Body Temperature and Thermoregulation

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Body Temperature and Thermoregulation

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Learning Objectives

• To understand the distinction between endothermy
  and ectothermy, and between homeothermy and
  poikilothermy
• To be familiar with the contribution of metabolic
  rate, surface insulation and circulatory
  adaptations to the maintenance of body
  temperature in endotherms
• To understand how some insects and mammals use
  endothermy on a temporary basis

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• Before we begin Definitions
• Importance of metabolism
• Endotherm - body temperature depends on heat
  production
• by animal metabolism
• Ectotherm - body temperature depends on heat
  acquired
• from the environment, and is little influenced
  by metabolic
• rate
• Importance of Body Temperature
• Homeotherm - body temperature constant (usually
  as a
• result of endothermy)
• Poikilotherm - body temperature variable and
  strongly
• influenced by environment

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Why keep warm ?

• surface / volume ratio of large animals
• more efficient

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Recall MR increases with body weight, but less
than proportionately
logMR a b . log bwt
MR a . bwtb
a
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Relationship between metabolic rate and body
weight for animals maintained at 20oC
Animal a b body temp (oC) Mammals 3.3 0.76 38
Birds 3.6 0.72 40 Lizards 0.13 0.8 20 Fish 0.43
0.81 20 Crabs 0.27 0.78 20 Lizards 0.68 0.82 38
at 38oC
Birds and mammals
Other animals
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Lizards and fish better of at warmer temperatures
Speed of passage of food through gut
Maximum speed
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• Mammals and birds are gas guzzlers
• Exploit elevated MR to maintain stable
  temperature
• gt Very costly (food, resources)
• gt Generates activity that is independent of
  external
• conditions

Birds and mammals
Other animals
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• Why arent lizards as efficient as birds or
  mammals?
• Body temperature - high and maintained in birds
• and mammals
• Intrinsically high metabolic rate if lizard
  placed at 38oC
• MR will increase, but not to same level as in
  birds
• and mammals
• Physiology of birds and mammals wired up to run
  faster
• than other animals
• Observed at level of O2 consumption
• Observed in organ system organisation lungs,
  gut
• absorptive surface, design of circulatory system

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Metabolic rate of cardinals reared at different
temp
oxygen consumption
too hot
below here need to use extra energy to keep warm
temperature, C
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Summary so far

• A large part of basal metabolism is to keep warm

Now onto how do we minimise heat loss?
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Surface insulation a barrier to loss of
metabolically generated heat
thermal conductivity (W m-2 oC-1) Water 0.6
1 Air 0.025 Muscle 0.48 Fur 0.04 Blubber
0.24 (subcutaneous fat) smaller numbers are
better
Heat lost through water more rapidly than through
air. Muscles are poor at retaining heat, but
subcutaneous fat is better Fur has low
conductivity terrestrial mammals can maintain a
30oC gradient between skin surface and external
temperature (i.e. trapping air of low
conductivity)
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seal v dog
external
Temp at skin surface 38oC
body 38oC
Temp at skin surface external temp
fur
Blubber has 6x conductivity of fur i.e. needs
6x thickness of fur to generate same insulation.
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Seal cross-section
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Blubber as surface insulation in seals
Skin surface in water water temp Skin surface
in air gt air temp Seals must lose heat to air to
avoid overheating
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Keeping extremities warm?
But what about the fins? countercurrent heat
exchanger!
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Retaining and maintaining heat the counter
current heat exchanger
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Countercurrent

• dolphin flippers
• hens legs

• Hens feet

2 mm
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Metabolic rate declines as temperature declines,
but down to 0oC, no heat loss from feet. When
temperature below freezing, vasodilators open
to prevent feet from freezing
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Keeping tuna muscle warm
Most fish, water temp body temp, as loss of
heat through gills. In tuna sustained fast
swimming requires temp of 30oC
Achieved by counter current blood flow
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tuna heat exchanger
a
a
v
a
v
v
0.1 mm
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Summary so far

• A large part of basal metabolism is to keep warm
• Keeping warm
• insulation
• fur, blubber
• countercurrent heat exchangers
• extremities
• regional temperature control

Now onto avoiding overheating
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Seal heat loss
Seals pump blood to body surface in air to
achieve cooling via vasodilator. i.e. pattern of
blood flow regulates heat loss
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Vasodilation

• jackrabbit ears before and after exercise

above 30C
all less than 10C
air temp 6C
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guanaco heat losses

• rate of heat loss 1/fur length

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Heat loss

• Actually, major heat loss is by evaporation
• sweat
• panting
• why evaporation uses a lot of energy
• 418 J to heat water from 0 to 100 C
• but 2443 J to boil it 1 g

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Heat loss in a hurry
Carotid artery in ungulates when blood temp
rises (during a chase to 44) danger to brain
venous blood cooled in rete and nasal cavity
(lt40C)
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Heat loss in the desert

• man (70 kg)
• BMR needs 0.12 l / hr evaporation
• heat from sun 1.2 l / hr
• camel (400 kg)
• temp goes from 34 to 41 C (2900kcal, 5 l water)
• store heat until night
• reduce difference between outside camel
• evaporation 0.9 l /hr
• fur reduces heat inflow (shearing doubles
  evaporation)

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Summary so far

• A large part of basal metabolism is to keep warm
• Keeping warm
• insulation
• fur, blubber
• countercurrent heat exchangers
• Heat loss by evaporation

Now onto Facultative endotherms
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Facultative endotherms
If there are so many advantages to
endothermy, why arent all species
endotherms? a) costly on resources, especially
food to maintain high metabolic rate b)
costly for small animals with high specific
metabolic rates Small animals can gain the best
of both worlds by employing endothermy only when
needed (facultative)
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Facultative endothermy in Insects Early season
bumble bees Night moths Achieved by
synchronized muscle activity (with no movement)
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countercurrent
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co-contraction
up/down alternate
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Summary so far

• A large part of basal metabolism is to keep warm
• Keeping warm
• insulation
• fur, blubber
• countercurrent heat exchangers
• Heat loss by evaporation
• Endothermy
• Facultative in insects

Now onto Torpor
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Torpor some birds and mammals exhibit
torpor/adaptive Hypothermia Reduces the
metabolic rate in response to low external
temperature And low food availability Torpor
is under physiological control
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Characteristics of Torpor Reduced metabolic
rate, but maintenance of control
(avoids freezing, i.e. during hibernation) Reduce
d motor and sensory function, more comatose than
Sleeping (low heart rate, low respiratory
rate) Can display arousal and return to normal
body temperature metabolic rate or endogenous
heat production
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Characteristics of Torpor (cont) Generally
small animals, small mammals, birds,
rodents hummingbirds due to? Energetic cost of
maintaining high body temp for small animals? Co
sts of arousal, costly for large animals However
bears in winter dormancy reduce MR by 50,
body temp by 5oC
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Torpor in birds
Eulampis
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Torpor in bats
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Torpor in mammals (marmot)
use fat rather than glucose
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??
Daily torpor in mammals
energysaved
Torpor in a pocket-mouse
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Summary to end

• A large part of basal metabolism is to keep warm
• Insulation
• and its control
• Endothermy
• Facultative in insects
• Torpor
• energy saving

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Reading

• PowerPoints on VLE or at http//biolpc22.york.ac.u
  k/303/
• Schmidt-Nielsen, K (1997) Animal Physiology CUP