Welcome to the first entry of Part 1 of my 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 produced some fascinating responses to these extremely chilly spaces 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.
That said, let’s set the Antarctic “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 Antarctic polar biome is essentially a rocky ice sheet. Between the reduced solar radiation load, high albedo, low temperatures, and lack of soil, it is extremely inhospitable.
There is almost no vegetation 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.
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 exist as 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.
That means that the fringe areas of the Arctic polar biome are capable of supporting a surprising amount of fauna, particularly invertebrates such as spiders, springtails, and enchytraeid oligochaete worms (aka Ice Worms, Mesenchytraeus solifugus) as well as larger vertebrate life like polar bears (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 a bit farther away 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.
In this environment, the deep ground remains so cold that it is in a constant state of permafrost covered by a thin layer of active soil during the summer months. The vegetation is therefore limited primarily to species that can survive and thrive on a thin layer of soil and rocks. Prominent examples are 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 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 surprisingly capable of calling the tundra home, including the common adder (Vipera berus) and the viviparous lizard (Zootoca vivipara).
Representing the planet’s invertebrate lineup are a collection of mites, springtails, nematodes, flies, and worms (among others.)
As you move farther away from the poles, however, conditions get even 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, acidic, and typically covered by a dense layer of decomposing conifer needles.
The landscape is dominated by coniferous trees where possible and by mosses where the substrate takes the form of peat bogs. This habitat is capable of supporting diverse vertebrate life, including moose, marmots, voles, martens, grouse, wolves, tiger, lynx, and the wood frog (Rana sylvatica).
Invertebrate life in the taiga 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
If you’ve ever heard of deep-sea gigantism or insular dwarfism, then you’re familiar with the fact that certain environments produce conserved morphological patterns. When the same strong evolutionary pressures are exerted across groups of organisms, certain traits can evolve convergently. For example, in the case of deep-sea gigantism, organisms tend to be larger than their relatives in shallower waters.
If you’re aware of those kinds of morphological trends, 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 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. It packs a lot of space into a volume with comparatively little surface area.
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 notice that the very cold climates are full of examples of this “fluffy football” body plan!
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 form 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 the animal’s core temperature (Willmer et. al 2009)! That’s a remarkable feat considering that the air temperature in very cold terrestrial climates is routinely well below freezing.
Aside from a thick coat, blubber (a thin layer of subcutaneous adipose tissue) provides a great way to conserve heat, but it comes with a steep price and is generally restricted in marine and amphibious cold-climate vertebrates.
For one, 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!) for most land animals.
Secondly, blubber is such a good insulator that animals will overheat when moving quickly. 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 way too hot and the risk of death from hyperthermia is considerable.
Aside: 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 and blubber are great, but what if it’s not enough. For instance, what if you’re a small mammal and even Allen’s Rule won’t adequately protect you from heat loss? Well, if this is your situation, then a burrow is a great option to help cope with the cold.
Burrows can be highly effective reservoirs of heat. When lined with grass, leaves, or other insulating materials, they can have 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 made in 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 subnivean tunnels and using communal aggregation (i.e. bunching together) to keep 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 warms peripheral tissues and aids in reducing heat loss. Basking also helps dry wet fur (as in the case of amphibious animals like polar bears). Water is a very good heat conductor in air and wet animals in the cold are fighting an uphill battle to stay warm.
Freeze Tolerance and Supercooling
Cold temperatures wreak havoc on physiological systems. It’s just physics and chemistry in action. As body temperature drops, all the molecular processes that sustain life begin to go awry. Enzymes dysfunction, cellular membranes experience less fluidity, etc.
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 animals, 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!
Animals with these capabilities appear to fall into one of two categories.
Animals such as some polar invertebrates (e.g. mites and springtails) are not able to survive freezing but 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, sorbitol, 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. It’s the same basic concept behind icing a road in the wintertime.
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 their supercooled bodies, they could begin to structurally destroy the animal’s 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 also employ cryoprotectants, but their bodies distribute them differently. Polyols, sugars, and other solutes accumulate in high concentrations intracellularly and in certain fluids which need to remain unfrozen. The remainder of the body, however, is 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 very structured and controlled ways 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 “cryostasis” for extended periods of time.
Of course, there’s still a catch. Since the animal is often semi-frozen and unable to move, it must rely on internal energy reserves for survival and 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 animals employing this technique, the arteries in peripheral tissues are often encircled by a network of veins. As warm blood from the body’s core flows through the arteries to these peripheral areas, heat radiates 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.
Torpor and Hibernation
Hibernation is probably a well-recognized strategy that elicits thoughts of cuddly animals snuggled up in their dens and sleeping until Spring. Torpor, however, is probably a less familiar term.
Both 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 decrease, body temperature falls, and the ability to wake 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 a period of hibernation, metabolic rates and body temperature plummet to spectacularly low levels. Respiratory rates may also fall as low as 1-2 breaths per minute with an accompanied reduction in heart rate. Sensory functions are generally suspended. Many protein synthesis reactions are inhibited. Glycolytic enzymes are phosphorylated (e.g. metabolic suppression) and most other “non-essential” physiological functions are suspended entirely during this prolonged period.
It should be mentioned, however, that hibernation is not a continuous event. It is periodic, consisting of several wake/sleep cycles which allow the animal to relocate, forage, and perform other necessary survival functions. Hedgehogs, for instance, wake every 11 days or so.
It is also worth mentioning 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 periods of inactivity.
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.
For these animals, the stability of the surrounding climate and the ability to forage for food along the route is paramount. Disruptions in clear, sustainable migrations vectors, like building a freeway through the middle of a migration route or converting plains to farmland, can be disastrous for migrating animals.
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 these approaches to survive, and this was certainly not an exhaustive list.
Regardless of the exact approach, all these adaptations have the same general goals: prevent the destruction of tissues and physiological functions caused by freezing, reduce metabolic expenses, and use the 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 carry on in spite of the odds.
This post was updated from an earlier version published in 2014