Life in the Cold



Chapter 1 

Winter Paths: Options for overwintering success.

  • Spiders remain active under the snow

  • Ermine line their nests with the fur of their victims

  • Ermine represents all that is 

  • 3 basic Strategies for Surviving Winter 

    • Migration

    • Hibernation

    • Resistance


Migration

  • Precarious due to energetic costs

  • Birds may carry 50% of body weight as fat 

    • High energy density 

  • Subject to hunting along their migration

  • New diseases, predators, parasites, and food availability 

  • The only option for many birds 

  • Not a viable option for many land animals

    • Too energetically expensive

Hibernation

  • Relatively uncommon winter strategy

  • No birds can hibernate

  • Requires animals to survive without food for an uncertain period

  • Den site availability can be a limiting factor

  • Energy demands can be too high for some species

  • All hibernators arouse periodically

    • Satisfy sleep debt 

    • Remove waste

  • High mortality for many rodents

    • 23-68% for adults

    • 41-91% for juveniles

  • Bears maintain a body temperature 6-7 degrees below normal

    • Reduces metabolic rate by ~½ 

    • Overwinter success rate >99% 

  • Reptiles and Amphibians

    • Hibernation is the only means of surviving the winter season 

    • Some 


Resistance

  • Plants

    • Produce no heat

    • Depend on biochemical changes 

      • Ice nucleating proteins 

      • Glycerols

  • Birds and mammals

    • Grouse grow more foot scales for better grip

    • Benefits to high foot-surface-to-body-weight ratio

    • Subnivian survival

      • Dependable annual snow is critical 

        • Acts as a thermal blanket

      • 20cm known as the “hiemal threshold”

        • This marks the true start of winter


Chapter 2

The Changing Snowpack


  • Snowflakes are destined for destruction

  • Outside forces

    • Wind

    • Fracturing

    • Compaction

    • Vaporized and recondensed 

    • Melted and refrozen

  • Three distinct processes affect the snowpack

    • Each process is influenced by the following:

      • Time

      • Internal snowpack characteristics

      • External weather conditions


Destructive Metamorphism

  • Formation of more or less rounded ice grains

  • “Equi-temperature” metamorphism

  • Reordering of water molecules on the surface of each snowflake

  • A net reduction in surface area 

  • Promoted by anything that promotes closer contact of snow crystals

    • Wind packing

    • Weight of overlying snow

    • Warm air temperature

  • Igloos and Quin-zhees take advantage of this stability


Constructive Metamorphism

  • “Temperature gradient” metamorphism

  • Snow surface is the same temperature as the air

  • Snowpack floor is closer to freezing 

  • Temperature affects the distribution of water vapor in the snowpack

  • The air inside of the snowpack is nearly 100% relative humidity

    • Warmer areas will have higher absolute humidity (more water molecules)

  • Upward migration of water vapor through the snowpack

    • Condensation onto the outside of the ice grains results in crystal growth 

    • Crystals on the bottom of the snowpack continually diminish in size

      • Termed “depth hoar”

      • The base of the snowpack becomes brittle

      • Facilitates the movement of small mammals

      • Increased avalanche danger


Melt Metamorphism

  • Meltwater acts as a heat pump gaining energy when it undergoes a state change and then transferring this to lower layers in the snow

  • Sno melts most quickly under foggy conditions 

  • Latent heat of condensation is very high (7x higher than condensation)


The insulative value of a Snowcover

  • The greatest effects felt in the subnivean

  • Density differences strongly influence the insulative value of snowcover 

  • As depth approaches 1.5 feet density becomes less important 


Snow and Radiant Energy

  • Earth receives ~60% of the energy in winter as it does in summer (mid-latitudes)

  • Snow is nature's best reflector

    • 75-95% reflection (albedo) 

  • Nearly a perfect absorber of long-wave radiation (IR or heat) 

    • Acts like a black body

    • This is evidenced by tree wells

  • Temperature inversion

    • Cold layer of air overtop of snowpack

    • Basically, the coldest air is at the snow surface at night 

    • Most comfortable overnight shelters for animals are in areas with a dense canopy

  • The coldest part of winter is after the solstice because the snow continues to reflect radiation 

  • Very low light levels under the snow influence a number of processes 

    • Chlorophyll production 

    • Photosynthesis 

    • Population dynamics of small mammals

    • Sexual maturation and reproductive behavior of some species

  • Penetration of light is dependent on the state of the snowpack

    • Density and depth 

    • Light transmission decreases with destructive metamorphism

    • Less than .1% of the light reaches the ground from January through April

      • Mammals roam the Subnivean in near-total darkness!

Chapter 3

Plants and the Winter Environment

  • Some species avoid winter by overwintering in seeds or a regenerative rootstock 

  • Plants completely covered by snow are protected from the harshest of winter 


Acclimating to the cold

  • Three types of plant adaptation stages 

    • Begins in late summer 

    • Peaks after just 2-3 weeks (protects to 20ish degree temps)

    • Active process as plants that are severely depleted in photosynthetic reserves do not acclimate

    • Abscisic acid (plant hormone) plays a critical role in acclimation 

      • Universal growth inhibitor 

      • Increases membrane permeability 

    • Decrease in the saturation of membrane lipids

      • More pliable membranes 

    • Third Phase after prolonged exposure to temps of ~-40F 

      • Takes 5-10 days

    • Dehardening can occur in just a day or two

  • Freezing of water in cells can be lethal to those cells 

    • Mechanically punctured/ membrane failure

    • Freezing and not low temperatures cause cold injury

  • Freeze injury is a strong selective pressure for many plant species

  • Dry winter winds promote severe desiccation of plants exposed above the snowpack

  • Exposed plants face their greatest water loss problems under bright sunshine and calm conditions

    • We most enjoy these days

  • Leaf stomates remain closed throughout most of the winter

    • Diffusion of water vapor through the protective cuticle is significant

    • Not unusual for leaves to be heated 20 degrees above ambient on sunny days 

    • Cuticles of conifers 10x more resistant to desiccation 

    • As long as the layer remains intact plants are relatively safe

  • Plants create a boundary layer by “huddling” needles 

    • Analogous to air trapped by animal fur and feathers

    • Wind currents reduce the boundary layer 

  • Desiccation damage is most notable at timberline 

    • Short growing season causes insufficient growth of the cuticle

    • Wind-carried ice causes abrasive damage 

  • Water moving in the xylem of a conifer must pass through bordered pits located at the ends of each conducting cell 

    • Water freezes and expands 

    • Hydrostatic pressure then builds to more than 900 psi

    • Air bubbles under extreme pressure dissolve immediately upon warming 

    • As the ice melts pressure reduces 

  • Broadleaved trees have perforated end walls, and cannot pressurize 

    • “Ring-porous” species 

    • This leads to cavitation bubbles 

    • This is why oaks and hickories are absent from boreal forests

  • Water movement in the winter is just 3% of the rate of movement in the summer

    • Sufficient to keep pace with water losses


The Evergreen Advantage

  • If foliage in the winter can be a liability why have it?

    • Continuance of photosynthesis

    • Mild Climates

      • 25% of summer rate on warm days mid-winter

      • 35% of summer rate on spring days

    • Harsh Climates

      • Negative C02 balance from November through April 

    • Reactivated surprisingly early in the spring 

  • Subnivean Plants

    • More benevolent environment 

    • No water stress 

    • Chlorophyll synthesis is found up to 80 cm below the snow surface

  • 50 deciduous tree species have chlorophyll in their bark

    • The majority of them are deciduous 

    • 15% of photosynthetic surface in aspens 

      • 50-75% of CO2 released is reutilized in bark photosynthesis 


Mechanical Problems 

  • Heavy snow

  • Blowing ice

    • The bark on the windward side is deeply pitted

    • Accelerated water loss

    • Development of distinct growth forms

      • Krumholtz

      • Flagging 

      • “Table trees”

      • Broomsticking - branchless windswept trunks

  • Browsing 

    • Accelerated water loss

  • Snow loading 

    • Spire shape in conifers helps minimize snow loading 

  • Snow drifting

    • Ribbon forest formation 




Chapter 4

Animals and the Winter Environment 


Animals remaining active must maintain their body temperature 

  • Heat is exchanged between animals and the environment readily

    • Conduction

    • Convection

    • Radiation 

    • Latent heat exchange 


The Basics of Energy Exchange

  • Conduction

    • Transfer of energy through molecular collisions 

    • Contact through media 

    • Depending upon the surface area exposed

    • High density = high conduction 

  • Convection

    • Transfer of energy through a moving fluid

      • Specifically air or water

  • Radiation

    • No molecular collisions

    • Any object whose temperature is above absolute zero radiates energy in direct proportion to its temperature 

  • Latent Heat of Exchange

    • Amount of energy either tied up or liberated with phase changes of water

    • Latent heat of fusion (Water to ice) 

      • 335 joules tied up in water (warmer)

    • Latent heat of evaporation (water to vapor)

      • 2450 joules required (colder)

  • Practical applications

    • Putting your feet in plastic bags before putting on your socks and boots works because it reduces the latent heat of vaporization 

    • Rate of transference by conduction also reduced 


Warm Bodies in Cold Environments 

  • Heat in = Heat out

  • Heat in

    • Metabolism of food or fat (only substantial input) 

    • Energy that is absorbed from the external environment 

  • Heat out 

    • Loss by the processes that we have just discussed 

    • Latent heat of exchange 

      • Small in most homeotherms 

      • <10% of energy loss 

  • Heat savings

    • Decrease surface area of exposure 

      • Curling or huddling 

        • Many small, solitary mammals become social during the winter

          • Nests are never totally vacant so animals always return to a warm nest!

          • Lower metabolic rates from communal animals 

          • Reduction in water loss 

          • Disadvantages 

            • Vulnerability to predation 

            • Competition

            • Disease and parasitism 

    • Increase the thickness of insulation (piloerection) 

    • Decreasing thermal conductivity 

      • Nest building or subnivean layer 

  • Lower critical temperature (LCT)

    • Represents the point at which the animal has done all it can to maintain a constant metabolic rate through physical regulation of heat loss

      • Must now generate additional heat to match 

    • An animal must at some point increase its metabolic rate

    • Seasonally adjusted in many mammals

      • Eg: Red Fox: 8*C summer, -12*C in winter 

      • Usually the result of increased fat thickness

      • Increase in basal metabolic rate

    • Nonshivering thermogenesis 

      • Common in many small mammals 

      • Increase in brown fat deposits 

        • Fat with numerous mitochondria 

        • Temperatures often exceed core temp

      • Concentrated in the interscapular region

      • Regulated by norepinephrine 

    • Increased efficiency of shivering heat production by muscle tissues

      • Increase in myoglobin 


Birds

  • Opportunities for heat savings are more limited in birds 

  • Increase total weight of feathers in winter by as much as 50%

  • Fluffing feathers while roosting 

    • Reduces conductivity by 30-50%

  • Grouse makes use ofthe  insulative value of snow by diving into snowpack 

  • No well-defined LCT

  • Lack brown fat

  • Even large birds must continually shiver during the winter when they aren’t generating heat through flight 

  • Chickadees lower body temperature during inactive periods

    • Torpor 

    • Minimizing the temperature gradients between body and air 

    • A drop of 10* saves 20% of basal metabolism 


The Problem with Appendages

  • Must be supplied with oxygen and kept from freezing 

  • Can’t be allowed to constantly drain heat from body 

  • Physiological shunting of blood through a heat exchanger that intercepts the heat 

    • Rete mirabile (miraculous net)

    • In cold air temps loss from the tail may drop 98% (2% total)

      • Over 25% of heat dissipates through the tail in warmer temps 

    • Bird legs, whale flippers 

  • Only when appendages are in imminent danger of feezing is blood supply increased

  • Spherical resting posture is efficient at reducing heat loss

  • Bergmann’s rule: northern “races” of a species tend to be larger than southern

    • Decreased surface area to volume ratio

    • This doesn’t hold true for small mammals 

    • It might only be the result of the release from competitive pressure


Coloration and Energetics

  • More complex than it is intuitively 

  • Black absorbs more sunlight

    • Also radiates more 

    • Net disadvantage

  • Gloger’s Rule

    • Pigmentation in animals tends to be reduced towards the poles 

    • What radiates less and thus more advantageous 

  • What is the advantage of an ermine being white?

    • Most of their time is spent hunting in the pitch black

    • Insulation of all white animals is much better than black animals 

    • White hair is highly vaculate (hollow) 

    • Air spaces in place of pigment reduces conductivity 

  • What is the stimulus for color change?

    • Both temperature and photoperiod serve to time molting 


The Cold-Blooded Gamble: To Freeze or Not to Freeze

  • With some exceptions, insects remain in a thermal equilibrium 

    • Same body temp as habitat temperature 

  • Do insects acclimate?

    • Yes

    • Sophisticated biochemical strategy


The Problem with Freezing 

  • The same dangers as with plant tissues

    • Cell lysis due to physical ice crystal damage 

    • Cold is not the issue

  • Must confine ice to intercellular spaces

    • Intracellular fluids made “unfreezable” 

  • Two categories of insects

    • Freeze tolerant

    • Freeze intolerant 


Freeze avoidance

  • Readily supercool their bodies 

  • Amplify this thought biochemical means 

  • Cannot tolerate ice formation in body tissues 

  • “Free” down to negative 20*C (-5*F)

    • Low volume of water 

    • Must actively lower the supercooling point past this 

  • Most insects totally evacuate their gut in the fall

  • Glycerol is the most common polyol synthesized during cold-hardening 

    • Sorbitol, mannitol, and ethylene glycol are also made

    • Sorbitol is an artificial sweetener

    • Ethelyen glycol is antifreeze (old school before we switched to propylene glycol due to its reduced toxicity) 


Freeze Tolerance 

  • Deliberate supercooling is a gamble

    • Any movement could trigger nucleation and instant death 

  • Avoided by promoting early and gradual freezing in the extracellular fluid 

  • Common in the following orders: Coleoptera (beetles), Diptera (Flies), Hymenoptera (Bees and ants), and Lepidoptera (Moths)

    • Also in barnacles and molluscs 

    • Frogs, garter snakes, hatchlings of painted turtles (different regulator functions)

  • Essential requirement 

    • Induce ice formation at a temperature only a few degrees below 0*C

    • Restrict ice to extracellular spaces

    • Limit the total amount of body ice and hence the concentration of solutes 

    • Protect membranes against the problems of structural damage and protein denaturation that accompany freeze dehydration 

  • Deliberate premature ice formation is the defining part of this strategy 

  • Freezing usually occurs slowly 

    • 48 hours or so 

    • Gives time to make cellular adjustments and limit osmotic stress 

    • Lethal freezing hits when 65% of body water is ice 

    • Limitations on ice growth due to thermal hysteresis proteins (antifreeze) 

  • Difficult to say under what environmental conditions one strategy is favored over another 

    • Some closely related species sharing the same habitat will display different modes of survival 

    • In general: freeze avoidance is favored where ambient temperatures are not extreme or where daily temperature fluctuation is not excessive 

      • Freeze tolerance is best in places that will not be disturbed


Freezing in Frogs

  • 4 species of land-hibernating frogs 

    • Spring peepers, chorus frogs, grey tree frogs, and wood frogs 

    • All freeze tolerant 

  • Initiated around -2*C

  • Only the grey tree frog produces glycerol 

    • We may still discover other cryoprotectants 

  • 200 time increase in glucose levels once freezing starts (within 8 hours)

  • Heartbeat doubles within minutes of the start of freezing 

    • Body temperature increases due to the latent heat of fusion 

  • No survival of frogs below -7*C

    • Importance of good snow cover 


Cold Shock 

  • Even cold-hardy insects will die if exposed to subfreezing temperatures that are above their supercooling point 

    • Product of rapid chilling 

    • This is what used to keep pine bark beetles out of whitebark pines 

  • Injury is due to membrane failure as lipids freeze 



Chapter 5

Life Under Ice 


Winter begins with a different phenomenon, overturn

  • The mixing of surface and bottom waters that redistributes resources in the aquatic environment 


Temperature/Density Relationships

  • The water reaches maximum density at 4*C

  • Most substances continue to increase in density as the temperature drops, if this were true our lakes would freeze from the bottom up, killing all life 

  • Through summer rich oxygenated water remains at the surface

  • Oxygen-poor waters settle toward the bottom

  • When the entire lake reaches the same temperature from top to bottom there is no longer any density differences to resist mixing 

  • The entire lake is saturated once again with oxygen

  • O2 saturation is 12.37ppm at 5*C and only 8.84ppm at 20*C

  • Winter begins as ice covers the lake 

    • Phytoplankton decline 

    • Oxygen gradually diminishes in the whole lake 

    • Fish migrate towards inlets where oxygen is higher 

  • Little threat of freezing 


Freezing Around the Edges

  • Fish can become entrapped in anchor ice when in shallow streams 

    • Must migrate or develop freeze tolerance 

  • Aquatic insects never developed freeze tolerance 

  • Saltwater can chill to -1.9*C

    • Fish only have a tolerance to -.8*C

    • Can increase salt concentration to reach goal

    • A few polar fishes have glycoproteins 


Dormancy vs Activity: Compensating for the Cold 

  • Two problems 

    • Fluidity of membrane proteins 

    • Rate of chemical reactions 

  • For every 10*C decrease in temperature, the chemically metabolic rate of the poikilotherm could be expected to decrease by over 2 time 

    • Many species show this over a 5*C period

      • Suggests a regulated metabolic drop

        • Restructuring proteins 

        • Reduced delivery of oxygen

  • “Semihibernation”

    • Can move but slowed respiration and lethargic

  • Winter acclimated species can maintain activity at low temperatures similar to warmer temperature 

    • Quantity and quality enzyme changes

    • Structural modifications of cells and organelles

      • Lipid concentration

    • Increase in red muscle fiber

    • Greater reliance on lipids for energy 

  • Bottom waters can become oxygen-deficient 

    • Fish use glycolysis for energy

      • Only 8% as efficient as respiration

      • Must build up lactic acid 

  • Phytoplankton remain active during winter 

    • Produce enough oxygen during daylight to pay back 60% of that consumed during the night

      • Net oxygen debt 

Winter ends with spring turnover 


Chapter 6

Food for Thought 


Plant-Animal Interactions

  • Where deer yard or moose or hare concentrations are high above snow browsing becomes a major driver of plant evolution

  • Changes in growth form

    • Retention of larger, chlorophyll-rich leaves for longer 

    • Stump sprouting in willow 

    • General compensatory growth 


Plant deterrent to winter browsing 

  • Herbivore preference for mature growth 

  • Strong chemical defenses 

  • May produce antiherbivore compounds only in their juvenile parts 

    • Juvenile stems at concentrations 25 times greater than adult parts 

    • Avoidance also related to lower digestibility and nitrogen content 

  • 8 to 11-year cycle of hares is thought to be the recovery time of vegetative memory


Plant Compound

Nature in Plants

Action on herbivore

Gibberellic acid

Hormones present at the highest levels during seed maturation and germination

A doubled number of mice producing litters in the lab possibly important in winter breeding

Tannins (soluble phenolics)

Defense present in flowers and in leaves of forces, trees, and shrubs

Precipitates plant proteins and GI system enzymes reduce protein availability and in some ruminants, cell wall digestion

Pinosylvin methyl ether (PME)

Defense (toxic phenol) present in buds and catkins of green alder

Repellant to snowshoe hares, the mechanism is probably similar to the above

Phenolic glycosides

Toxic phenols in willow

Repellent to snowshoe hare

6-MBOA (methoxybenzoxazolinone)

Glycoside derivative in vegetatively growing young plants

accelerates sexual maturity and breeding in voles

Camphor

Toxic phenol in white spruce

Deters snowshoe hares

Papyriferic acid

Toxic phenol in birch

Deters snowshoe hares

3-0 Malonylbetulafolientriol oxide

Toxic phenol in birch

Deters snowshoe hares

Trihydroxydihydrochalcone

Defense compound in balsam poplar

Deters snowshoe hares

Coevolution of Plants and Browsers

  • The effectiveness of plant secondary compounds as feeding deterrents is related to the coevolutionary history of plants and consumers

  • If animals were given a chance they would preferentially browse, in order, Icelandic plants, Finnish plants, and lastly Siberian and Alaskan Plants

    • And Alaskan hares would show a greater tolerance for chemical deterrents 

    • This is what experiments have shown


Plants and the Quality of Subnivean Life

  • Roots of perennial herbs are high in carbohydrates before flowering

    • Provides an important food source for subnivean mammals 

  • Stimulous for breeding in small mammals may come from plant compounds ingested by the animal

    • Berellic acid- a plant growth hormone that is most abundant during germination and seed maturation


The Carbon Dioxide Debate

  • Subnivean CO2 levels are 3 to 10 times the ambient 

  • Small mammals move out of winter habitats in which CO2 buildup consistently occurred



Chapter 7 

Winter Profiles: A seas in the lives of selected animals 


There are Few Absolutes in Winter Ecology

  • Overwintering success depends not upon the perfect solution to low temperatures or deep snowcover but upon the entire suit of adaptations by which animals are able to profitably exploit all the resources in their environment 


Northern Cervids (Deer Family)

  • Moose and caribou experience less stress from winter than they do from heat and biting insects

  • Being a ruminant helps

    • The bulk of the diet is cellulose

    • Dietary constraints are balanced in part by increased heat production 

    • “Specific dynamic action of food” may depress the lower critical temperature in ruminants 

    • Heat production 10-50% higher than fasting heat production 

    • The amount of food a ruminant can eat is constrained by how long it takes to digest

      • Deer have been known to die of starvation with full stomachs 

  • Size advantage 

    • The ability to store fat is directly related to an animal's mass

    • The rate at which an animal utilizes energy to maintain body temperature is proportional to ¾ of their size

      • Optimized with larger animals  

  • Two different winter survival strategies 

    • Increase foraging efforts to compensate for energy needs

    • Reduce foraging efforts to conserve energy 

    • Choose what is optimal at the time


Mule Deer and White-Tailed Deer

  • Exist in habitats that range from -60*C to 50*F

  • Some degree of physiological adaptation

    • Insulation thickness

    • Metabolic rate

    • Water requirements

  • Both are dependent upon behavioral adjustments 

  • Neither species is endowed with any special physical ability to cope with snow

    • 25-30cm greatly impede mobility 

    • Migrate to lower, more open, south-facing slopes

    • Preferentially seek old growth 

      • Old Douglas-fir produce more litterfall and arboreal lichens which are preferred by deer over younger trees 

    • “Yard” in areas of dense conifers

    • Confines deer to 10-20% of normal foraging territory 

  • Energy use

    • Bedding 75% of the time vs 25% of the time

      • 33% increase in caloric demand 

    • Standing increases metabolic rate by 20%

    • Foraging increases rate 28-33% over bedding 

    • Voluntary reduction in activity appears to be the best means of energy conservation

      • Deer cannot satisfy their energy requirements with extra browse 

  • Heat loss

    • Unsurpassed ability to conserve heat

    • Constriction of the surface blood vessels 

    • Peripheral cooling 

    • Seasonal pelage changes 

      • 5cm long

      • 1,000 per cm2 

    • Thermoregulation accomplished by

      • Posture

      • Piloerection 

      • Bedding site selection 

        • Optimized for both day and night 

        • Used year after year 

        • Night beds in dense conifers with good longwave radiation 

        • Day beds along packed trails and with southerly or westerly exposure (max solar gain) 

      • Shivering heat production 

  • Weather is the primary regulator of long-term deer populations in northern regions 


Elk

  • Gregarious behavior 

    • Herding behavior is a cow elk's primary predator avoidance mechanism

    • Reduced food quality and accessibility in winter 

  • Rely heavily on dried grasses 

    • Must dig or “crater” in the snow to get access 

  • “Caloric bankruptcy” following the rut often forces bulls to wander, singly or in small groups, in search of high-quality forage

    • Areas are often associated with recent burns

    • Sacrifice safety for food quality

    • Bulls tend to suffer greater winter mortality 

  • Locomotion in snow is energetically costly for elk as well

    • Less well adapted to making and using trails in deep snow

    • Snow of 40cm or deeper causes elk to cease cratering and switch to browse 


Moose 

  • Winter acclimatized moose begin to heat stress at ~26*F

  • Tolerant of relatively deep snow

  • Highest insulative value of known land mammals 

  • Lower critical threshold near -40*F

  • Winter range shrinks to half of other seasons

  • Good winter range determined by concentrations of food supplies 

    • Streamside habitats are strongly preferred 

  • In food-rich areas 6 hours a day is spent foraging 

    • 1 hour long bouts 

    • 6 cycles of bedding and feeding each day

  • Daily energy requirements are around 30-35lbs of food per day


Caribou (Rangifer tarandus)

  • Exceptional foot surface area

  • The gut flora of caribou is uniquely adapted to its diet

    • Lichens! 

  • Feet of caribou stay flexible at cold temperatures because fatty tissues deposited therein remain soft

    • Switching from saturated fat to unsaturated fat

  • 45% of each day spent laying 

    • Saved 25-35 days worth of forage


Semiaquatic Mammals 

  • The thermal conductivity of water is many times greater than air (23 times!)

  • All show a seasonal increase in fur

  • Compression by water pressure dramatically reduces the effectiveness of insulating fur 


Muskrat

  • Exploit shallow water

    • Abundance of emergent vegetation

  • Do not cache food

  • Construct and maintain feeding shelters

    • Resemble lodges

    • Smaller and constructed of emergent plant materials 

    • Plunge hole through ice is kept open

    • Their feeding platform is situated above the water

  • LCT of muskrats in water is 30*C

    • Rarely experienced an internal temp drop of more than 2*C

  • Meticulous groomers

    • Sebaceous glands spread oil on the entire pelt

    • The Hardarian gland, situated in the orbital socket of the eye provides oil

    • These secretions are hydrophobic 

  • Prior to entering the water, they raise their body temp 1.2*C

    • Hyperventilate just prior to diving 

  • Well endowed with interscapular brown fat

  • The underwater endurance of muskrats is no more than 58 seconds 

    • A straight line distance of 44 meters 

  • Burrows excavated into the shoreline had a higher probability of remaining occupied in winter than stick shelters


Beaver (Castor canadensis) 

  • Prefer shallow water with emergent plants 

  • Usually denied access to terrestrial food supplies by persistent ice cover 

    • Limited underwater caches 

    • Occasional submerged aquatic plants 

  • As many as 200 willow stems cached

    • Under 6cm in diameter 

    • Constructed in autumn

    • “Planting the cut stems in the bottom mud 

  • Wintertime drop in water levels often leaves hanging ice that beavers can use to extend foraging times

  • Kits and young beavers gained weight during winter while adults lost weight

    • Caches were adequate to support only the number of young in the colony and that adults “overwinter in poverty”

  • Beavers' tails store fat and shrink as winter progresses

  • Ice-bound beavers left the lodge only once every two weeks on average 

    • Caused deviation in circadian rhythm 

    • Moved to 26-29 hour cycles 

  • Desynchronize with other members of the lodge

    • Ensures that they always return to a warm lodge

  • Lower critical temperature between 20 and 25*C

  • Behaviorally avoids many of the problems associated with cold-water immersion and oxygen limitations by caching food close by


River Otters (Lutra canadensis)

  • Good winter habitat alone may establish the carrying capacity of a region 

  • Fish are the bulk of an otter's diet

  • Improved catchability of some fish species under the ice

  • Heavily dependent on ponds and streams with steeply banked shorelines that provide denning opportunities with access to water under the ice 

    • Rely on beaver activity 

    • Deliberately rift beaver dams, dropping water levels and creating air space

  • Winter food shortages generally force otters to forage for themselves

  • Use numerous shelters rather than one den site

  • 88 different resting locations found in 16 month period

  • Singularly adapted to the environment 

    • Depends almost entirely on insulation  

    • Four times denser fur than muskrats

    • Guard hairs have spike-like scales that interlock 

  • Sea otters have the most dense fur in the animal kingdom 

    • Twice that of river otters

    • Lacks arrector pili muscles 

      • Reflects a nearly complete adaptation to aquatic life

  • Modest subcutaneous fat

    • 10% of total body weight 

  • Higher metabolism than predicted by standard weight scale 

    • Slightly lowered LCT


Mink (Mustela vision)

  • Minimizing direct competition through dietary habits

  • Mink often fare better in the vicinity of muskrat 

    • Similar to the beaver-otter relationship

    • Smaller abandon burrows

  • Have been known to coexist with muskrats in larger burrows

  • Will cache excess food

  • Activity of mink increases in the winter 

  • Partially web feet 

  • Maladaptive body shape for cold exposure 

    • Proven good for predatory efficiency and flexibility 

  • Can lower their resting body temperatures

    • So can martens!


Gallinaceous Birds 

  • “Fowl-like birds (Grouse and Ptarmigan)

  • Birds generally have a higher body temperature than mammals (40-43*C)

  • Small body size leads to an increased cooling rate

  • Never evolved brown fat

  • Many birds are nearly hyperactive on the coldest days of winter 

  • Adapted to feeding on coarse, fibrous plant material 

    • primarily buds, conifer needles

  • All the advantages of a ruminant without being confined to the ground 

    • Enlarged caecum

    • High concentration of bacterial flora 

    • Food is ground in the gizzard

  • As winter approaches these birds shift to a diet of nearly entirely conifer needles 

  • Strong drive to seek out grit

    • Roadsides, lakeshores, streambanks, uprooted trees 

  • Gradual increase in the size of the bird’s crop, large intestine, and caecum 

  • After feathers are a secondary structure produced from the main axis of a feather 

    • The main axis is called a rachis 

  • A habit of roosting beneath the snow 


Blue Grouse 

  • Migrate from lower aspen groves to higher conifer stands in winter 

  • Positive energy balance!

    • Gain weight 

    • Little winter mortality

      • Except hit by cars 🙁

    • Daily winter movements in winter average only 70m

  • Fasting birds have a lower critical temperature of -5*C

  • Metabolic rates uncorrelated with ambient temperature 




Chapter 8

Humans in the Cold

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