the risk of death from hyperthermia is considerable.
Note: For some more information on Polar bear coats, see my post Polar Bear Fur Isn’t (Technically) White
Use of Microenvironments (Burrows and Basking)
Fur is great, but what if it’s not enough (i.e. particularly in the case of small terrestrial animals)? Well, if this is your situation, than a burrow is a great option to help cope with the cold. Burrows can be highly effective reservoirs of heat and when lined with grass, leaves, or other materials can create upwards of a 30 degrees Celsius increase in temperature relative to the air at the surface (Davenport 1992, Willmer et. al. 2009). Furthermore, if the burrows are dug into a translucent material like snow, they can create entire subnivean (“under the snow) habitats capable of sustaining vegetation (which derives energy from solar radiation penetrating through the snow). This is a boon for animals like lemmings who use subnivean habitats as a strategy for remaining active throughout the winter; feeding on the vegetation that grows inside and using communal aggregation to keep the space (and their bodies) warm.
If a burrow isn’t your thing, basking is always an option (not that burrowing and basking are mutually exclusive). While not nearly as effective a burrow, basking does warm peripheral tissues/structures and thus aids in reducing the thermodynamic gradient between the skin and the air. Basking also helps dry wet fur (as in the case of amphibious animals like polar bears), thus reducing heat-loss derived from contact with water.
Freeze Tolerance and Supercooling
Cold temperatures absolutely wreak havoc on physiological systems. As the body temperature drops, enzymes begin to dysfunction, cellular membranes begin to experience less fluidity, etc. Now, while there is a certain level of temperature tolerance for animals acclimated to cold climates, it’s safe to say that freezing is usually not an option. For most animal, reaching a temperature near (or at/below) freezing means certain death, but some animals have actually developed mechanisms that allow them to survive body temperatures in that range! (Yes, that’s an exclamation point. I know you don’t see it in “science writing” too often because of the importance of objectivity, but I’m going to use one here because this adaptation is awesome.)
Animals with these capabilities appear to fall into one of two categories. Freeze intolerant animals such as as some polar invertebrates (e.g. mites and springtails) are able to exploit the physical process of supercooling to lower their body’s freezing point well below 0 degrees Celsius. This is accomplished by increasing the concentration of various antifreeze compounds like glycerol, sorbital, sugars, and sodium chloride in their blood and tissues. At high concentrations, these compounds act as cryoprotectants that allow these animals to remain active even with a sub-zero body temperature. Of course, there’s a catch. The formation of ice is still a very destructive (if not deadly) risk. Should even the tiniest of ice crystals begin to form in these supercooled bodies, they could begin to structurally destroy the animals tissues at the cellular level and lead to death. Thus, animals employing this strategy must do everything they can, including often greatly dehydrating their bodies, to prevent the formation of any ice in their tissues.
Freeze tolerant animals on the other hand don’t need to prevent ice formation, at least in some parts of their body. These animals also employ cryoprotectants, but their bodies distribute them differently. Polyols, sugars, and other solutes accumulate in high concentrations intracellularly and in certain fluids which are to be kept unfrozen. This, of course, means that the remainder of the body, such as certain fluids and the extracellular space(s), are subject to freezing and ice crystal formation. To cope with this, many of these animals employ ice-nucleating agents (INA’s) which allow for ice formation in a very structured and controlled manner at the molecular level and restrict it to specific areas within the body. The combined effect allows the animal remain in a sort of semi-frozen “cryo-stasis” for extended periods of time. But there’s a catch again. Because the animal is often semi-frozen and unable to move about, it must rely on internal energy reserves for survival and therefore has a limited amount of time before it must thaw again and find food.
Countercurrent blood flow
While we’re on the topic of physiological responses to very cold environments, I thought I’d mention the role of countercurrent blood flow in many cold climate terrestrial animals. In these animals, the arteries in peripheral tissues are often encircled by a network of veins. As warmed blood from the body’s core flows through the arteries to these peripheral areas, heat inevitable begins to radiate into the cooler surrounding tissues. As this heat escapes the arterial walls, much of it is recaptured by the surrounding veins which conduct the heat back toward the core; conserving energy and protecting the animal’s vital organs.
Hibernation, Torpor, and Migration
Sometimes, rather than reckon with the stresses of the inhospitable cold, animals elect to leave the area or skip entire time segments during the coldest part of the year altogether. I think everyone is familiar with migration in polar animals. The annual migration of caribou, for instance, from tundra to taiga habitats is an absolutely spectacular annual occurrence that occurs across the upper portions of North America and Asia. Hibernation is probably a similarly well-recognized strategy which elicits thoughts of cuddly animals snuggled up in their dens and sleeping until Spring. Torpor, however, is probably a less familiar term. Torpor and hibernation represent temporal and physiological variations of what is essentially the same process of “controlled hypothermia.” Torpor is the “short-term approach” and is often an overnight energy conservation measure in small terrestrial endotherms (rodents, etc). During torpor, the animal enters a deep state of “sleep”; metabolic rates are reduced, body temperature falls, and the ability to awaken in response to external stimuli is greatly inhibited. Arousal from torpor can take upwards of a few hours and is often begins with non-shivering thermogenesis followed by often violent shivering as the muscles regain function.
Hibernation on the other hand is far more extreme and represents periods of greatly extended torpor. During this period, metabolic rates and body temperature plummet to spectacularly low levels. Respiratory rates may fall as low as 1-2 breaths per minute with an accompanied reduction in heart rate, sensory functions begin to cease, many protein synthesis reactions are inhibited, glycolytic enzymes are phosphorylated (metabolic suppression), and most “non-essential” functions are suspended during this prolonged period. It should be mentioned, however, that hibernation is not a continues event; it consists of several wake/sleep cycles which allow the animal to relocate, forage, and perform other necessary survival functions. For a hedgehog, for instance, this may mean waking every 11 days or so. It should also be mentioned that the hibernation strategy appears to be limited primarily to animals inhabiting boreal forest habitats, since the thermodynamic stresses of the polar and tundra climates are too great to make hibernation a viable strategy. Animals in those regions would simply experience too much heat loss to survive the extended period of inactivity.
In this “episode”, we covered a variety of strategies and adaptations employed by animals in extremely cold climates. Animals often use some or all of them in combination to survive and this was certainly not an exhaustive list. Regardless of the exact approach, all are aimed at the same general goal: prevent the destruction of tissues and physiological functions caused by freezing, reduce metabolic expenses, and use the standing conditions of the surrounding environment to the animal’s advantage as much as possible. It’s undeniable that extremely cold habitats are extremely stressful for terrestrial animals, but I think it’s remarkable how well they seem to manage in spite of the odds.
- Davenport, Animal Life at Low Temperature, Springer, 1992
- Willmer et. al. Environmental Physiology of Animals 2nd Ed., John Wiley & Sons, 2009
- F. Harvey Pough, Vertebrate Life 9th Ed., Pearson Higher Ed, 2013