Temperature Chapter 8

Temperature Average kinetic energy of a system Arguably the most important aspect of the physical environment for life –Influences geographic distributions of species –Influences interspecific competition

Temperature Ranges For most living, active animals: –-2 °C (polar aquatic species) to 50 ° C (desert spp.) –Few species can survive entire range

Terminology of Thermal Biology Various terms used to describe thermal biology of animals: 1.Cold-blooded vs. Warm-blooded 2.Poikilothermic vs Homeothermic 3.Ectothermic vs. Endothermic

Cold-blooded vs. Warm-blooded Description of relative body temperature –“Warm-blooded” = high body temperature Mammals and birds –“Cold-blooded” = low body temperature Reptiles, amphibians, fish, invertebrates Tend to be inaccurate –Many “cold-blooded” animals have high body temperatures (e.g. desert reptiles and invertebrates) –Many “warm-blooded” animals may have low body temperatures (e.g. hibernating mammals)

Homeothermic vs. Poikilothermic Description of variation in body temperature –Homeothermic – body temp. strictly regulated Mammals and birds –Poikilothermic – body temp. may fluctuate widely “Lower” vertebrates and invertebrates Problems –Many “poikilotherms” normally have stable body temperatures under natural conditions –Some “homeotherms” have broad seasonal changes in body temperature (hibernation, etc.)

Endothermic vs. Ectothermic Description of heat production –Endothermic – most body heat is physiologically generated (muscle contraction) –Ectothermic – most body heat derived from the environment (solar radiation, etc.) Problems –Some animals generate lots of heat but do not use it to regulate body temperature

Determinants of Body Temperature Temperature depends on the amount of heat (calories) contained per unit mass tissue –# calories contained per °C determined by the heat capacity of the tissues (~ 0.8 cal*°C -1 *g -1 ) Amount of heat in body depends on… 1.Rate of heat production 2.Rate of external heat gain 3.Rate of heat loss to the environment Body heat = heat produced + heat transferred

Heat Transfer Three ways of transferring heat 1.Conduction – transfer of heat between objects in contact with one another 2.Radiation – transfer of heat by electromagnetic radiation 3.Evaporation – transfer of heat to water as it changes from liquid to gaseous phase

Conduction Transfer of kinetic energy between two objects in contact Heat moves from warmer region to cooler region Rate of transfer: H = rate of heat transfer per cross sectional area k = thermal conductivity of the conductor d = distance between two points T 1 and T 2 = temperature at points 1 and 2 H = k × T 1 – T 2 X

Conduction and Convection Conduction rate is increased by convection –Movement of gas or liquid over the surface of transfer –Continuous replacement of fluid maximizes temperature difference and facilitates heat transfer

Radiation Transfer of heat via electromagnetic emission Objects do not require contact Stefan-Boltzmann law: H = εσT s 4 –H = rate of heat exchange per unit area –ε = emissivity (wavelengths at which EM radiation is emitted,~3-4 μm for most objects on earth) –σ = Stefan-Boltzmann constant –T s = surface temperature of the object Net heat exchange is from the object with the higher T s to the one with the lower T s

Evaporation Only means by which heat can be lost to a hot environment Vaporization of water requires heat –~ 2400 kJ per g water absorbed from the surface of the animal –Evaporative cooling used to dissipate heat Sweating, panting

Heat Storage Animals can store heat in their bodies by moderating heat transfer to the environment Factors Affecting Heat Transfer –Surface Area/Volume Ratio Larger animals have proportionately lower heat flux –Temperature Gradient Between body and environment Lower gradient, slower heat transfer –Specific Heat Conductance Insulation – reduces heat conductance

Total Body Heat H total = H v + H c + H r + H e + H s H v = heat produced by metabolism (+) H c = heat loss/gain by conduction and convection (+/-) H r = heat transfer via radiation (+/-) H e = heat loss by evaporation (-) H s = stored heat (+)

Effects of Body Temperature Change Temperature affects the rate of chemical reactions –Affects chemical reactions needed to maintain homeostasis –Too low metabolism not fast enough to maintain homeostasis –Too high reactions in metabolic pathways uncouple, enzymes denature, etc.

Changes in Metabolism with Body Temperature Temperature Coefficient (Q 10 ) – factorial increase in a rate with a 10 °C increase in temperature Q 10 = (R 2 /R 1 ) 10/T 2 -T 1 –R 2 and R 1 reaction rates at temperatures T 2 and T 1 respectively –If T 2 and T 1 differ by 10 °C, Q 10 = (R 2 /R 1 ) –Typical Q 10 values for biological rates (metabolic rate, etc) range from 2 to 3 (doubling or tripling of rate)

Changes in Metabolism with Temperature Q 10 values often change across a range of temperatures Performance curves –Initial large increase, followed by smaller increases