Thermoregulation

Temperature Gradient for Kingsnake

Temperature Gradient for Varanid

Basic Thermodynamics

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

Note: this is a physical system and follows both the first and second laws of thermodynamics.

Basic Thermodynamics

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

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

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

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

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

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

Magnitude of Infrared Radiative Exchange: R– Depends on temperature difference

between animal and object (Ts4 - Te

4)

– 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

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

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

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

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

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

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

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

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

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.

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

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).