Download Thermoregulation

Thermoregulation
Temperature Gradient for Kingsnake
Temperature Gradient for Varanid
Basic Thermodynamics
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Overall, heat gained must = heat lost.
E = Qabs+ M ± R ± C ± LE ± G
Where
– Qabs = absorbed surface radiation
– M = heat of metabolism
– R = infrared radiation
– C = convective heat exchange
Basic Thermodynamics

E = Qabs+ M ± R ± C ± LE ± G
– LE = condensation/evaporation
– G = conduction w/ substrate
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Note: this is a physical system and
follows both the first and second laws of
thermodynamics.
Basic Thermodynamics
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Absorbtion of solar radiation: Qabs
– primary absorbtion is of visible and infrared
light (400 - 1500nm).
– There is to little UV light to be of any
consequence for Herp Thermoregulation.
Basic Thermodynamics
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Rate of solar energy absorption:
– Qabs = S·A·vfs·a
• S = intensity of solar radiation
• A = surface area of the animal
• vfs = view factor : portion of animal presented to
radiation source.
• Absorptivity = a.
Basic Thermodynamics
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The animal actually has considerable
control over many facets of this
equation.
– The animal can control surface area to
some extent
– The animal can control view factor.
– The animal can control absorptivity.
– S can be modified through position.
Basic Thermodynamics
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Metabolic Heat Production
– Recall, for the most part, reptiles are
extotherms.
• However, some larger forms (that is, those with
low SA/V ratios) are capable ofgenerating
metaboic heat in sufficient quantities to make a
difference.
• Being able to retain metabolic heat is a big
deal.
Basic Thermodynamics
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Metabolic Heat Production cont.
– Leatherback sea turtles weigh as much as
850kg.
• They occupy water as cold as 8ºC.
• Yet, their body temperatere is about 18ºC
warmer. This heat is a consequence of
metabolic production during swimming.
– Female Indian Pythons generate metabolic
heat to 32 ºC while incubating eggs.
Basic Thermodynamics

Female Indian Python
– Muscular thermogenesis is different than
shivering thermogenesis in mammals.
– Occurs only in brooding females.
– Behavior is temperature dependent. As
temperature drops, thermogenic behavior
increases.
Basic Thermodynamics
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Infrared Radiative Exchange: R
– Continuous exchange w/ environment
(700-1500nm), as long as environment is
above 0ºK.
– heat transfer occurs from object w/ more
energy to that with less energy.
Basic Thermodynamics
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Magnitude of Infrared Radiative
Exchange: R
– Depends on temperature difference
between animal and object (Ts4 - Te4)
– Depends on area of animal exposed to
radiation: A·vfs
– Depends on emissivity of skin (how readily
surface radiates/absorbs IR radiation.
Basic Thermodynamics

Emissivity of skin:
– Not dependent on color of skin obviously.
– Matte surfaces have higher emissivity than
smooth, shiny surfaces.
– Example: Uma sp. (Fringe toed lizards)
• Uma burries is a psamnophilous lizard.
• It buries itself just below the surface of the
sand, with only the pineal eye exposed (BTW,
the pineal has a retina).
Basic Thermodynamics
• At mid-day, sand surface temperature exceeds
60ºC, well above CTM, and above BT of 38ºC.
• Dorsal surface scales of animal are matte, and
have high emmisivity.
• Ventral surface scales are smooth and shiny,
and have low emmisivity.
• Thus, dorsum has IR exchange w/
environment, but ventrum reflects IR back to
sand.
Basic Thermodynamics
• Thus, during the cool AM, Uma can gain energy
while basking in the sun, and at mid-day, Uma
can get into the shade and use the dorsum to
re-radiate energy back to the sky (cloudless sky
behaves like an object w/ a surface temp of
23ºC, so net movement of energy is from
animal @ 38 to sky @ 23).
Basic Thermodynamics
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Convective Heat Exchange
– Convection occurs between object and a
fluid (air is a fluid).
– Depends on
• Temp. dif. Btwn animal and air.
• Surface area exposed to air (modifiable by
animal).
• Convective coefficient, itself dependent on air
velocity and diameter of animal parallel to
airflow.
Basic Thermodynamics
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Convective Heat Exchange
• Animal can change position w/ repsect to
airflow (higher cooler air moves faster than
hotter, slow air) - this can be dramatic within 1m
of surface.
• Reduced thickness of boundary layer increases
convective heat exchange - smaller animals
have smaller boundary layers.
Basic Thermodynamics
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Small animals influenced most strongly
by convective heat exchange.
Large animals influences most strongly
by radiative heat exchange.
Thus, we expect fundamental
differences in structure and function of
large and small herps.
Basic Thermodynamics
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Compare 2g Uta stansburiana w/ 500g
Sauromalus obesus.
– In the cool AM, Uta is subject to
convection, and inserts itself in the
boundary layer of a large rock, while
Sauromalus moves about freely.
– At mid-day, Uta climbs a shrub and uses
convection to cool, while Sauromalus
avoids radiative heat exchange and finds
its rock pile retreat.
Basic Thermodynamics
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Evaporative Cooling
– This is a problem for desert herps. In the
desert, the objective is to minimize water
loss.
– Lack of sebaceous glands reduces
presence of water on epidermis. Thus, it is
difficult to use external skin to help cool.
Basic Thermodynamics
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However,
– during periods of thermal stress, there are
a few options (used by birds as well).
• Panting enables the animal to evaporate water
from surface of lungs.
• Gular fluttering
• Urohydrosis.
Basic Thermodynamics
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Conduction
– Movement of heat energy via direct
contact.
• Many herps use warm roads at night. They use
rocks and boulders as well.
• Area of surface contact is primary factor.
• During thermal stress, minimizing area of
thermal contact (Callisaurus) is the key.
Behavioral Thermoregulation
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How do we regulate
body temperature?
– Hypothalamus is
temperature
sensitive element of
brain.
– Supplied by carotid
arteries, and can
therefore evaluate
core body temp.
– In Lipidosaurs, the
Pineal eye is
assoicated with the
hypothalamus.
– When you are cold,
you 1) get
goosebumps, 2)
initiate shivering
thermogenesis, 3)
find someplace
warm, or 4) put on a
coat etc.
Behavioral Thermoregulation
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When you get hot:
– sweat, pant, take off
clothing, find a cool
place etc.
– When a lepidosaur
gets hot, it too
exhibits a series of
behaviors and
physiological tricks.
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How does the herp
know when it is hot?
– Set points.
– Set points are
dependent on recent
thermal history,
physiological status,
reproductive status,
social status, age,
etc.
Behavioral Thermoregulation
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Set points can be very wide or very
narrow. In fact, set points in some
desert herps are narrower than those in
some mammals (Duck-billed platypus,
tree sloths).