Thermoregulation

Thermal Strategies

Animals must survive thermal extremes

- the highest and lowest TA in their niche

Animals must survive thermal change

Thermal Strategies

Many ecosystems exhibit spatial variation in

temperature:

Underground refuges buffered from thermal

extremes on the surface

Large bodies of water decrease in temp with

increasing depth

Daily cycles of cold and heat.

Thermal Strategies

Heat Exchange

The most important physiological

parameter in an animal’s thermal

physiology is body temperature (TB)

An animal’s thermal strategy serves to

control the transfer of energy between

animal and environment.

Metabolism = major source of thermal E

for many animals.

Other routes for thermal energy, into and

out of an animal:

◦ Conduction

◦ Convection

◦ Radiation

◦ Evaporation

Conduction: transfer of thermal E from

one region of an object or fluid to another

Convection: transfer of thermal energy

between an external fluid that is moving

Radiation: emission of electromagnetic

energy from an object.

Evaporation: loss of water molecules from

the surface of an object (absorbing thermal E)

Controlling Heat Fluxes

Thermal Energy (H)

Htotal = Hmetabolism + Hconduction +

Hconvection +Hradiation + Hevaporation

◦ If Htotal = 0 TB will remain constant

◦ If Htotal = + TB will increase

◦ If Htotal = - TB will decrease

Thermal Conduction

Heat is conducted from internal tissues, thru

other tissues and fluids, and into surroundings.

High thermal conductivity = heat sink

◦ Water has a higher thermal conductivity than air.

An animal will lose heat much faster in water

than in air due to interplay of conduction and

convection.

Convective Heat Loss

Body works to warm “boundary layer”

Strength of gradient between animal and

environment determines heat loss.

Heat lost to a moving fluid (air or water)

is convective heat loss

Radiant Energy

Sun = most important source of radiant heat:

Photons from the sun excite molecules in the

atmosphere, warming them by radiant heat.

Evaporation

Evaporative cooling:

◦ Fluid draws thermal energy from the body

surface as water molecules make transition

from liquid to vapor.

Magnitude of heat loss dependent upon

volume of water and heat of vaporization.

◦ Requires more energy to evaporate salty

sweat than pure water

Surface Area to Volume Ratio

Surface Area to Volume Ratio

Surface Area to Volume Ratio

Bergmann’s Rule:

◦ States that animals living in cold environments

tend to be larger than animals in warmer

environments

Allen’s Rule:

◦ States that animals in colder climates tend to

have shorter extremities than animals in

warmer climates.

Insulation

Internal and external insulation reduce

heat losses.

Fur and feathers:

◦ restricts movement of molecules between the

surface of the animal and the environment.

Blubber:

◦ lipid layer disrupts the flow of thermal energy

from the core to external surface of animal.

Insulation

External Insulation:

◦ Molecules of air in the insulation layer are

warmed by the animal and then trapped

within the insulation

External Insulation

External Insulation

Some species change thickness of

external insulation seasonally.

Thick coats are a thermoregulatory

burden in the warm season:

◦ Beneficial to shed fur in the spring

◦ Cost of rebuilding coat when temperatures

cool is much less than cost of trying to stay

cool in the warm season

Internal Insulation - Blubber Fur

Uncommon to be main

form of insulation for MMs

Compressible

Energetically expensive

Blubber

Common insulation

for marine mammals

Non-compressible

Energy store

Thermal Strategies

Poikilotherm:

◦ Animal with a variable TB

◦ Varies with environmental conditions

Homeotherm:

◦ Animal with a relatively constant TB

Distinction depends on both physiology

of animal and nature of environment.

Thermal Strategies

Ectotherm: environment determines TB

Endotherm: generates internal heat to maintain TB within a narrow range.

The terms ectotherm and endotherm distinguish animals by the physiological mechanisms that determine TB.

Homeothermy

Homeotherms maintain their CNS and internal organs at a more constant temperature.

Core temperature: temperature of deep internal regions.

Regional Endothermy

Many homeotherms experience some

sort of temperature variation.

Regional endothermy: keep core

temperature near-constant, while other

regions of the body (ex. extremities) can

experience temperatures much lower

than core.

Temporal Heterotherms

Many mammals and some

birds undergo dramatic,

prolonged changes in TB

Although their bodies cool,

considered homeotherms b/c

produce and maintain metabolic

heat to keep TB above TA

Regional Heterotherm

Regional heteroterm:

can retain heat in certain

regions of the body.

Heater organs: produce enough

heat near eyes and optic nerves to

improve visual clarity when diving

deep into cold waters

MARLIN

SWORDFISH

Thermotolerance

Animals have a characteristic degree of thermotolerance.

Ectotherms: changes in TA alter TB, directly changing rate of many biological processes.

◦ Preferred Temperatures

Endotherms respond to changes in TA by inducing regulatory responses.

◦ Thermal Neutral Zones

Thermal Neutral Zone

Environmental temperature (°C)

Meta

bolic

Rat

e

Lower critical

temperature

Upper critical

temperature

BMR

(Adapted from Eckert 2001)

Metabolic Regulation

Heat Production Active Heat

Dissipation

Thermal Windows

Thermal window: area of an animal’s

body that has the ability to radiate a

considerable amount of body heat relative

to other areas.

Extremities, areas with high SA:V ratios,

highly vascularized areas, may all serve as

thermal windows.

Thermal Windows

http://news.bbc.co.uk/2/hi/science/nature/8165895.stm

Counter Current Heat Exchange

Counter Current Heat Exchange

Shivering Thermogenesis

Bodily response to the early stages of hypothermia.

◦ Triggered by drop in core body temperature

◦ Primary motor center for shivering in hypothalamus

Smallest neurons recruited first, followed by the larger neurons.

◦ individual myofibers contract, but motor units are uncoordinated

Antifreeze Proteins (AFP)

The solutes in animal tissues reduce the

freezing point of water, but generally not lower than about -2°C.

Antifreeze Proteins - disrupt ice crystal

formation by binding to the surface of

small ice crystals to prevent their growth