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How Life Survives Winter
Life Must Go On
As the temperature drops and the bitter silence of winter takes hold, it often seems like much of the life that once inhabited a natural landscape has ceased to be. Fortunately, the organisms that live in climates that endure freezing temperatures have evolved a plethora of adaptions that allow them to survive such harsh environmental conditions. Here I have assembled specific information on how various kinds of organisms survive the winter chill.
Perhaps the most well-known winter survival strategy used by birds is their ability to escape it. The capacity to fly enables birds to avoid the harsh reality of winter and vacation in a more temperate climate. Such a grueling journey however requires a vast store of energy that some birds just can’t muster. For the birds that can’t make the trip and all those that have evolved to wait out winter, energy conservation becomes their game. In winter, food resources are often scarce and frigid temperatures push a bird’s capacity to keep warm to the limits. A bird must forage all day in the hope of building an energy store large enough to survive a night of shivering. Luckily, birds have developed many strategies to get the most out of every calorie.
The first and perhaps most obvious way birds save energy is by adding insulation onto their body through heavy layers of feathers. Fluffing those feathers out can trap more air inside, adding to their insulation value. Surface area is further reduced by a tucked body posture when inactive. Some birds allow their body temperature to fall during periods of inactivity, decreasing the gradient between them and air temperature. A lower body temperature saves some energy that would have been required to maintain a higher one. Finally a problem arises with excess amounts of heat escaping through body appendages like legs that have a large surface area and are left to perch on cold tree branches all winter. A process known as countercurrent heat exchange places arteries entering an appendage next to veins leaving an appendage so that heat can be transferred between them. Heat moves from the warm arteries to the cold veins so that the arteries are cooled by the time they reach the appendage and the veins are warmed as they enter the body core. This has a net effect of cooling the appendage down so that less heat is allowed to escape.
With the physiological aspects aside there are many behavioral adaptations to consider as well. Birds often look for some sort of shelter before a long cold night. Ruffled grouse often burrow deep into the snow to avoid particularly viscous winter storms. Other birds such as the gold-crowned kinglet huddle with their peers to minimize heat loss. Still other birds make food caches to help balance their energy budget.
Mammals employ many of the same tricks of winter survival as birds. They pack on the insulation through fur and often are able to pack on much larger stores of fat than birds. Some mammals also create food caches to utilize in food shortages. They also use countercurrent circulation and reductions in body temperature to minimize heat escape. Many small mammals also seek out shelter from the cold and huddle with others to minimize their exposed surface area.
The ability to store up body fat seems to allow more mammals to wait out the winter in their respective dens than birds. Skunks for example are content overwintering in a burrow undergoing daily torpor and often huddling to limit energy costs.
Mammals also have the ability to not only undergo shivering to raise their body temperature, but also non-shivering thermogenesis. They do this by metabolizing brown fat stores (a fat with greater amounts of mitochondria able to produce more heat).
With the shortages of food faced in winter many mammalian herbivores resort to other sources of food such as deer eating the bark of trees. Habitat preference can also be influenced because of winter challenges such as walking through deep snow. Coyotes for example have been found to prefer coniferous habitats during winter because they have the lowest snow sinking depth.
An insect no matter what life stage it is in (egg, pupae, larvae or adult) must do one of two things to survive winter, give into freezing or resist it (some can do both however). Freeze avoidance insects adjust the physiology of their body to lower the freezing point of blood and body fluid. Many insects are able to supercool up to -20 C without any adjustments simply because of the small volume of water held in their body. Many insects avoid contact with foreign ice crystals (which would lead to tissue freezing) by choosing a dry, protected hibernation site.
Further lowering the supercooling limit can be done in a number of ways, but mainly by limiting the amount of free water in their bodies, the production of antifreeze proteins and peptides, and the elimination of ice nucleation points. Many insects expel gut contents to minimize the amount of ice nucleators. Some also remove lipoprotein nucleators from their hemolymph. Any additional nucleartors must become associated with organelles or cell membranes in order not to allow ice nucleation.
In the addition of anti-freeze protein production, some insects produce low molecular weight compounds, often polyhydric alcohols, that aid in lowering the freezing and super cooling points. The most common of these compounds is glycerol, but sorbitol, mannitol, and ethylene glycol can be produced alone or in addition to each other.
The other option, freeze tolerance, seems a much safer bet for those insects that are able to do so. Supercooled insects must over winter in the shadow of a flash freeze that would instantly kill them. Freeze tolerant insects are in a much more stable state, but still face the danger of temperatures dipping low enough to freeze their intracellular fluid also.
In order to avoid the flash freeze that spells the demise for freeze avoidance insects, freeze tolerance insects actually produce nucleation points instead of removing them. By freezing early, they avoid supercooling and have a more controlled, slow freezing. This slow and controlled freeze also allows for natural osmosis to occur as water moves from inside to the cell to the growing ice outside of it. This osmosis however must be checked or else fatal cellular dehydration can occur when about 65% of total body water is turned to ice. Antifreeze proteins, just as in freeze avoidance insects, control the amount of water aloud to freeze. The protection of antifreeze proteins is supplemented in freeze tolerant insects by the production of polyols that bind to and prevent some water from freezing.
A final category of insects that must be addressed is somewhat of an exception to the rule of freeze avoidance. Bees, unlike most other insects are endothermic, being able to produce heat through their wing muscles. Colonial insects such as bees can swarm together and huddle out the winter. Living on energy stores in their honey and the heat produced from their wings, a coordinated thermostat develops within the hive. The hive consists of a prime over wintering site for bees, that if left during winter, spells disaster for a bee.
Winter survival for freeze-tolerant amphibians is very similar to that of insects, both being cold blooded. Land-hibernating frogs overwinter just under the organic litter of soil and under a blanket of snow. In such an exposed location freezing is inevitable and occurs at -2 to -3 C. With such a large body mass (compared to an insect) frogs have no problem freezing early and have no need to produce artificial nucleators. While frogs also use antifreeze, the most common cryoprotectant is glucose. In this case it seems the main function of glucose is in the protection of cell membranes. This incredible feat however does not come with much preparation for the most part. Blood glucose levels can be increased to levels 200 times more than previous levels within 8-hours. In order to distribute the excess glucose throughout the body in time, a frog’s heart rate can double. When ice content within a frog’s body reaches 60 to 65 percent all breathing and cardiac activity ceases. From this point on the frog is kept barely alive by the small amount of anaerobic metabolism of its energy stores. Despite these measures, no frog has been found to survive if temperatures in their subnivean lair fall below -7 C. Only an hour after thawing the heart begins to beat again, and within 6 hours the heart rate can be back to normal. Frogs may thaw and refreeze several times throughout its hibernation with no ill effects.
Those amphibians that cannot tolerate freezing escape the ice by overwintering in aquatic habitats. There are exceptions to this rule however with some frogs staying in soil pockets under leaf liter and in caves. Whichever site they choose it must not expose them to freezing conditions and must have abundant oxygen. Toads escape freezing temperatures by borrowing deep into soil or following rodent tunnels to below the freezing line of soil.
Herbaceous plants + Trees
The two major problems faced by plants in winter are desiccation and the threat of intracellular freezing from cold temperatures. In plants there is no way to avoid freezing, plants do not bother to produce antifreeze compounds. Instead they rely heavily on osmosis and the natural flow of water to draw water outside of its cells to freeze. The freezing process beings with ice formation in the nonliving xylem and extra-cellular spaces. The freezing of this extra-cellular ice creates a pull on the water inside of the cell (because of an energy gradient), moving it out through the cell membrane. As the water leaves the cell, the solute concentration within the cell increases causing a freezing point depression. The growing ice can even penetrate the cell wall, pushing the cell membrane back. The only problem with ice growth can occur when cells dehydrate to a degree that irreversible damage occurs. The key to the process of freezing is that it occurs at a slow rate so that the water moving out of the cell can keep up, if not intracellular freezing will occur causing cellular death. It is believed that the plant hormone abcisic acid plays a major role in the plant acclimatization process. The hormone (ABA) likely causes biochemical responses such as changes in specific sugars and proteins as well as increasing lipid unsaturation. In a later stage macromolecules are rearranged to resist dehydration (entering a depolymerized state where water can be bound to proteins).
Coniferous plants address the issue of winter dessication from their leaves through a waxy cuticle covering the outside of the leaf. A thin layer of air that forms around the leaf aids this resistance to water loss. The air around the leaf acts similar to the air used within the insulation of feathers to add to their effectiveness. Decedious trees have already lost their leaves and avoid this problem, but some water is still lost through other means. What water that is lost by the tree however seems to be replaced in part by movement through the xylem despite ice formation within it. More water may be absorbed from areas of the plant that lie under the snow pack. Air within the snow pack can have a relative humidity exceeding 99 percent, in which case water can move into the cells of the plant (this however may be very limited).
Trees must also deal with the problem of snow loading causing damage. Deciduous trees avoid much of this problem by allowing their leaves to fall off before the winter months and regenerate in spring. Coniferous trees risk the additional damage from ice in order to save the energy of re-leafing and having the ability to photosynthesis whenever weather permits. It appears that some trees (most being deciduous) also have the ability to photosynthesis through their bark.
Some herbaceous plants known as annuals die out every year and leave their neatly packaged seeds behind to overwinter and carry on their genes. Perennials often let their above ground presence die away and choose to retreat back to their bulb and roots to survive winter in a more controlled subterranean environment.
It is my understanding that Lichens may be the most winter tolerant species of all. They are extremely hearty, even being able to survive a trip in space. Compared to the extreme pressures of open space, winter is nothing. Lichens can simply shut down and wait out ill weather. They are able to dehydrate their cells to prevent intercellular freezing and lie dormant until they are freed from the ice. Lichens are even known to photosynthesis at temperatures as low as -20 C (http://www.jon-nelson.com/lichens.asp).
Fungi can survive winter either as resting spores, hyphal bodies, conidia, or in anholocyclic aphid populations depending on species (http://glwww.mst.dk/udgiv/
publications/2001/87-7944-343-5/html/kap04_eng.htm#4.1). I can only assume that fungus dehydrate their cells as temperatures drop to avoid intracellular freezing and mortality. They may also employ antifreeze compounds to further lower their freezing point.
Again mosses must evacuate excess water from their cells to avoid intracellular freezing. It also appears that they may utilize sugars such as sucrose for their antifreeze properties. The cold tolerance of bryophates appears to increase between summer and winter as the mosses make physiological changes in preparation for winter (http://www.springerlink.com/content/j1l66062l050585n/).