Endothermy & Thermoregulation

1. Endothermy & Thermoregulation Endothermy: maintain appreciable difference between body temp (TB) & ambient temp (TA); due to cellular respiration Homeothermy: maintain relatively stable TB

2. Evolution of Endothermy Endotherms = structures within their nasal passages (respiratory turbinates) = intermittent countercurrent heat and water exchangers. Enable them to have increased ventilatory rates associated with high resting O2 consumption. Ectotherms = lack respiratory turbinates, have relatively low lung ventilation and O2 consumption rates. As a result, they have relatively narrow nasal passages

3. Evolution of Endothermy 2 Late Permian fossil lineages of synapsids = have respiratory turbinates, whereas Dinosaurs did not have them

4. Energy Costs of Endothermy • High energetic costs • As difference TB to TA increases, more heat lost • Newton’s Law of Cooling • Heat loss varies directly proportional to gradient difference between TB and TA • e.g., TB = 40oC and TA = 20oC losses heat twice as fast as TB = 30oC

5. Endothermy & Thermoregulation • Tremendous cost • Foraging mouse = 20-30 times more energy than lizard of same mass • Small mammals = 80-90% energy to thermoregulation • Tremendous benefits • High activity levels (faster, quicker…) • Active in fluctuating & diverse environments • Increased niche exploitation, especially nocturnal niche • Efficient chemical reactions; high enzymatic efficiency

6. Variation in TB Increase sophistication Lethal temp ~6oC

7. Endothermy & Thermoregulation • basal rate of metabolism: measure of minimal cost to maintain normal TB during rest & post absorptive (since last meal previous night) *Allometric relationship – scales with body mass

8. THE CONCEPT OF METABOLISM • Maintenance Metabolism Field Metabolism (FMR) • sum of resting metabolism and energy used for all other activities

10. ENERGY REQUIREMENTS DEPEND ON BODY SIZE Deviations in Energy Required for Maintenance • aquatic mammals: higher-than-expected RMR

11. ENERGY REQUIREMENTS DEPEND ON BODY SIZE Deviations in Energy Required for Maintenance • aquatic mammals: higher-than-expected RMRs • sloths: lower-than-expected RMRs

12. ENERGY REQUIREMENTS DEPEND ON BODY SIZE Deviations in Energy Required for Maintenance • energetic tradeoffs: • high RMR: rapid development and population growth • low RMR: better survival and parental care

13. ENERGY REQUIREMENTS DEPEND ON BODY SIZE Energy Required for Normal Daily Activity

14. THE BRAIN AS AN ENERGY DRAIN Brain size is determined by: • body size • energy expended for brain support

15. THE BRAIN AS AN ENERGY DRAIN Brain Size and Energy Demand

16. THE BRAIN AS AN ENERGY DRAIN Mammals Sleep A Lot - two-toed sloth = 20 hours - armadillo, bat, opossum = 19 hrs - lemur, tree shrew = 16 hrs - hamster, squirrel = 14 hrs - rat, cat, mouse = 13 hrs

17. THE BRAIN AS AN ENERGY DRAIN Why Do We Sleep? • Restore glycogen...? • Brain development…? • Memory consolidation…? • Conserve energy…?

18. Metabolism-Temperature Curves • If TA < TB = lose heat passively via dryheattransfer (i.e., conduction, convection, radiation), and/or evaporation • Rate of heat loss positively correlated to increased disparity between TB and TA

19. Thermoneutral zone (TN): zone over which basal rate of metabolism is independent of TA • upper & lower limits of TN = upper and lower critical temps; TUC & TLC, respectively

21. small mammals tend to have narrow TN, e.g., mice = 30-35oC • Large mammals tend to have wider TN , e.g., eskimo dogs = -25 to 30oC • Principal mechanism of thermoregulation in the TN zone is the degree of insulation (thick fur, fat, or blubber)

22. Endothermy & Thermoregulation • Temperatures above thermoneutrality • Approaching TUC • Lose excess heat via evaporative cooling • e.g., sweating, panting, spread saliva on fur

23. Temperatures above thermoneutrality • Become hyperthermic by raising TB to near TA, thereby reducing water loss and continuing dry heat transfer • e.g., many desert mammals

24. Temperatures above thermoneutrality • Daily cycling of TB relative to availability of water • e.g., common in camels

25. Temperatures below thermoneutrality • Approaching TLC • Increase basal metabolic rate • TLC = point where increasing insulation fails to prevent drop in TB; must increase metabolic rate

26. Endothermy & Thermoregulation • Metabolism & Insulation • Factors that affect insulation (from heat or cold) • piloerection: elevation or flattening of fur; involuntary response to temp changes (“goose bumps”) • Alter peripheral/superficial blood flow vasoconstriction: constrict blood vessels resulting in reduced convective movement of heat to body surface

27. Endothermy & Thermoregulation • Metabolism & Insulation • Factors that affect insulation (from heat or cold) B) Alter peripheral/superficial blood flow vasodilation: dilate blood vessels resulting in increased convective heat movement to body surface

28. Metabolism & Insulation • Factors that affect insulation (from heat or cold) C) Variable distribution of fur on body heat windows: regions of body with thin fur; excellent convection, conduction, and radiation regions (e.g., groin region)

29. Endothermy & Thermoregulation • Metabolism & Insulation • Factors that affect insulation (from heat or cold) D) Changes in body posture (examples of behavioral thermoregulation) • Warm temps = reduce surface area (SA) exposure to solar radiation e.g., camels face into sun & huddle in long rows

30. Interesting Side Note • Hyraxes (Family Procaviidae) = 7 living species. • Sub-Saharan Africa and Middle East • Hyracoids usually grouped with as elephants and sirenians as "subungulates,“ because of common ancestor

31. Endothermy & Thermoregulation • Metabolism & Insulation • Factors that affect insulation (from heat or cold) D) Changes in body posture (examples of behavioral thermoregulation) • cold temps = reduce SA exposure to surroundings & wind chill e.g., curl into ball; tuck nose (reduce evaporative heat loss); huddle in group

32. Highest value • Metabolism & Insulation • Factors that affect insulation (from heat or cold) E) Fur thickness & insulation Quality/Value • Positive correlation between fur thickness & insulation quality of fur

33. Endothermy & Thermoregulation • Metabolism & Insulation • Factors that affect insulation (from heat or cold) E) Insulation - Blubber

34. Metabolism & Insulation • Factors that affect insulation (from heat or cold) F) Body Size **** (primary character influencing basal metabolic rate) • Small body size = larger SA exposed; heats faster, cools faster • Large body size = smaller SA exposed; heats slower, cools slower