Biology 2672a: Comparative Animal Physiology

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Biology 2672a Comparative Animal Physiology

• Endothermy
• Dr Brent Sinclair
• bsincla7_at_uwo.ca

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What happens in the thermoneutral zone?
Thermoneutral Zone
Metabolic Rate
Tb
Ambient temperature
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Endothermy Homeothermy

• Endotherms generate their own heat
• Homeotherms regulate their body temperature by
  physiological or behavioural means

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Endothermy

• Heat production
• Heat retention
• Regulation

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Producing Heat I All biochemical processes
produce heat
Fig. 6.4
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Producing Heat II Shivering Thermogenesis

• Uncoordinated muscular contractions
• Performs no locomotor function
• Uses ATP, therefore generates heat

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Producing Heat III Non-shivering Thermogenesis

• Finding other ways to do biochemical work and
  to produce heat

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Acclimation to 6C
After acclimation, they maintained their body
temperature without shivering
Initially, rats shivered uncontrollably
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Brown adipose tissue

• Highly vascularized tissue with lots of
  mitochondria
• Abundant in newborn mammals, declines with age
• Can increase with acclimation/ acclimatisation
  (rats)

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Localisation of Brown Adipose Tissue
Fig. 9.29
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Generating heat through futile cycles

• Circular biochemical reactions that use up ATP
  and produce heat without achieving anything else
• Mechanisms not well-understood

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Futile cycles

• Ion cycling across membrane
• Breakdown and building of triglycerides

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Uncoupling protein (thermogenin, UCP1)
Box 7.1
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Uncoupling protein (thermogenin, UCP1)
Box 7.1
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Uncoupling protein (thermogenin, UCP1)

• When thermogenin is activated, proton transport
  into the mitochondrion is uncoupled from ATP
  synthesis
• ATP is expended pumping protons out again,
  generating heat
• The numerous mitochondria in BAT are essentially
  futile pumping engines

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But birds dont have BAT?!

• A UCP2/3 homolog has been found in hummingbird
  muscle
• Another tissue with lots of mitochondria
• Dual use of muscles because of weight restriction?

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Being an endotherm is expensive!!
Clarke, 2006 Funct. Ecol. 20 405-412.
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Counter-current heat exchange
Fig 9.34
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Countercurrent heat exchange
Cold
Hot
No transfer of heat between outgoing and incoming
vessels

• Heat is lost to the outside world
• More heat is then necessary to increase
  temperature upon return to the body
• A NET LOSS OF HEAT ENERGY

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Countercurrent heat exchange
Cold
Hot
Allow heat transfer between outgoing and incoming
vessels

• Heat transfers to the incoming vessel
• Less heat is lost to the outside world
• Less energy is required to re-heat the incoming
  blood

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Counter-current heat exchange
Fig 9.33
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Regional heterothermy

• Exposed body parts are often cooler than core
  temperature
• Heat energy conservation
• Testes and brains are often also kept a little
  cooler

Fig. 9.30
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Why fish are ectotherms

• Fishes can generate heat just like anything else
  with metabolism, but they have problems keeping
  it
• Surrounded by thermally-conductive water

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Fish gills act as a heat sink
Fish Gills
Fig. 22.10c
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Why Fish are Ectotherms

• Fish are ectotherms because the high blood flow
  across their large gill surface means that they
  lose heat to the environment really quickly
• Heat retention is a major issue

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Tuna temperature gradients in 19C Water
Regional Endothermy
23 C
29 C
31 C
27 C
23 C
19 C
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Rete mirabile

• A recurring theme especially in (but not
  exclusive to) fish
• Swim bladder
• Heater organs
• Oxygen delivery to retina
• Allows for VERY effective countercurrent exchange
  of heat (and other things)

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Red muscle in Tuna
Salmon
Tuna
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Area of Rete
Fig. 9.43
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Red muscle temperature is elevated above water
temperature
Thermal Mass
Fig. 9.44
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Where does the heat come from?

• Heat comes from the normal heat produced by
  contractile activity of the red muscles
• The only difference is that the heat is retained

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Why?

• Good question!
• To allow long migration through water of
  different temperatures?
• To allow better performance as a predator chasing
  prey into colder water?
• Improvements in power output of muscles?

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Brain heaters in billfish
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Heater organ in Billfish
8.5C
7.2C
Brain
Eye
Heater Organ
Carotid Rete
4.8C
1.3C
Carotid Artery
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Heater organ in Billfish

• Derived from an extra-ocular eye muscle
• No contractile ability left
• 63 mitochondria (by volume)
• Another 30 is sarcoplasmic reticulum
• Heat is produced by futile cycling of Ca2 pumping

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Large sharks are also regional endotherms
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Regional endothermy evolved in bony fishes on at
least three separate occasions
Fig 9.45
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Brain heaters evolved in bony fishes on at least
two separate occasions
Lateral rectus muscle developed into a brain
heater
Superior rectus muscle developed into a brain
heater
Fig. 9.45
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Reading for next week(s)

• Two Guest Lectures
• Hibernation
• Migration
• (are examinable)
• Note Lecture Quiz 2 material ends NEXT Thursday
• Reading to be announced on OWL
• Reading for Tues Oct 7 Thu Oct 9 Circulation
• Pp 611-641