Freeze Tolerant Animals

For many animals that live in climates with extreme winter temperatures, the ability to survive freezing of body fluids is a necessary part of their existance. Natural freeze tolerance occurs in aquatic animals such as polar fishes and intertidal invertebrates, terrestrial amphibians and reptiles, and various polar and temperature insects. Various strategies have been worked out by these disparate animals to withstand the rigours associated with ice formation at low temperatures.

Freeze Tolerance in Amphibians and Reptiles

There are many examples of lower vertebrates that hibernate in temperature regions where ice growth in the extracellular fluid is tolerated within certain limits. For example, the wood frog, Rana sylvatica, is capable of withstanding temperatures as low as -8C, with 65% of its body water converted to ice, or at temperatures of -2.5C for periods of up to 2 weeks. Ice formation of this magnitude causes the cessation of all muscle movements (heart, breathing, vasoconstriction, skeletal), the onset of ischemia, and large changes in the volume of cells and organs. Other terrestrial frogs and some turtles display similarly advanced freeze tolerance while there are also many other reptiles and amphibians that are able to withstand short, mild freezing exposures typical of overnight frosts.

There are several factors that influence the ability of a vertebrate to survive extracellular ice formation (there are no examples of vertebrates that can withstand intracellular ice formation).

Aquatic Animals at Low Temperatures

There are many intertidal invertebrates that survive brief periods of extracellular ice formation. This occurs primarily by the mechanism of colligative cryoprotection, as in terrestrial animals. The real action here is the strategy that polar fishes have evolved for dealing with the freezing environment.

Seawater freezes at -1.9C, a temperature that is reached during the winter in polar and temperature ocean regions. This is well below the melting point of the body fluids of marine fishes (-0.8C). Although it is conceivable for an organism to exist with 1 degree of supercooling, the marine environment is one in which there are small ice crystals suspended throughout the seawater when it reaches the freezing point. Since it is necessary for fishes to live in this environment, and pass this water (with its ice crystals) through their gills, it is impossible for these organisms to avoid ice growth within their bodies. Yet they appear not to freeze until the temperature drops below -2C, at which point they will freeze and die.

The melting point of the fluids within these polar fishes is -0.8C, but the apparent freezing point (the temperature where ice begins to grow) is significantly lower. This freezing point depression is not colligative (although the depression of the melting point is) and is lost when the fluid is dialyzed through a molecular sieve with a cutoff of about 2500. The agents responsible for this freezing point depression are either glycoproteins (Antarctic and North-temperate fish) or proteins (Arctic fish). These compounds are generically referred to as antifreeze proteins.

Insect Adaptations to Low Temperatures

The ability of insects to adapt to diverse ecological conditions is legendary. This tremendous diversity is justly illustrated by their ability to withstand the intense cold of arctic and alpine environments. Indeed, the Arctic spring is accompanied by a veritable deluge of biting insects; a grim but unmistakable testament to their overwintering capabilities.

The adaptations that have evolved to allow insects to survive low temperatures are legion, but they can be classified along two general lines, freeze tolerance (the ability to survive following ice formation within the body cavity) and freeze avoidance (the prevention of ice formation within the body cavity at temperatures where such freezing would normall occur).

In addition to the problems posed by ice formation, there are also significant problems that must be solved for normal metabolism to occur at low temperatures. The maintenance of neural function, fluidity of cell membranes, pH control, activity of enzymes, adaptation to hypoxia, dehydration of body fluids, etc. all present obstacles to low temperature survival. Although these difficulties are formidable, they will not be discussed further here, as the adaptations to freezing temperatures are the focus of this chapter.

Very few insect species are actually exposed to the full rigors of winter temperatures as most choose an overwintering microhabitat that provides a buffered temperature. The habitats provided inside vegetation (logs, stumps, etc.) or under the soil provide thermal buffering, especially when covered with snow. In many climates, however, the organisms are still exposed to potentially lethal conditions throughout the winter. The particular adaptations associated with freeze tolerance and freeze avoidance allow these organisms to survive in such harsh environments.

  • Freeze Tolerance

    Many species of insects have developed a tolerance for ice formation within their body fluids. The degree to which these species withstand freezing varies widely, from just a few degrees below freezing to -87C for an Alaskan beetle. The strategies employed are legion, although the principle means for minimizing injury from ice formation is the use of cryoprotectants to reduce the amount of ice formed and the salt concentration at a given temperature.

  • Freeze Avoidance

    Insects that are freeze-susceptible (ice formation in their body fluids is lethal) need to avoid freezing during the winter months. There are three basic strategies that insects employ to avoid ice formation within their body cavity: 1. Colligative depression of the freezing point through the concentration of a low molecular weight solute; 2. Production of an antifreeze protein (AFP) to lower the crystal-growth temperature non-colligatively; 3. Lowering of the nucleation temperature by removal of ice nucleation sites.

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    Document last updated Mar. 1, 1999.
    Copyright © 1999, Ken Muldrew.