Energetics 2 Really Temperature 4

1
Energetics (2)(Really Temperature 4)

• Endothermy
• Regional heterothermy
• Facultative ectothermy (NOTE this is actually
  correct endotherms become more like ectotherm
  facultatively under certain conditions)
• Torpor
• Hibernation
• Regulating Body temperature
• Generating heat
• Shivering thermogenesis
• Nonshivering thermogenesis
• Measuring energy consumption

2
Regional Heterothermy

• Different regions of body different
  temperatures
• Countercurrent heat exchangers
• Bird and Mammal legs
• Flippers
• Ungulate antlers
• Beaver tails
• Ears of arctic hares
• Tuna swimming muscles!

3
Counter-current heat exchange
Cold Region 10
Hot Region 37
37 32 28 23 18 13
34 29 24 19 14 10
Counter current exchange Exchange of heat (or
other things) between two vessels that 1) run in
opposite directions 2) are in close contact
4
Bird legs have countercurrent exchangers
BODY IS HOT
FEET ARE COLD!
5
Why dont their feet freeze?
3

• 1) Heat loss is low and constant until 0ºC
• 2) Below 0ºC blood flow speeds up
• countercurrent exchange is less efficient!
• 3) Birds pay higher energetic cost to keep feet
  slightly warm

2
1
6
Why dont their feet freeze?
3
metabolic rate
2
1
7
Rete mirabile

• Anatomical arrangement of circulatory system
• Consists of bundles of arterioles and venules
• In close appostion (stuck together)
• Running in opposite directions
• Countercurrent exchanger of both gas and heat

8
Regional endothermy in Tunas
Rete mirabile
30ºC
Arterioles leading into swimming muscle carry
cold blood (from gills) Venules carry warm blood
(from muscles) Heat is transferred into
arterial blood Heat is retained in muscles
9
Regional heterothermy

• In bird example, arteries into legs carry warmer
  blood, veins returning from feet carry cooler
  blood
• In tuna example, arterioles into muscle
  capillaries carry cool blood, venules carry warm
  blood
• In both cases countercurrent exchange is used to
  maintain distinct temperature zones in body

10
Costs of endothermy

• When is it most expensive to be a homeothermic
  endotherm?
• When its REALLY COLD
• When youre SMALL
• Rate of heat loss is proportional to surface area
• Rate of heat production is proportional to VOLUME
• Heat Loss/Heat production SA/volume

11
Surface area to volume ratioCorrections in BOLD
Generally
Surface area Length2
Volume Length3
SA/Volume Length -1
If length 2, SA/V 4/8 6 3.0 If length
4, SA/V 16/64 6 1.5 If length 8,
SA/V 64/512 6 0.75
Surface area to volume ratio higher in smaller
endotherms Ratio of heat loss/heat production is
also higher!
12
Hibernation and Torpor

• Both are ways of turning down the thermostat to
  save energy
• Can be distinguished by the length of bouts
• Hibernation tends to be seasonal
• Deep hibernators are small mammals
• Bears undergo shallow hibernation
• Too much energy to warm up their big mass
• Torpor tends to be daily (or nightly)
• Tend to be very small birds and mammals
• These animals can re-warm fairly quickly

13
Torpor
Nightly hibernation - small endotherms conserve
energy by switching thermostat off during their
inactive time of day
Tb 40C
Tb 40C
Insert torpor trace?
Tb low and variable
Data from Hiebert 1990
Not completely off. Wont go below Tb 13C
14
Torpor in small mammals bats
Torpid bats are true ectotherms! Body temp air
temp!
15
Torpor in small mammals bats
Torpid bats have very low rates of O2 use (And
therefore, ATP demand!)
16
Torpor in the daily cycle
Note how body T Overshoots target! Very typical
of rewarming
17
Hibernation

• Term is used to refer to two different states
• Winter sleep shallow hibernation
• Slightly lower body temperature
• Slightly lower metabolic costs
• Most physiological functions continue normally
• Characteristic of bigger mammals
• Skunks, beavers, bears
• True or deep hibernation
• Dramatically lowered temperature setpoint
• Dramatically lower metabolic cost
• Profound metabolic suppression
• Characteristic of smaller mammals lt 200 g.
• Biggest example are marmots (8 kg)

18
Arctic ground squirrels

• Spend 6-7 months a year in obligate
  (non-optional!) hibernation
• Body temperatures dip below zero and show very
  slight supercooling before the thermostat kicks
  on the heat.
• Thermostat is ON, just turned WAY down
• Takes intense 12-15 hours of shivering to arouse,
  they do this every 2-3 weeks. (VERY expensive)
• Lots of research on these guys here at UAF!

19
Temperature setpoints

• Thermostat anterior and posterior hypothalamus
• Both endotherms and ectotherms have thermostats
• Heating or cooling an animals hypothalamus will
  induce strong thermoregulatory responses
• If heated
• Panting and vasodilation, shade seeking behavior
• If cooled
• Increase in metabolic rate, vasoconstriction,
  shivering, basking behavior
• Inputs temperature sensitive neurons
• brain, spinal cord, sites in body core, skin
  (peripheral)

20
Temperature setpoints

• These can be variable
• Day vs. night core temperature varies by 1-3ºC
• In nocturnal animals, night temperature is higher
• In diurnal animals, day temperature is higher
• Decreased setpoint energy savings
• Fever represents a controlled increase in
  setpoint
• In birds and mammals, ranges from 1-4ºC
• In ectotherms, can be much higher
• Test response using pyrogens bacterial
  antigens
• Hibernation and torpor are dramatic changes in
  setpoint

21
Adjusting heat production

• Muscle activity
• Voluntary
• Involuntary shivering
• Lots of animals shiver even (occasionally)
  ectotherms!

22
Endothermic insects

• Big furry fliers Large bees and hawkmoths
• Increase flight muscle T before takeoff by
  shivering
• Thorax temps are high and constant during flight

takeoff
23
Adjusting heat production

• Muscle activity
• Voluntary
• Involuntary shivering
• Lots of animals shiver even (occasionally)
  ectotherms!
• Non-shivering thermogenesis
• Metabolic pathways that dont produce anything
• Way to use metabolic fuel to make heat
• Brown adipose tissue
• Leaky membranes

24
Brown Adipose Tissue

• Who has it?
• Infants, small mammals, hibernators
• What is it?
• Specialized adipose tissue (fat)
• Highly vascularized (lots of blood supply)
• Packed with mitochondria (hence brown)
• WHY?
• It breaks down triglycerides to make heat, and
  only heat! (NOT ATP!)

25
Brown Adipose Tissue
M
M
M
Lipid Droplet
M
M
M
M
How does it work???????
26
Memory Lane Oxidative metabolism
Fatty acids
27
ATP production in normal mitochondrion
Proton gradient is formed Harnessed to make ATP
28
What would happen if you poked a hole in this
membrane?

• No ATP synthesis
• Enhanced proton pumping
• No ATP buildup to inhibit metabolism!
• System just cranks away
• Producing heat but no ATP

29
Uncoupling protein (UCP)
UCP membrane channel that allows H gradient to
dissipate no ATP production
Electron transport is uncoupled from ATP and
not inhibited
This is an important heat producing futile cycle
a.k.a. thermogenin
30
Might UCP be a mechanism for endothermy?
3 have been discovered in mammals
UCP 1 and 3 Brown fat UCP 2 expressed
everywhere!
Vianna et al 2001 Physiological Genomics 5
137-145
31
ENERGETICS
Energy Lifes most central currency
32
The Cost of Living
Can an animal get enough food to persist in an
environment?
Why does it budget its time the way it does?
Metabolic costs are fundamental to understanding
animal ecology and behavior
33
For example
Study of white crowned sparrows found that they
expended more energy sitting still in the cold
than flying around
Behavior only makes sense in terms of costs and
benefits (energy is an important currency)
How do we measure them?
Paladino and King 1984. Phys. Zool. 57 226-236