16-1 d. thermoregulation 1. animals show two types of responses to changing environmental...
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D. Thermoregulation
1. Animals show two types of responses to changing environmental temperature
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TB
BodyTemp.
TA Ambient Temp
b. Thermoregulator
a. ThermoconformerTB will be stable only ifTA is constant
maintains relativelyconstant TB
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2. Can classify animals by their source of body heat:
a. endotherms: derive heat from internal source
b. ectotherms: derive heat from outside sources
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3. Alternatively, animal are classified by stability of heat:
a. homeotherms: constant TB over time
b. poikilotherms: variable TB over time
Does not imply animal has no control
Most vertebrates exhibit some degree of control of TB in the wild
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E. Temperature regulation achieved by controlling mechanisms of heat gain and loss
Mechanisms of Heat Transfer
1. Conduction
Heat gain or loss through direct contact
Depends on thermal conductivity of object
rate at which heat passes through it
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air: 0.00006 cal/cm2 sec °C
water: 0.00144 cal/cm2 sec °C
Air is good insulator
to prevent conduction, trap air next to body
Water is good conductor
in water, heat will be transferred between animal and environment very quickly
aquatic ectotherms must be thermoconformers
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2. Convection
transfer of heat through bulk movement of medium
e.g., currents, wind, blood flow
mechanism to gain or lose heat
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3. Radiation
transfer of heat in the form of electromagnetic radiation
mechanism to gain heat when radiation absorbed by tissue
mechanism to lose heat when radiation emitted by tissue
If TB constant over time, 100% of absorbed radiation must eventually be re-emitted
but not necessarily at the same wavelength
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very hot objects: emit short wavelengths
cool objects: emit long wavelengths
animals, earth: absorb short, emit long
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4. Evaporation
heat loss by an organism through evaporation of water
585 cal to vaporize 1g at 22° C
can be unfortunate or intentional loss
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5. Metabolism
heat gain by metabolic burning of food
“calorigenesis”
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Ectotherms
Obtain heat by using external mechanisms
conduction, convection, radiation
Endotherms
Obtain heat using internal mechanisms
metabolism
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F. Ectotherms can utilize these to regulate TB
Behavioral Thermoregulation
Lizard in thermal gradient will move until it finds “preferred temperature” and stay there
Will “defend” preferred temperature if gradient changes
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Things to note:
1. Maximal activity of enzymes matches preferred temperature
e.g. lizards
2. Preferred temperature matches environment
3. TB can be as high as endotherms
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4. How is preferred temperature achieved in wild?
a. internal and external T perceived by thermoreceptors in CNS and skin
b. hypothalamus maintains “temperature set point”
c. hypothalamus activates behaviors to thermoregulate by taking advantage of 5 mechanisms
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If TB < set point, work to warm up
bask (absorb radiation)
sit on hot rock (conduction)
increase mr (metabolism)
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If TB < set point, work to warm up
bask (absorb radiation)
sit on hot rock (conduction)
increase mr (metabolism)
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If TB > set point, work to cool off
retreat to burrow (emit radiation)
sit on cool substrate (conduct away)
pant (evaporation)
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5. Advantages of Behavioral Thermoregulation
Can survive on little E
Ectotherm uses 1/500th the E to survive as endotherm
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6. Disadvantages
a. No environmental E, can’t warm up
enzymes need broad ranges
b. If cold or warming, exposed to predators
c. Difficult in aquatic environment
high conductance makes it impossible to be different from TA
d. Restricted to animals of small size
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Heat is gained across skin to warm tissue
skin = surface area
tissue = volume
surface/volume ratio will vary with animal size
surface area of an object increases as a function of length2
volume of an object increases as a function of length3
Therefore: as size increases, volume increases more rapidly than surface area
small animal has high surface:volume ratio
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Alternative strategy:
G. Metabolic Thermoregulation
1. Characteristics
obtain heat by high mr
conserve heat by good insulation
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2. Advantages:
a. increase aerobic capacity
enzymatic processes are always functioning at optimum levels
adapted for immediate responses, sustained activity, endurance
b. niche expansion
allows maintenance of metabolic activity as animals visit habitats of varying T
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3. Disadvantages
a. requires huge amounts of E/g BW
BMR in 10 g mouse: 10 cal/g hr
SMR in 10 g lizard at 37°: 1 cal/g hr
especially a problem in smallest animals
theoretical 5 gram minimum for endotherms
b. high mr requires high ventilation rate leading to high rate of respiratory water exchange