1. Biology 2672a: Comparative Animal Physiology
Endothermy
Dr Brent Sinclair

2. What happens in the thermoneutral zone?
Metabolic Rate Tb
Ambient temperature

3. Endothermy & Homeothermy
Endotherms generate their own heat
Homeotherms regulate their body temperature by physiological or behavioural means

4. Endothermy
Heat production
Heat retention
Regulation

5. Producing Heat I: All biochemical processes produce heat
Fig. 6.4

6. Producing Heat II: Shivering Thermogenesis
Uncoordinated muscular contractions
Performs no locomotor function
Uses ATP, therefore generates heat

7. Producing Heat III: Non-shivering Thermogenesis
Finding other ways to do biochemical ‘work’ and to produce heat

8. Acclimation to 6°C
After acclimation, they maintained their body temperature without shivering
Initially, rats shivered uncontrollably…

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

10. Localisation of Brown Adipose Tissue
Fig. 9.29

11. Generating heat through futile cycles
Circular biochemical reactions that use up ATP and produce heat without achieving anything else
Mechanisms not well-understood…

12. Futile cycles Ion cycling across membrane
Breakdown and building of triglycerides

13. Uncoupling protein (thermogenin, UCP1)
Box 7.1

14. Uncoupling protein (thermogenin, UCP1)
Box 7.1

15. 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

16. But birds don’t 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?

17. Being an endotherm is expensive!!
Clarke, 2006 Funct. Ecol. 20:

18. Counter-current heat exchange
Fig 9.34

19. 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

20. 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

21. Counter-current heat exchange
Fig 9.33

22. 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

23. 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

24. Fish gills act as a heat sink
Fig c

25. 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

26. Tuna temperature gradients in 19°C Water – Regional Endothermy

27. 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)

28. Red muscle in Tuna
Tuna
Salmon

29. Area of Rete
Fig. 9.43

30. Red muscle temperature is elevated above water temperature
Thermal Mass
Fig. 9.44

31. 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

32. 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?

33. Brain heaters in billfish

34. Heater organ in Billfish
8.5°C
7.2°C
Brain
Eye
Heater Organ
Carotid Rete
4.8°C
1.3°C
Carotid Artery

35. 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

36. Large sharks are also regional endotherms

37. Regional endothermy evolved in bony fishes on at least three separate occasions
Fig 9.45

38. 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

39. 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