Thermal Relations

 

Hill et al., Ch 8

 

 

Updated 10 September 2005

 

Students: If you rely on printed copies of these notes, make sure that you have the latest copy (check date stamp) and remember to study the linked figures and graphs online.

 

 

Introduction

A.   Temperature affects most physiological processes (Q10 = 2-3) and thus most organisms

1.     Physiological processes (and enzymes) often exhibit optimal response curves; prediction re Tb?

2.     Animals live in a relatively narrow range of body temperatures (cf environmental temps); WFig. 8.1

a.      predict optimal response within the range

 

B.    Maintenance of Tb range requires heat transfer between body and environment

1.     Tb not uniform throughout body; importance of shell/core concept (HFig. 8.4)

a.      during activity, metabolic rate increases; more heat must be transferred to the surface to maintain core Tb

2.     Responses to temperature variation are variable

a.      with thermal strategy

-       endotherms (physiological heat source); generally higher Tb, less variable

-       ectotherms (environmental heat source); generally lower Tb, more variable

b.     with phylogeny within a strategy (Table)

Endotherm taxon

Normal Tb

Lethal Tb

Monotremes

31 2

37

Marsupials

36 2

40-41

Eutherians

38 2

42-44

Passerine birds

40 2

47

 

c.     with adaptation (species-specific thermal strategies; e.g., stenothermal vs. eurythermal)

d.     with habitat (e.g., aquatic vs. terrestrial organisms); most terrestrial environments are more thermally extreme

-       aquatic animals (Tb usually similar to environment)

-       terrestrial animals (may maintain substantial difference between Tb and environment

e.      with ontogeny (reproductive stages usually most sensitive)

f.       with body size (scaling responses)

-       small animals, fast response

-       large animals, thermal lag

 

C.   Terminology

1.     Cold blooded vs. warm-blooded - rarely used by biologists often inaccurate, misleading, and biologically uninformative (terms are relative to human condition)

2.     Poikilothermic (variable Tb) vs. homeothermic (constant Tb)

3.     Ectothermic (environmental heat gain) vs. endothermic (physiological heat gain); preferred - focus on processes rather than results

4.     Limitations of all terminology (large physiological diversity; not all organisms can be conveniently categorized); e.g., some insects, birds, and mammals routinely generate heat by physiological processes but also bask to gain environmental heat)

 

D.   Physics of heat and heat transfer

1.     Heat

a.      heat measured in calories (amount of heat required to raise the temperature of 1g of water 1C)

b.     specific heat capacity is the heat required to raise 1g of substance 1C

c.     specific heat of pure water is 1.0 cal/g (relatively high); animals are mostly water (mean specific heat ~0.8)

2.     Heat transfer between animals and their environment

a.      to maintain constant body temperature, heat loss must equal heat gain (metabolism + environmental)

b.     thermal environment and metabolism highly variable→ must move heat

c.     methods of heat transfer (HFig. 8.03)

3.     Conduction

a.      occurs where physical bodies are in contact (solids, liquids, or gases)

b.     result of molecular motion (=heat difussion)

c.     rate dependent on

-         temperature gradient

-         area of contact

-         thermal conductivity coefficient; e.g., high (metals); intermediate (water, most animal tissues); low (air, hair, fat insulators)

4.     Convection

a.      result of mass flow of fluid (gases, liquids); renews fluid at boundary layer

b.     much faster than conduction; accelerates conductive heat flow

c.     natural convection (caused by density changes due to differential heating) vs. forced convection (caused by external forces, e.g., wind, water currents, locomotion)

5.     Radiation

a.      physics

-       all objects above absolute zero emit electromagnetic radiation (emit original source vs. reflection)

-       no direct physical contact

-       travels at speed of light

-       higher temperatures shorter wavelengths (HFig. 8.5) and greater energy (intensity) over all wavelengths

-       energy may be reflected, transmitted, or absorbed (converted to heat at surface)

b.     animals exchange radiant heat with all objects in their environment (ubiquitous; most important heat exchange process for most terrestrial animals)

c.     most important wavelength→ middle infrared (5000-10,000 nm); animals behave as black bodies (i.e., absorb and emit 100% regardless of visible color; WFig. 8.18)

d.     visible wavelengths (color; 400-700 nm); dark surfaces absorb more radiation than light surfaces (thermoregulatory consequences)

e.      near infrared reflectance (Fig-frogs); functions? thermoregulation, predator avoidance (cryptic)

6.     Evaporation

a.      heat of vaporization of water ~580 cal/g (large amount of heat); highly effective strategy for heat loss if water is not limiting

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Thermal challenges

A.   Concept of operative temperature (Te; measure with models; HFig. 8.03)

 

B.    Free-ranging animals must:

1.     Cope with acute temperature extremes (hot/cold)

2.     Respond to chronic changes in environmental temperature

3.     Regulate body temperature (homeostasis)

 

C.   Tolerance to acute high temperatures

1.     Methods of determining lethal temperatures

a.      static: maintain animals at various constant temperatures; determine body temperature at which 50% of animals die within a specified time (endpoint is death Lethal Maximum)

b.     dynamic: subject animals to slowly increasing temperatures; determine body temperature at which animals are incapable of removing themselves from situation (behavioral endpoint: loss of locomotion, muscle spasms; ecological death; manheat stroke); Critical Thermal Maximum (CTMax)

 

2.     Variation in upper lethal temperature

a.      max: animals ~50C, plants ~75C, bacteria ~100C; most have much lower thermal limits

b.     terrestrial animals

-   highest: various invertebrates (desert ant >45C)

-   terrestrial vertebrates: salamanders ~30C reptiles/birds/mammals ~40C

-   species differences are often reflected in various aspects of ecology; e.g., habitat selection (intertidal snails), species interactions (Japanese honey bees mob/kill hornet predators by increasing its Tb above lethal)

-   some terrestrial animals normally live close to their CTM (whiptail lizard, mean activity temp = 41C, highly stenothermal, CTM ~43C

c.     aquatic/marine animals generally have lower lethal temps

-   lowest is Antarctic icefish (4-6C); normally lives in water 2C year round

-   exception: desert pupfish ~34C; hot springs

 

3.     Causes of death at high Tbs

a.      insufficient oxygen (hypoxia)

b.     depletion or accumulation of intermediary metabolic products (due to Q10 differences among reactions in a biochemical pathway)

c.     disrupt protein conformation (inactivate)

 

4.     Response to high non-lethal temperatures

a.      protective role of heat-shock proteins (HSPs)

b.     occur in all animal groups (highly conserved)

c.     usually absent; quickly induced by acute non-lethal temperatures

d.     help repair and maintain 3D protein structure (folding)

e.      high energetic cost (uses ATP)

 

D.      Tolerance to cold and freezing

1.     Most animals cannot withstand extensive sub-zero Tbs (intracellular freezing usually fatal)

2.     Most normally freeze at -0.1 to -1.9C; 3 options for avoidance:

a.      escape (migration, habitat selection)

b.     increase metabolic heat production (endotherms)

c.     develop cold hardiness (ectotherms)

3.     Cold hardiness strategies (Table); both effective

Characteristic

Freeze-intolerant (FI)

Freeze-tolerators (FT)

Ice formation

Lethal must avoid freezing; cannot tolerate

Extracellular water freezes; increases osmolarity of extracellular water

1.      withdraws water from cell

2.      increases cell osmolarity

3.      depresses freezing point in cell

Lower Critical Temperature

average -20 to -25C; lowest -65C

-20 to -70C

Supercooling capacity

High: many reduce ice-nucleating agents; ice formation cannot be initiated

Little/none: no reduction of ice-nucleating agents; some synthesize inas to promote freezing, e.g., various alcohols (insects), glucose (vertebrates)

Antifreezes

Types:

1.         depress FP by increase solute conc. (alcohols: e.g., sorbitol, glycerol)

2.         chemically depress FP (antifreeze proteins); more effective

None

Occurrence

-     Many insects (antifreeze + supercooling)

 

 

 

Vertebrates:

-     Marine teleost fish (body fluids omotically dilute); various antifreeze proteins in >10 orders (independently evolved several times)

-     Various terrestrial amphibians and reptiles

-     Many invertebrates (esp., intertidal invertebrates, some withstand 60-80% body water frozen)

 

Vertebrates:

-     No fish

-     Few frogs that overwinter terrestrially

-     Some turtle hatchlings overwintering in terrestrial nests

 

E.    Temporal changes in thermal tolerances/relations

1.     Ontogenetic response (tolerance polygon; WFig. 8.29)

2.     Temperature acclimation - individual ectotherms respond to their thermal history

a.      acute response (HFig. 8.9; Q10 = 2-3)

b.     chronic response: acclimation (HFig. 8.10); compensation (HFig. 8.11)

c.     biological significance?

-       reduces acute effect (HFig. 8.12)

-       results from lag in enzyme production

3.     Evolutionary responses

a.      organismal adaptation: preference↔performance (HFig. 8.14)

b.     affects rate of tissue processes

c.     affects functional properties of molecules

-       protein conformity: enzyme-substrate affinity (HFig. 8.16); LDH (330 AA), homologs differ 1-4 AA

-       membrane fluidity: homoviscous adaptation (HFig. 8.18)

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Body temperature regulation

A.   Why regulate Tb?

1.     Necessary to

a.      avoid lethal temperatures (re environmental extremes)

b.     optimize physiological performance (Fig.-Dipsosaurus)

2.     Thermoregulation vs. thermoconformity (HueyModel); both regulators and conformers must avoid thermal extremes and maintain favorable Tbs

3.     Maintain Tb between upper and lower set points (negative feedback model)

-       peripheral sensors (thermosensitive neurons) thermoregulatory control center (hypothalamus) effectors (muscles, sweat glands, etc.) feedback etc.

 

B.      Fundamental thermal strategies

1.     Endotherms (mostly thermoregulators, homeothermic)

a.      diel variation; diurnal spp. (higher during day) vs. nocturnal spp. (higher during night)

2.     Ectotherms (thermoregulators or thermoconformers, homeothermic or poikilothermic)

a.      more variation in body shape and size; can be elongate/very small

b.     thermal gradients in body may be reversed compared to endotherms; heat sources/sinks

 

C.      Hypothermia

1.     Evolutionary conservative trait; widespread response to reduced energy supplies resulting from environmental conditions (including man; fast→ reduce MR); permits escape from demands of homeothermy

2.     Controlled response to winter (cold, hibernation; Fig. Citellus), summer (heat, aestivation), or daily (torpor)

a.      best known physiologically hibernation

b.     hibernation occurence

-         small endotherms: many mammals, few birds (e.g., hummingbirds, swifts, mouse birds)

-         ectotherms: all terrestrial species in cold climates; many also display summer aestivation

3.     Student responsibility (pp. 228-232 and ch. 9)

 

D.      Hyperthermia

1.     Raising of normal Tb above the upper set point (not a passive failure of temperature regulation)

2.     Occurrence

a.      bacterial/viral infection (fever); physiological fever (endotherms); vs. behavioral fever (ectotherms)

b.     increase heat loss to environment (important in endotherm thermoregulation)

 

E.    Thermoregulation in endotherms

1.     Endothermy

a.      benefits: independent of Te, specialized cellular function, high aerobic capacity

b.     costs: very high energetic cost (~90% assimilated)

 

2.     Relationship of MR and ambient temperature (HFig. 8.22)

a.      constant Tb in thermoneutral zone (TNZ)

b.     below LCT: compare species (WFig. 8.34) note slope and relative width of TNZ

c.     above UCT: active evaporative cooling (increases MR) and hyperthermia (increases heat loss)

d.     regulate by changing conductance

-       regulate insulation (hair, feathers); changes convection at skin surface (creates boundary layer)

-       vasomotor regulation (circulation to surface; WFig. 8.36)

-       regulate behavioral exposure of thinly-furred (posturing); especially important in elongate endotherms (e.g., weasels MR = 2x normally proportioned mammals)

-       group posturing (huddling); especially important in cold climates (penguins) and in neonate litter mates

e.      thermogenesis (heat generated by metabolism)

-       voluntary muscular activity or involuntary shivering (all mammals/birds)

-       non-shivering thermogenesis (NST)

     placental mammals; few hatchling birds (no adults)

     brown fat (highly vascular, large no. mitochondria, high MR); thermogenin (protein) uncouples ATP synthesis from respiration in mitochondria produces large amounts of heat; localized anteriorly (heat vital organs); found in 3 kinds of placental mammals (cold-acclimated, hibernators, newborn)

 

3.     Evaporative cooling

a.      last line of defense for terrestrial animals with evolutionary histories in hot, dry climates (water scarce)

b.     cutaneous water loss (skin lipids inhibit)

c.     sweating (glands; whole or parts of body); mammals only

d.     respiratory and oral evaporation; may be augmented by associated behaviors (panting, gular fluttering (hyoid))

-       panting (shallow breathing, increase convection); pant at species-specific natural (resonant) frequencies→ minimizes calories expended (heat produced<heat lost); no electrolyte loss (cf sweating)

-       may induce respiratory alkalosis (elevated pH); shallow breathing (HFig. 8.32) reduces respiratory gas exchange

-       many panting species have developed high tolerance to alkalosis

a.      saliva spreading (some rodents, many marsupials)

 

4.     Regional heterothermy (energetic savings)

a.      example: cool temperatures of heat-sensitive organs (brain; HFig. 8.31); heat flow from arterial blood venous blood (many ungulates)

b.     example: reduce appendage temperature (whale flippers, bird legs); little insulation, high SA; potential for large heat flux

c.     regulate heat flux with heat exchangers (increase regulation/efficiency; arctic canid footpads regulated near 0C)

-  structure: vascular bundles (SNFig-birdleg)

-  function: countercurrent flow (HFig. 8.29; KFig. 4.43); avoid excessive Tb how regulate? (WFig. 8.38)

 

5.     Aquatic mammals (water: high conductance, high heat capacity; large potential heat loss)

a.      Tb similar to terrestrial mammals (36-38C); LCT below 0C

b.     MR 2x that of similar-sized species (partial solution)

c.     insulation (SNFig-blubber); lose little heat to water

d.     how avoid overheating?; bypass insulation (SNFig. 7.14), increase surface temp and transfer heat to water (method not available to terrestrial animals); polar bears - semiaquatic lifestyle made possible by blubber

 

6.     Effects of body size

a.      observations

-       SA/V ratio: small animals gain/lose heat faster than large animals

-       MR: small animals have higher metabolic rates than large animals

-       VMI (scales with body mass): large mammals use Ts more for heat loss

-       insulation: small animals cannot have thick fur/feathers

b.       small animal strategy (desert rodents): avoid extreme heat/cold (subterranean, nocturnal) )

-       cannot have/afford high enough MRs to keep warm (HFig. 5.9)

-       cannot afford to evaporate water to keep cool (SNFig. 7.22)

-       desert ground squirrel (diurnal, cyclic avoidance activity; unload heat gain by conduction in burrow)

c.     large animal strategy (camel): regulate conductance, EWL

-       increase insulation (dorsal fur insulation; shave fur increase water loss 50%; cf human desert dwellers)

-       tolerate water depletion (20-25%); large capacity for rapid drinking

-       increase Tb (daily cycling; SNFig. 7.23); reduces thermal gradient between body and environment (reduce heat gain); also gazelles (47C); many birds (45C)

 

7.     Chronic responses to summer/winter temperatures

a.      Insulatory acclimatization (HFig. 8.33.2; HFig. 8.34.2); fur/feather thickness, vasomotor control

b.     Metabolic acclimatization (HFig. 8.33.1; HFig. 8.34.1)

 

E.    Thermoregulation in ectotherms

1.     Many thermoconformers; some thermoregulators (HFig. 8.7)

2.     Sources of heat flux - aquatic ectotherms

a.      infrared absorbed by water, no evaporation, high specific heat of water (conduction, convection)

b.     small animals→ thermoconform (no choice)

a.      large air-breathing animals

-         thick superficial tissues with reduced circulation (insulation)

-         leatherback sea turtle (tropics-arctic circle), maintains constant Tb (~25C in 7.5C water); gigantothermic

b.     large water-breathing animals (tuna, some sharks, billfishes)

-         maintain high Tbs (gigantothermic); permits fast-swimming predatory life style

-         how maintain high Tb (HFig. 8. 40) and compensate for heat loss through gills? heat exchanger between gills and body (SNFig-tuna); heat only strategic parts (swimming muscles, gut, eyes, brain)

-         independent origins of endothermy in fish (3; HFig. 8.41) and in tetrapods (2)

 

3.     Sources of heat flux - terrestrial ectotherms

a.      metabolism (rare): egg brooding pythons; increase MR ~10x with muscle contractions

b.     radiation: diurnal heliotherms

-         darken/lighten skin (dispersion of melanophores); visible light only - black body absorption in infrared region

-         change body/limb orientation and posturing

-         used especially by many insects and reptiles; Liolaemus SNFig. 7.33; heating and plateau phases

c.     conduction: thigmothermic (many small skinks, fossorial ectotherms)

d.     evaporation (cutaneous water loss): WFig. 8.21 (water limited in terrestrial environment); many insects

e.      regulate heating/cooling rates (marine iguana feeds in cold water <pref. Tb, emerges to bask terrestrially); retard cooling and accelerate heating); SNFig. 7.34

 

G.   Opportunistic endothermy: large flying insects (some moths/butterflies, bees/wasps, crickets/katydids, dragonflies); also some beetles)

1.     Temporal and spatial endothermy

a.        warm up (metabolism) only when necessary (foraging, egg brooding, singing, dung rolling)

b.       thoracic endothermy - simultaneous contraction of all flight muscles (shiver/vibrate); 38-43C; HFig. 8.42; generate more heat/g than birds/mammals; bumblebees shiver while on flower

c.        homeothermy (thermoregulation)

-     modulate insulation; regulate heat flow to abdomen (conserve or release heat); maintain thoracic heat/avoid overheating

-     some modulate MR (HFig. 8.43)

d.       benefits:

-     permits flight (requires near-maximum energetic output at min; ~30-35C; aerobic)

-     much lower overall energetic cost (temporal/spatial)

 

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