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Body Temperature and
Thermoregulation
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
Before we begin: Definitions
1. Importance of metabolismEndotherm - body temperature depends on heat productionby animal metabolism
Ectotherm - body temperature depends on heat acquired from the environment, and is little influenced by metabolicrate
• Importance of Body TemperatureHomeotherm - body temperature constant (usually as a result of endothermy)
Poikilotherm - body temperature variable and strongly influenced by environment
Recall: MR increases with body weight, but less thanproportionately
log MR
log body weight
a
0.75 = b
logMR = a + b . log bwt
MR = a . bwtb
a
Animal a b bodytemp (oC)
Mammals 3.3 0.76 38Birds 3.6 0.72 40Lizards 0.13 0.8 20Fish 0.43 0.81 20Crabs 0.27 0.78 20
Lizards 0.68 0.82 38at 38oC
Relationship between metabolicrate and body weight for animalsmaintained at 20oC
Birds andmammals
Other animals
Mammals and birds are ‘gas guzzlers’
Exploit elevated MR to maintain stable temperature
=> Very costly (food, resources)
=> Generates activity that is independent of externalconditions
Birds andmammals
Other animals
Why aren’t lizards as efficient as birds or mammals?
a. Body temperature - high and maintained in birds and mammals
b. Intrinsically high metabolic rate: if lizard placed at 38oCMR will increase, but not to same level as in birdsand mammals
Physiology of birds and mammals ‘wired up’ to run fasterthan other animals
Observed at level of O2 consumption
Observed in organ system organisation: lungs, gut absorptive surface, design of circulatory system
Metabolic rate of cardinals reared at different temp
too hot
below here need to use extra energy
to keep warm temperature, °C
oxyg
en c
onsu
mpt
ion
Summary so far A large part of basal metabolism is to
keep warm
Now onto: how do we minimise heat loss?
Surface insulation: a barrier to loss of metabolically generatedheat
thermal conductivity (W m-2 oC-1)
Water 0.61Air 0.025Muscle 0.48Fur 0.04Blubber 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 betterFur has low conductivity: terrestrial mammals can maintain a 30oC gradientbetween skin surface and external temperature (i.e. trapping air of low conductivity)
Blubber has 6x conductivity of fur: i.e. needs 6x thickness of fur to generate same insulation.
body38oC
external
fur
Temp at skin surface38oCbody
38oC
blubber
external
Temp atskin surface= externaltemp
seal v dog…
Blubber as surface insulation in seals
Skin surface in water = water tempSkin surface in air > air tempSeals must lose heat to air to avoid overheating
Metabolic rate declines as temperature declines, but down to 0oC, no heatloss from feet. When temperature below freezing, vasodilators open toprevent feet from freezing
Most fish, water temp = body temp, as loss of heat through gills.
In tuna: sustained fast swimming requires temp of ~30oC
Keeping tuna muscle warm
Achieved by counter current blood flow
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
Seals pump blood to body surface in air to achieve coolingvia vasodilator. i.e. pattern of blood flow regulates heat loss
Seal heat loss
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]
Carotid artery in ungulates: when blood temp rises (during a chase to 44°) danger to brain: venous blood cooled in rete and nasal cavity (=<40°C)
Heat loss in a hurry
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)
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
Facultative endotherms
If there are so many advantages to endothermy,why aren’t 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 employingendothermy only when needed (facultative)
Facultative endothermy inInsects:
Early season bumble beesNight moths
Achieved by synchronizedmuscle activity (with nomovement)
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
Torpor: some birds and mammals exhibit torpor/adaptiveHypothermia
Reduces the metabolic rate in response to:
low external temperature
And low food availability
Torpor is under physiological control
Characteristics of Torpor:
Reduced metabolic rate, but maintenance of control (avoidsfreezing, i.e. during hibernation)
Reduced motor and sensory function, more comatose than Sleeping (low heart rate, low respiratory rate)
Can display arousal and return to ‘normal’ body temperaturemetabolic rate or endogenous heat production
Characteristics of Torpor (cont):
Generally small animals, small mammals, birds, rodentshummingbirds: due to?
Energetic cost of maintaining high body temp for smallanimals?Costs of arousal, costly for large animals
However: bears in winter dormancy: reduce MR by 50%, bodytemp by 5oC
Summary to end A large part of basal metabolism is to
keep warm Insulation
and its control Endothermy
Facultative in insects Torpor
energy saving