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Extreme Cold Hardiness in Ectotherms
By: Jon P. Costanzo (Deptartment of Zoology, Miami University) © 2012 Nature Education
Citation: Costanzo, J. P. (2012) Extreme Cold Hardiness in Ectotherms. Nature
Education Knowledge 3(10):3
Ectotherms, animals whose body temperature closely tracks ambient temperature, occur in virtually
every ecological niche on Earth. By virtue of some remarkable adaptations, they thrive even at high
latitudes and altitudes in habitats characterized by seasonal or continuous cold (Addo-Bediako et al.
2000). Because aquatic habitats tend to be relatively warm and thermally stable, even in the Polar
Regions, this article focuses on ectotherms that occupy terrestrial, arboreal, or intertidal habitats where
temperatures may fall appreciably below the freezing point (FP) of body fluids.
For any ectotherm, even brief exposure to subzero temperatures carries the risk of irreparable injury or
death. Cold impairs cellular functions by rigidifying membranes, slowing ion pumps, inducing oxidative
damage, denaturing proteins, and altering energy balance. Freezing of tissues also provokes myriad
stresses that are injurious and potentially lethal to most species. Even for cold-hardy ectotherms,
survival depends on cooling rate, exposure temperature, and the duration and frequency of subzero
chilling episodes.
Not surprisingly, a species' capacity for cold hardiness is well matched to the thermal regimen to which
it has adapted. It may exhibit a latitudinal or altitudinal cline such that populations inhabiting colder
regions are adequately protected. Within a given population, survival limits, and even the cold
hardiness mechanism (i.e., freeze avoidance or freeze tolerance), can change from year to year (Kukal &
Duman 1989). For ectotherms with complex life cycles, such as holometabolous insects, cold hardiness
commonly is most pronounced in the overwintering life stages (Salt 1961).
Thermal Environments of Cold-hardy Ectotherms
Avoidance often is an animal's primary means to protection from extreme temperatures. If migration to
warmer climes is not an option, survival may depend on finding an overwintering site, or hibernaculum,
that insulates from damaging cold. For example, some toads and terrestrial turtles, being proficient
excavators, descend into the soil column and overwinter below the reach of frost. Various snakes and
woodland salamanders evade frost by following abandoned rodent burrows or root channels to
underground lairs. Nevertheless, many species encounter subzero cold, either because their winter
refuge lacks adequate insulation or because they are behaviorally active during cold weather (Figure 1).
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Figure 1: Garter snakes (Thamnophis sirtalis) emerging from a subterranean hibernaculum in late February.
T. sirtalis is the most northerly distributed, and the earliest and latest active, of all North American snakes.
Hibernation usually is below the frost line, but transient exposure to subzero temperatures can occur during the
colder portions of the activity season; these it survives by virtue of its freeze tolerance (Costanzo et al. 1988).
To survive in winter, even cold-hardy ectotherms must seek thermally buffered sites. The ideal
hibernaculum also conceals its occupant from potential predators, permits gas exchange, and prevents
excessive desiccation. Some species prefer relatively exposed sites from which they can readily detect
environmental cues stimulating spring emergence. For others, such as plant gall-inhabiting insects
(Baust et al. 1979) and the hatchlings of some turtles (Costanzo et al. 2008), there is no choice in the
matter: winter is passed in the very place where one hatches.
The thermal regimens to which overwintering ectotherms are exposed vary geographically, with the
intensity and frequency of subzero exposures increasing with altitude and latitude. Even locally,
temperatures can range from mild to severe, depending on site characteristics and the physical
features of individual hibernacula, and, being subject to the vagaries of the weather, can vary markedly
from year to year (Figure 2). Within hibernacula, prevailing temperatures follow a pronounced seasonal
rhythm, usually attaining the lowest values at high winter, and also oscillate on a diel cycle.
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Figure 2: Regional, local, and annual variation in winter temperatures encountered by hatchlings of the
painted turtle (Chrysemys picta).
In North America, C. picta occurs farther north than any other turtle. Although their eggs hatch in late summer,
neonates commonly remain inside the nest, in the company of siblings, until the following spring. Temperatures
inside these nests, which are constructed in open areas near water bodies, commonly fall below zero and, in
extreme cases, may approach -15°C (Costanzo et al. 2003). Winter minima within nests tend to be higher in locales
where air temperature is temperate and snow cover is extensive and frequent, such as the upper Great Lakes
region, and lower in colder locales where snow cover is scarce, such as the Great Plains region. Shown are minimum
temperatures recorded during winter 2000-01 inside 13 nests in the Sandhills of west-central Nebraska and seven
nests near a millpond in northern Indiana (adapted from Costanzo et al. 2004). Thermal minima within individual
nests vary markedly due to differences in topography, aspect, and other environmental factors. Data collected at
the Indiana site over five winters illustrate pronounced annual variation in thermal regimens owing to the vagaries
of weather. The graph depicts the winter minimum temperature attained in the warmest and coldest nests, as well
as the average for all nests studied in each winter (N as shown). Adapted from Baker et al. (2010).
Freeze Avoidance
Many biologists would be surprised to learn that a solution or organism does not necessarily freeze at
its FP, but under certain conditions can cool much further whilst remaining unfrozen (liquid), or
"supercooled." Indeed, a small volume of pure water can be chilled to nearly -40°C before it
spontaneously freezes at its so-called "supercooling point," or, more accurately, temperature of
crystallization (T ).
c
To exploit the purely physical phenomenon of supercooling in a freeze-avoidance strategy, an
organism must remain free of potent ice-nucleating agents (INAs), any of various inorganic
particulates, microorganisms, proteins, and organic residues that can organize water molecules into a
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crystalline arrangement. Ubiquitous in nature, INAs of various potencies occur in diverse habitats,
including the overwintering sites of molluscs (Ansart et al. 2010), insects (Zachariassen & Kristiansen
2000), amphibians (Costanzo et al. 1999), and reptiles (Costanzo et al. 2000), and may enter the body
through orifices or be inadvertently ingested with food. Many freeze-avoiding ectotherms prepare for
dormancy by eliminating ingested INAs, and also by masking or inhibiting endogenous ice-nucleating
proteins (Costanzo et al. 2003, Duman 2001). It is crucial that they avoid physical contact with ice,
which potentially can invade the body and initiate freezing. Species that rely on supercooling for winter
survival can reduce the risk of such "inoculative freezing" by selecting hibernacula that limit their
exposure to environmental ice. Some harbor in their tissues antifreeze proteins (AFPs) that effectively
inhibit inoculation (Duman 2001).
To avoid ice nucleation, many cold-hardy ectotherms accumulate one or more cryoprotectants in
advance of winter (Zachariassen & Kristiansen 2000). Representing several classes of compounds, these
solutes vary by species, but all are of low molecular mass and benign in high concentrations (Table 1).
They not only colligatively depress the organism's FP, much as automotive antifreeze (e.g., ethylene
glycol) prevents radiator fluid from freezing, but also can enhance supercooling (Figure 3).
Supercooling capacity is further increased by partial dehydration of the body, which occurs preparatory
to winter in many insects (Lee 2010).
Class
Examples
Known From
Carbohydrates
polyhydric alcohols (glycerol,
sorbitol, ethylene glycol);
sugars (glucose, trehalose);
cyclitols (myo-Inositol)
bacteria, marine
and terrestrial
invertebrates,
amphibians,
reptiles
Amino acids &
derivatives
taurine, glycine, proline,
alanine, asparagine, glutamic
acid, lysine
bacteria, marine
and terrestrial
invertebrates
Methylamines
glycine betaine,
glycerophosphorylcholine,
trimethylamine oxide
bacteria, marine
invertebrates,
beetles
Urea
Terrestrial
gastropods,
amphibians,
reptiles
Table 1: Cryoprotectants used in animal freeze-avoidance and
freeze-tolerance.
Permutations of the freeze-avoidance strategy include vitrification, a process in which body fluids form
a glass when cooled to extreme temperatures (Sformo et al. 2010), and cryoprotective dehydration, a
survival adaptation of various invertebrates that overwinter in frozen substrata (Holmstrup et al. 2002,
Sørensen & Holmstrup 2011). Because the vapor pressure of supercooled water exceeds that of ice,
water tends to leave the unfrozen body until the internal vapor pressure reaches that of its frozen
environment. Concomitantly, tissue FP drops as solutes become concentrated in a reduced water
volume (and, commonly, as cryoprotectants are synthesized), thereby totally eliminating the risk of
freezing. For this mechanism to be effective-and for ice inoculation to be avoided-dehydration must
proceed quickly enough that the organism's FP consistently remains near the temperature of its cooling
environment. Consequently, this strategy can be useful only to small ectotherms with a highlypermeable integument and profound desiccation tolerance.
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Excessively deep or prolonged chilling can result in transient or permanent neuromuscular dysfunction
or cold narcosis ("chill coma"), and even death (Salt 1961). Underlying mechanisms of chilling injury are
not well understood, but probably include disturbance of ion homeostasis and metabolic functions,
oxidative stress, and adverse phase changes in membranes (Kostal et al. 2006, MacMillan & Sinclair
2011).
Figure 3: Effect of solute concentration on a solution’s
freezing point (FP) and supercooling capacity, as
represented by its temperature of crystallization (T )
c
Upper (red) line depicts the theoretical FP depression of
-1.86°C per osmole. Lower lines are regressions fitted to
empirically determined Tc values for 5-µl samples of
solutions containing various quantities of solute, with or
without hemolymph (5% v/v) from the cold-hardy beetle,
Eleodes blanchardi. Note that a potent ice-nucleating agent
in the hemolymph markedly raised the solution’s T but did
c
not alter the effect of solute concentration on supercooling
capacity. Modified from Lee et al. (1981).
Freeze Tolerance
Found in a small number of arthropods, molluscs, nematodes, annelids, amphibians, and reptiles,
freeze tolerance is an adaptation for the survival of tissue freezing under ecologically-relevant thermal
and temporal conditions. Freezing and thawing of the body, whilst potentially lethal, can be managed
by mounting a diverse array of molecular and physiological responses that limit injury to cells and
tissues.
For many species, freeze tolerance is expressed seasonally, usually developing in autumn in response
to environmental cues and becoming most robust during the coldest months. In the laboratory,
capacity for freezing survival can be enhanced by acclimation to high salinity and low environmental
temperature, oxygen tension, and water potential (Lee 2010, Murphy 1983).
Control of the freezing process is key to survival in freeze-tolerant ectotherms (Lee & Costanzo 1998).
In many invertebrates, which are prone to supercool owing to their small size, freezing is initiated by
INAs in the hemolymph or other tissues. In other taxa, particularly species (e.g., amphibians) having a
permeable integument, inoculation by external ice ensures that tissues freeze at relatively high
temperatures (Costanzo & Lee 1996, Layne et al. 1990). Slow cooling enhances freezing survival,
perhaps by allowing the organism adequate time to mount adaptive responses and by restricting ice to
extracellular spaces; intracellular freezing is lethal for all but a few species (Lee 2010, Murphy 1983).
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Ectotherms vary markedly in their limits of freeze tolerance. Known from laboratory studies, freeze
endurance extends from perhaps a few days to several months of continuous freezing, although
multiple freeze-thaw episodes are commonly experienced in nature (Layne et al. 1999). Depending on
the taxon, the minimum survivable temperature ranges from a degree or two below 0°C to about -80°C
(Table 2). Despite this variation, apparently no ectotherm can withstand the freezing of more than
50-80% of their body water.
Lower Lethal
Temperature
(°C)
Reference
-20
Loomis
1991,
Murphy
1983
earthworms,
enchytraeids
-20
Holmstrup
2007,
Slotsbo et
al. 2008
Terrestrial
molluscs
slugs, snails
-5
Ansart and
Vernon
2003
Other
terrestrial
invertebrates
free-living
nematodes,
centipedes, flies,
beetles,
butterflies and
moths, wasps
-80
Lee 1991,
Wharton
2002
Amphibians
salamander,
frogs
-40 (frogs, -6)
Storey and
Storey 1992
Reptiles
turtles, lizard,
snake
-4
Storey and
Storey 1992
Taxon
Examples
Marine
invertebrates
(intertidal)
barnacles,
bivalves,
gastropods
Terrestrial
annelids
Table 2: Thermal limits of freeze tolerance in animals
Molecular and physiological adaptations to freezing and thawing help prevent cell death from
dehydration, mechanical distortion, damage to macromolecules and membranes, metabolic
perturbation, and ischemia/reoxygenation (Storey & Storey 1992). Dehydration is a particularly onerous
problem. As ice accumulates in extracellular spaces, water is lost from the supercooled cells to the
interstitium where solutes, rejected by the forming ice crystals, become increasingly concentrated in
the as yet unfrozen solution. The rising osmotic potential of this solution withdraws water from inside
the cells, progressively shrinking them and concentrating cytosolic solutes to potentially harmful levels
(Mazur 1984). Oxidative damage, precipitated by the return of oxygen to thawed tissues, also
contributes to cryoinjury (Storey & Storey 2010, Swartz 1972).
Freezing/thawing stress can be ameliorated by invoking mechanisms of anoxia tolerance, depressing
metabolism, and accumulating one or more cryoprotectants, either in anticipation of or during
freezing. Cryoprotectants used in freezing adaptation include a host of sugars and sugar alcohols,
amino acids, and even the "waste product," urea (Table 1). By colligatively depressing the FP of body
fl uids, these solutes permit the cytoplasm to remain supercooled whilst the extracellular fl uids free
ze,
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and also limit ice formation (Storey 1997). Many play supportive roles in antioxidation, energy supply,
macromolecular stabilization, and counteraction of perturbing solutes (Yancey 2005).
Freeze Avoidance or Freeze Tolerance?
Freeze avoidance is an effective survival strategy in small ectotherms that either use dry hibernacula or
possess an impermeable integument. Although exposure to extreme cold or prolonged chilling can be
harmful, cryoinjury can be minimized if freezing is avoided. Accordingly, freeze-avoiding species
generally tolerate a relatively broad range of subzero temperatures. On the other hand, overwintering
in the supercooled state is a precarious proposition, as death is the likely consequence of an
inadvertent nucleation event.
Freeze-tolerant ectotherms may be of any size and utilize myriad hibernacula, including ones that
harbor ice. For those overwintering in an icy environment, tissue freezing eliminates the vapor pressure
gradient across the integument and thereby prevents the organism from desiccating. On the downside,
freezing/thawing stresses can be highly deleterious and, therefore, survival is possible over a relatively
limited range of temperatures and exposure durations.
Although most species rely on freeze avoidance or freeze tolerance, a very few are uniquely adapted to
employ either strategy, where the mode used during any particular chilling episode depends on
prevailing physiological and/or environmental conditions (Costanzo et al. 2008, Sformo et al. 2009,
Sinclair et al. 2004). Intensive study of these species may provide new insights into how the various
selective pressures that shape life history traits drive the evolutionary development of cold-hardiness
strategies (Sinclair et al. 2003).
Acknowledgements
The support of the National Science Foundation (Grant Number IOS-1022788) is gratefully
acknowledged. The manuscript was improved by comments by M. C. do Amaral, R. E. Lee, T. J. Muir,
and two anonymous reviewers. Graphics are a courtesy of M. F. Wright.
Glossary
antifreeze proteins: Class of specialized polypeptides or glycopolypeptides, also known as
ice-structuring proteins (ISPs), that bind to minute ice crystals to inhibit the growth and
recrystallization of ice in a solution.
antioxidation: Mounting defenses against the accumulation of free radicals and development of
oxidative stress.
chill coma: Physiological state of being immobilized by the absence of neuromuscular function from the
effects of low temperature on cell and tissue function.
cryoprotectants: A substance that acts through colligative or other properties to limit damage from
exposure to low temperature and/or the freezing of biological tissue.
cryoinjury: A general term referring to perturbation or damage associated directly or indirectly from
exposure to low temperature and/or the freezing of biological tissue.
hibernaculum: The protective shelter of a hibernating animal.
holometabolous: Adjective referring to an insect that undergoes complete metamorphosis, which
includes four life stages: egg or embryo, larva, pupa, and imago or adult.
ice-nucleating proteins: Specialized protein with an ability to initiate the freezing of a solution,
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apparently by orienting water molecules into a crystalline lattice.
oxidative stress: A process in which free radicals accumulate in cells with deleterious consequences.
Caused by an imbalance between the production of reactive oxygen species and the ability to detoxify
the reactive intermediates or repair the resulting damage.
supercooling point: Supercooling, sometimes referred to as undercooling, is the tendency for a solution
to remain in the liquid state when cooled below its freezing point. The supercooling point is taken as
the lowest temperature achieved before the solution begins to freeze.
temperature of crystallization: Temperature at which a substance, supercooled or not, begins to freeze.
vapor pressure: The pressure exerted by a vapor in thermodynamic equilibrium with its solid or liquid
phase. Relates to the propensity of particles to escape by evaporation (liquids) or sublimation (solids).
vitrification: The process by which a substance is converted at its glass transition temperature into an
amorphous solid that is devoid of crystalline structure.
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