16-1

D. Thermoregulation

1. Animals show two types of responses to changing environmental temperature

16-2

TB

BodyTemp.

TA Ambient Temp

b. Thermoregulator

a. ThermoconformerTB will be stable only ifTA is constant

maintains relativelyconstant TB

16-3

2. Can classify animals by their source of body heat:

a. endotherms: derive heat from internal source

b. ectotherms: derive heat from outside sources

16-4

3. Alternatively, animal are classified by stability of heat:

a. homeotherms: constant TB over time

b. poikilotherms: variable TB over time

Does not imply animal has no control

Most vertebrates exhibit some degree of control of TB in the wild

16-5

E. Temperature regulation achieved by controlling mechanisms of heat gain and loss

Mechanisms of Heat Transfer

1. Conduction

Heat gain or loss through direct contact

Depends on thermal conductivity of object

rate at which heat passes through it

16-6

air: 0.00006 cal/cm2 sec °C

water: 0.00144 cal/cm2 sec °C

Air is good insulator

to prevent conduction, trap air next to body

Water is good conductor

in water, heat will be transferred between animal and environment very quickly

aquatic ectotherms must be thermoconformers

16-7

2. Convection

transfer of heat through bulk movement of medium

e.g., currents, wind, blood flow

mechanism to gain or lose heat

16-8

3. Radiation

transfer of heat in the form of electromagnetic radiation

mechanism to gain heat when radiation absorbed by tissue

mechanism to lose heat when radiation emitted by tissue

If TB constant over time, 100% of absorbed radiation must eventually be re-emitted

but not necessarily at the same wavelength

16-9

very hot objects: emit short wavelengths

cool objects: emit long wavelengths

animals, earth: absorb short, emit long

16-10

4. Evaporation

heat loss by an organism through evaporation of water

585 cal to vaporize 1g at 22° C

can be unfortunate or intentional loss

16-11

5. Metabolism

heat gain by metabolic burning of food

“calorigenesis”

16-12

Ectotherms

Obtain heat by using external mechanisms

conduction, convection, radiation

Endotherms

Obtain heat using internal mechanisms

metabolism

16-13

F. Ectotherms can utilize these to regulate TB

Behavioral Thermoregulation

Lizard in thermal gradient will move until it finds “preferred temperature” and stay there

Will “defend” preferred temperature if gradient changes

16-14

Things to note:

1. Maximal activity of enzymes matches preferred temperature

e.g. lizards

2. Preferred temperature matches environment

3. TB can be as high as endotherms

16-15

4. How is preferred temperature achieved in wild?

a. internal and external T perceived by thermoreceptors in CNS and skin

b. hypothalamus maintains “temperature set point”

c. hypothalamus activates behaviors to thermoregulate by taking advantage of 5 mechanisms

16-16

If TB < set point, work to warm up

bask (absorb radiation)

sit on hot rock (conduction)

increase mr (metabolism)

16-17

If TB < set point, work to warm up

bask (absorb radiation)

sit on hot rock (conduction)

increase mr (metabolism)

16-18

If TB > set point, work to cool off

retreat to burrow (emit radiation)

sit on cool substrate (conduct away)

pant (evaporation)

16-19

5. Advantages of Behavioral Thermoregulation

Can survive on little E

Ectotherm uses 1/500th the E to survive as endotherm

16-20

6. Disadvantages

a. No environmental E, can’t warm up

enzymes need broad ranges

b. If cold or warming, exposed to predators

c. Difficult in aquatic environment

high conductance makes it impossible to be different from TA

d. Restricted to animals of small size

16-21

Heat is gained across skin to warm tissue

skin = surface area

tissue = volume

surface/volume ratio will vary with animal size

surface area of an object increases as a function of length2

volume of an object increases as a function of length3

Therefore: as size increases, volume increases more rapidly than surface area

small animal has high surface:volume ratio

16-22

Alternative strategy:

G. Metabolic Thermoregulation

1. Characteristics

obtain heat by high mr

conserve heat by good insulation

16-23

2. Advantages:

a. increase aerobic capacity

enzymatic processes are always functioning at optimum levels

adapted for immediate responses, sustained activity, endurance

b. niche expansion

allows maintenance of metabolic activity as animals visit habitats of varying T

16-24

3. Disadvantages

a. requires huge amounts of E/g BW

BMR in 10 g mouse: 10 cal/g hr

SMR in 10 g lizard at 37°: 1 cal/g hr

especially a problem in smallest animals

theoretical 5 gram minimum for endotherms

b. high mr requires high ventilation rate leading to high rate of respiratory water exchange