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
Blubber
Fur

Common insulation
for marine mammals

Uncommon to be main
form of insulation for MMs

Non-compressible

Compressible

Energy store

Energetically expensive
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
MARLIN
Heater organs: produce enough
heat near eyes and optic nerves to
improve visual clarity when diving
deep into cold waters
Regional heteroterm:
can retain heat in certain
regions of the body.
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
Metabolic Rate
Thermal Neutral Zone
Metabolic Regulation
Heat Production
Active Heat
Dissipation
BMR
Lower critical
temperature
Environmental temperature (°C)
Upper critical
temperature
(Adapted from Eckert 2001)
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.
http://news.bbc.co.uk/2/hi/science/nature/8165895.stm
Thermal Windows
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