Thermal environments (thermal niches) on the planet
temperate zone
tropical zone
polar zone,, …
p
Fig.13.1
THERMOREGULATION
challenge of fluctuating environmental temperatures
effects on animal’s metabolism
energetic
g
costs of meeting
g this challenge
g
•Temperature-dependance of biochemical and molecular processes
temperature quotient
Q10 = ( k2 / k1))10/(t2-t1)
where:
k1 = reaction rate at t1
k2 = rate at t2
if (t2 - t1) = 10o C
for metabolic rate (MR)
Q10 = MR(t+10) / MR(t)
Fig. 17.2 Eckertl
Thermal physiology of ectotherms
Acclimatization (natural setting) to changes in Ta
Acclimation (lab.)
compensatory changes in physiology
e.g. in enzymatic activities
Q10 effect?
Fig. 13.10
METABOLISM
Anabolism synthesis
growth, repair, regeneration
Catabolism degradation
release of chemical energy
storage
g of energy
gy as ATP
Basal metabolic rate BMR (homeotherms)
minimum metabolic rate necessary for maintenance
at rest (awake)
post-absorptive (fasting)
in thermoneutral zone
measured as:
heat released
oxygen consumed
(direct calorimetry)
(indirect calorimetry; respirometry)
Metabolic Rates
Change in internal conditions as external conditions vary
Classification of thermoregulatory modes of animals
By source of heat:
Endothermy - internally generated from metabolism
Ectothermy external,
external environmental
By variability of Tb:
Homeothermy - constant core Tb
activity independent of Ta
Poikilothermy
y - core Tb fluctuates
activity dependent on Ta
metabolic heat < heat loss
may regulate Tb behaviourally
Heterothermy
Th
Thermal
l strategy
t t
in
i which
hi h Tb varies
i either
ith
spatially or temporally
Heterothermy (e.g. insectes)
why
heterothermy?
Fig. 13.8
Regional Heterothermy in Insects
When inactive, Tb = Ta
For flight, thoracic muscles pre-warmed to >30°C
Fig. 17.18 Eckert
Fig. 14.17
Regional Heterothermy in Ectotherms
e.g. swimming muscles in blue-fin
blue fin tuna
Swimming muscles do not thermoconform
Tmuscle > Twater
Able to swim fast in cold water
Fig. 13.22
Gas exchange vs heat exchange (water/blood)
Tarterial blood = Twater
Ectothermic fish
(e.g. trout)
Heterothermic fish
(e g blue-fin
(e.g.
blue fin tuna
tuna, mako shark)
counter-current heat
exchanger
Regional heterothermy in endotherms
limbs poorly insulated, long & thin
•high heat loss at cool Ta
•major
j route
t off heat
h t loss
l
ffrom
body if Tlimb = Tcore
SOLUTION
Tlimb < Tcore
Wolf at -30oC
Foot pads
Paws
Legs
Core
0o
8o
14o
38o
NEW PROBLEM
Cold venous blood returning from limb to core
SOLUTION?
Allow extremities to cool
Protect core Tb by:
warming blood returning to core
Fig. 13.23
Daily heterothermy
for small endotherms
at low Ta, cost of maintaining constant Tb high
Solution:
during inactive/cold part of day, Tb declines
Problem:
must rewarm to normal Tb before resuming activity
Fig. 13.7 Heterothermy in birds
Heat budget & physics of heat gain and heat loss
body heat = metabolic heat (Hmet) + thermal flux
= metabolic heat + (heat gain – heat loss)
ΔHtot
t t = total body heat
= ΔHmet + ( ΔHc + ΔHr + ΔHe + ΔHs)
if Tb is
i constant,
t t Hs
H (heat
(h t stored)
t
d) = 0
Hmet always +ve
Hc (heat of conduction or convection,
+ or -)
Hr (heat of radiation, + or -)
H (heat
He
(h t off evaporation,
ti
always
l
-))
Fig.13.2
C d ti
Conduction
C
Convection
ti
Evaporation
Challenge
g of freezing
g environments (T<0C)
(
)
(formation of ice crystals in ICF is lethal to cells)
Strategies used by ectotherms
•Avoidance of freezing T
migration
"warm" microenvironments
•Resist freezing
(of ICF, may tolerate freezing of ECF)
super
p cooling
g
absence of nucleating agents
presence of anti-nucleating agents
lower FP
antifreeze agents (proteins, glycerol, sorbitol,..)
colligative properties, low toxicity, inert
•Controlled ice formation
ice-nucleating
g proteins
p
slow rate of ice formation, reduce osmotic stress
Thermal Biology of ENDOTHERMS
Fig. 13.9
Thermoneutral Zones
oC
Human
White-footed mouse
Mongolian gerbil
Franklin ground squirrel
Harvest mouse
24-31
26-34
28-34
27-31
31-35
•Within the thermoneutral zone:
body T is regulated by energetically inexpensive adjustments
vasomotor responses
postural changes
insulation adjustments
•Below thermoneutral zone:
Metabolic regulation to generate heat (energetically expensive)
•Above thermoneutral zone:
Active heat dissipation (energetically expensive)
Ta
20o
30o
40o
e.g. unclothed human
Tb Skin
30o
34o
36o
Tb Rectal
37.1o
37.1o
37.2o
Cold death typically occurs before tissues freeze
energy production < maintenance needs
Heat death associated with:
denaturation of proteins
liquefaction of fats
dessication
Biological processes (chemical reactions)
typically occur 2-3x faster with 10oC increase in Tb (Q10 effect)
Range of Tambient
Air
-60oC Æ +50oC
Water -2oC Æ +40oC
Range of Tbody for activity
acti it
Upper 47-49
47 49oC intertidal invertebrates
in ertebrates
Lower -2oC arctic and antarctic fish
Human?
Hypothermia in Humans (Davenport 1992)
Core temp oC
36
intense shivering muscular weakness
increased metabolism
35
reduced shivering HYPOTHERMIA
34
heart rate & BP ↓
33
shivering stops
amnesia
muscular rigidity
31
irregular respiration
semiconscious
pain resistant
30
irregular heart beat
unconscious
28
ventricular fibrillation
26
respiration ceases
22
heart stops
Core Temperatures in Homeotherms
Normal
Core oC
Monotremes
30-31
Marsupials
35 36
35-36
Eutherians
36-38
Humans
37
Nonpasserine
39-40
Passerine
40-41
(Schmidt-Nielsen, Table 8.2)
Upper Lethal
Lower Lethal
Core oC
Core oC
37
40 41
40-41
≈28oC in all
42-44
homeotherms
43
except temporal
heterotherms
46
47
(Endothermy in cold environments)
(N.B. vasomotor responses, postural changes, insulation adjustments
within thermoneutral zone)
Thermogenesis : production of heat in cold (below thermoneutral zone)
Shivering thermogenesis
contractions of antagonistic muscles
inefficient muscle movement (work)
ATP hydrolyzed for muscle contraction
chemical energy released as heat
Nonshivering thermogenesis
metabolism of fats (fats oxidized, heat is produced)
white fat and brown fat
Membrane adaptations to changes in T
Lipids at low temperatures
As T decreases, viscosity ↑ and fluidity ↓
cool extremities of homeotherms in cold climates require lipids
with low MP
MP α saturation
# carbons Saturated
Unsaturated
16
palmitic 63°C
palmitoleic -0.5°
18
stearic 70°
oleic (1=)
13°
linolenic (3=) -11
11°
Composition of subcutaneous fat
Saturated
Mono-unsat. Poly-unsat.
Grey seal
17%
55%
28%
Sheep
53%
45%
2%
In cold-adapted
p
species,
p
, lipids
p
in cool extremities are more
unsaturated than in warm core ∴ remain fluid at low T
In small hibernators with low Tb during torpor,
entire body ≈0°C
↑ unsaturated fats pre-hibernation
mammals cannot synthesize polyunsaturated
f tt acids
fatty
id (PUFA)
fat in hibernating golden-mantled ground squirrels
25% linoleic
10% linolenic
PUFA must be obtained from diet
captive study:
↑ polyunsaturates in diet
↓ mortality in hibernation
ENERGY STORAGE
Fat vs. Glycogen
1 g glycogen Æ 4.1 kcal
1 g fat Æ 9.3
9 3 kcal
thus fat is least bulky way to store energy
Advantages of glycogen:
quickly metabolized
anaerobic glycolysis
WAT (WHITE ADIPOSE TISSUE)
Glycogen storage:
in sedentary organisms
in low PO2 environment
e.g.
bivalve mollusks
intestinal parasites
•energy reserve and heat production
independence from day-to-day variability in food availability
non shivering thermogenesis
non-shivering
triglycerides in fat stores → fatty acids → circulation
•insulative
low thermal conductivity
poorly vascularized
•buoyancy
less dense than water
nitrogen sink
•nitrogen
in deep diving mammals
N more soluble in fat than water
Heat Production
by
BAT
Fig. 13.18
BAT (brown adipose tissue)
•located between scapulae,
scapulae in axillae
axillae, in throat
on heart and aorta
on cervical & thoracic blood vessels
•extensively vascularized
many mitochondria, multiple fat droplets
•in
in situ oxidation & heat production
in BAT cells specialized for non-shivering
thermogenesis
•uncoupling
li off ATP synthesis
th i in
i mitochondria
it h d i
energy released as heat instead
of stored as ATP
Thus blood p
passing
g through
g BAT is warmed
Fig.13.18
Thermoregulation and specialized metabolic states
Dormancy energy saving strategy
lowers food ((and water)) requirements
q
avoidance of energy-consuming
thermal regimens
•Sleep
Sl
•Torpor
•Hibernation and
winter sleep
•Estivation
•Fever
(in endotherms
ectotherms)
Fig. 17.35
Endothermy in hot environments
Challenge:
eliminate excess heat
conserve water
H t lloss b
Heat
by radiation,
di ti
conduction,
d ti
evaporative
ti cooling
li
Evaporative cooling
- sweating, panting
- how to prevent respiratory alkalosis during
panting (canines, birds)
Limited heterothermy (camel vs antelope ground squirrel)
Countercurrent cooling of carotid blood
Thermoregulatory centers
Thermostatic regulation
g
of body
y temperature
p
Hypothalamus : thermostat (Tset)
Thermoreceptors : T-sensitive neurons and nerve endings
in brain, spinal cord, skin, and sites in body
core
Thermostat controls
heat production
heat exchange
g mechanisms
Heat-conserving versus heat-dissipating
Comparison of Tb and Tset
Fig. 17.29
Fig. 13.19
Figure 13.26
Thermoregulation and specialized metabolic state
•Fever
Fever
(in endotherms
and
ectotherms)
Pyrogens
Endogenous
Exogenous
(endotoxins
from bacteria)
Fig. 17.35