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

Why keep warm ? surface / volume ratio of large animals more “efficient”

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

Maximum speed Speed of passage of foodthrough gut

Lizards and fish better of at warmer temperatures

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…

Seal cross-section

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

Keeping extremities warm?

But what about the fins?countercurrent heat exchanger!

Retaining and maintaining heat: the counter current heatexchanger

Countercurrent dolphin flippers

hens’ legs

Hen’s feet

2 mm

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

tuna heat exchanger

v

v

v

aa

a

0.1 mm

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

Vasodilation jackrabbit ears before and after

exercise

all less than 10°Cabove 30°C

air temp 6°C

guanaco heat losses rate of heat loss = 1/fur length

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)

countercurrent

co-contraction

up/down alternate

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

Torpor in birds

Eulampis

Torpor in bats

Torpor in mammals (marmot)

use fat rather than glucose

energysaved

Daily torpor in mammals

Torpor in a pocket-mouse

??

Summary to end A large part of basal metabolism is to

keep warm Insulation

and its control Endothermy

Facultative in insects Torpor

energy saving

Reading … PowerPoints on VLE or at

http://biolpc22.york.ac.uk/303/

Schmidt-Nielsen, K (1997) Animal Physiology CUP