|By Janet M. Storey||(Click links for more images.)|
Winter arrives and the mercury once again plunges below 0 C. We humans retreat into the shelter of our heated homes and warm coats and make plans for spring break on a sunny beach. We are all aware that most animals disappear from our sight over the winter. Those that remain are, like ourselves, of the warm-blooded sort -- various birds, squirrels, rabbits, etc. However, we don't often stop to think about what winter means to cold-blooded animals like frogs and turtles, insects and spiders. Where do they go and how do they cope with freezing temperatures? We have only to look at the effects of the first hard frost on a once-glorious autumn garden to be reminded that, for most organisms on Earth, freezing is lethal. So, how do cold-blooded animals make it through a harsh Manitoba winter?
Surviving Winter with different strategies:
Mourning Cloak (Nymphalis antiopa) Winters as an adult in cracks in tree bark or under roof shingles.
Wood Frog (Rana sylvatica) Can freeze beneath forest litter.
Garter Snakes spend winter below the frost line in holes or rock caves.
Snapping Turtle (Chelydra serpentina) Can breathe underwater during winter!
As a first approach, most animals do whatever it takes to avoid exposure to subzero temperatures. This can mean migrating to warmer climates, going deep underground below the frost line, or spending the winter underwater, which is generally safe unless a pond or lake freezes to the bottom. Many birds, some bats and even some insects, like the monarch butterfly head south to avoid the cold. Garter snakes may travel several miles to mass by the hundreds in warm underground dens, toads may dig down in the soil as much as 3-4 feet to stay below the frost line, and many other small animals (salamanders, centipedes, etc.) descend down tunnels created by rodents or tree roots to spend the winter underground. Many common insects, such as dragonflies, spend the winter as aquatic larvae that emerge into the air again in early summer. Many frogs and turtles also choose the relative warmth of the aquatic environment as their winter home. Here, temperature is typically stable between 0 and +4 C all winter. For these lung-breathing animals, however, this poses a new problem - how to get oxygen! Frogs solve this by absorbing oxygen and releasing carbon dioxide across their skin. The tougher skin of turtles is not so obliging but some species, like snapping turtles, can take up some oxygen from the water across the skin lining the throat. Others, like painted turtles, have altered their metabolism to survive without oxygen. In cold water, painted turtles can stay submerged for as long as 3 months with zero blood oxygen! Humans, by contrast, begin to suffer irreversible nerve damage if oxygen supply to the brain is cut off for more than 4 minutes.
Other animals, however, don't have a foolproof way to avoid exposures to subzero temperatures. Some insects hibernate in places, such as under tree bark, where they get little or no relief from the full blast of winter air temperatures Many other cold-blooded animals spend the winter at or near the soil surface. This includes lots of invertebrates such as insects, spiders, centipedes, and snails as well as other kinds of frogs and turtles that winter on land. Grass cover or leaf litter, coupled with a thick layer of snow and the "heat sink" of the unfrozen earth beneath, can hold temperatures in the subnivean (under the snow) environment between about 0 and 5 C for long stretches of the winter. However, the protracted deep cold of January-February drives frost through the leaf litter and temperatures under the snow also fall to subzero values, although rarely lower than about -8 C.
So, many kinds of cold-blooded animals have to deal with the fact that the temperature of their environment is much lower than the temperature at which their body fluids normally freeze. What do they do about this? There are only two choices - either find a way to stay liquid or find a way to survive being frozen solid. Both choices have been mastered by different animals and the general strategies are named "freeze avoidance" and "freeze tolerance". Freeze avoidance is the much more common strategy of winter survival because it is a lot simpler to find ways to keep body water from freezing than it is to find solutions to the many different types of injuries that can be caused by ice forming in a body. Both strategies, however, involve some pretty amazing adaptations.
Many insects and other small animals have perfected methods for staying liquid even at very low subzero temperatures. Most insects that winter under the snow in the leaf litter can cool to least -10 to -15 C before they freeze whereas those that winter above the snow pack can often cool to -40 C without freezing. Arctic insects can do even better, some staying liquid down to -55 C. How do they do this?
The freeze avoidance strategy has three main parts. First, special antifreeze proteins are produced and circulated in the blood. These proteins are specially designed so that they bind to the surface of embryonic ice crystals and prevent them from growing any further. So, any ice that forms stays in tiny crystals that can't do any harm to tissues. Antifreeze proteins are also present in the blood of cold water marine fish so that they will not freeze when they swim in -2 C polar waters.
Secondly, the animals build up extremely high concentrations of sugars or sugar alcohols in their blood and tissues and this lowers the freezing point of their body water. This is exactly the same strategy that we use to prevent the water in a car radiator from freezing and indeed, some insects actually synthesize and accumulate the same chemical, ethylene glycol, that we put in the car! More commonly, however, a related chemical called glycerol is used which is much less toxic. By midwinter, the amount of glycerol in an insect's body is so high that it often makes up 20-25% of its entire body weight!
The third part of the freeze avoidance strategy is to prevent contact with molecules that can act as "ice nucleators", molecules that "seed" the formation of ice in body fluids. Ice itself is the most potent ice nucleator and so animals take steps to avoid contact with environmental ice. Some insects do this by wintering in very dry places, but most rely on wrapping themselves in water-proofing. This can be done by increasing the thickness of the waxy cuticle on an insect's body or by spinning a water-proof cocoon. Potential nucleators inside the insect's body are also eliminated such as by emptying the gut to get rid of foreign particles and bacteria and by seasonal changes to the types of proteins present in their blood to remove any whose structure could stimulate ice crystallization.
Carry on for More about Freeze Tolerance
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