Temperature

1. Temperature Chapter 8

2. 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

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

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

5. 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)

6. 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.)

7. 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

8. 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… • Rate of heat production • Rate of external heat gain • Rate of heat loss to the environment • Body heat = heat produced + heat transferred

9. Heat Transfer Three ways of transferring heat • Conduction – transfer of heat between objects in contact with one another • Radiation – transfer of heat by electromagnetic radiation • Evaporation – transfer of heat to water as it changes from liquid to gaseous phase Fig 8.3

10. 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 • T1 and T2 = temperature at points 1 and 2

11. 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

12. Radiation • Transfer of heat via electromagnetic emission • Objects do not require contact • Stefan-Boltzmann law: H = εσTs4 • 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 • Ts = surface temperature of the object • Net heat exchange is from the object with the higher Ts to the one with the lower Ts

13. 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

14. 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

15. Total Body Heat Htotal = Hv + Hc + Hr + He + Hs Hv = heat produced by metabolism (+) Hc = heat loss/gain by conduction and convection (+/-) Hr = heat transfer via radiation (+/-) He = heat loss by evaporation (-) Hs = stored heat (+)

16. 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.

17. Changes in Metabolism with Body Temperature • Temperature Coefficient (Q10) – factorial increase in a rate with a 10 °C increase in temperature Q10 = (R2/R1) 10/T2-T1 • R2 and R1 • reaction rates at temperatures T2 and T1 respectively • If T2 and T1 differ by 10 °C, Q10 = (R2/R1) • Typical Q10 values for biological rates (metabolic rate, etc) range from 2 to 3 (doubling or tripling of rate)

18. Changes in Metabolism with Temperature • Q10 values often change across a range of temperatures • Performance curves • Initial large increase, followed by smaller increases