Endothermy & Thermoregulation
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Endothermy & Thermoregulation
Endothermy: maintain appreciable difference between body temp (TB) & ambient temp (TA); due to cellular respiration
Homeothermy: maintain relatively stable TB
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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
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Evolution of Endothermy
2 Late Permian fossil lineages of synapsids = have respiratory turbinates, whereasDinosaurs did not have them
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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
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Endothermy & 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
• Tremendous cost
– Foraging mouse = 20-30 times more energy than lizard of same mass
– Small mammals = 80-90% energy to thermoregulation
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Variation in TB
Approx. normal core temp (C)
Approx. lethal core temp (C)
Monotremes 30-31 37
Marsupials 35-36 40-41
Insectivores 34-36 41
Other Eutherians
36-38 42-44
Humans 37 43
Increase sophistication Lethal temp ~6oC
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Mass (kg)
0.01 0.1 1 10 100 1000 10000
RM
R (
kc
al/
da
y)
100
101
102
103
104
105
106
Placental
Marsupial
Isometric (1:1)
RMR = aM3/4
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
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THE CONCEPT OF METABOLISM
• Maintenance Metabolism
Field Metabolism (FMR)– sum of resting metabolism and energy
used for all other activities
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ENERGY REQUIREMENTS DEPEND ON BODY SIZE
Deviations in Energy Required for Maintenance– aquatic mammals: higher-than-expected RMR
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ENERGY REQUIREMENTS DEPEND ON BODY SIZE
Deviations in Energy Required for Maintenance– aquatic mammals: higher-than-expected RMRs– sloths: lower-than-expected RMRs
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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
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Mass (kg)
0.01 0.1 1 10 100 1000 10000
Me
tab
oli
c r
ate
(k
ca
l/d
ay
)
100
101
102
103
104
105
106
RMR = kM0.75
FMR = aM0.81
Placental Mammals
ENERGY REQUIREMENTS DEPEND ON BODY SIZE
Energy Required for Normal Daily Activity
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THE BRAIN AS AN ENERGY DRAIN
Brain size is determined by:– body size– energy expended
for brain support
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THE BRAIN AS AN ENERGY DRAIN
Brain Size and Energy Demand
Mass (kg)
0.01 0.1 1 10 100 1000 10000
Bra
in s
ize
(k
g)
10-2
10-1
100
101
102
RM
R (
kc
al/
da
y)
100
101
102
103
104
105
Brain Size = 0.01M0.70
Brain
RMR (slope = 0.75)
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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
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THE BRAIN AS AN ENERGY DRAIN
Why Do We Sleep?– Restore glycogen...?– Brain development…?– Memory consolidation…?– Conserve energy…?
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• Metabolism-Temperature Curves
– If TA < TB = lose heat passively via dry heat transfer (i.e., conduction, convection, radiation), and/or evaporation
– Rate of heat loss positively correlated to increased disparity between TB and TA
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• 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
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– 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)
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Endothermy & Thermoregulation
• Temperatures above thermoneutrality
– Approaching TUC
– Lose excess heat via evaporative cooling• e.g., sweating,
panting, spread saliva on fur
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• 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
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• Temperatures above thermoneutrality
– Daily cycling of TB relative to availability of water• e.g., common in camels
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• Temperatures below thermoneutrality
– Approaching TLC
– Increase basal metabolic rate
– TLC = point where increasing insulation fails to prevent drop in TB; must increase metabolic rate
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Endothermy & Thermoregulation
• Metabolism & Insulation
1) Factors that affect insulation (from heat or cold)
A) piloerection: elevation or flattening of fur; involuntary response to temp changes (“goose bumps”)
B) Alter peripheral/superficial blood flow
vasoconstriction: constrict blood vessels resulting in reduced convective movement of heat to body surface
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Endothermy & Thermoregulation
• Metabolism & Insulation
1) 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
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• Metabolism & Insulation
1) 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)
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Endothermy & Thermoregulation
• Metabolism & Insulation
1) 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
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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
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Endothermy & Thermoregulation
• Metabolism & Insulation
1) 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
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• Metabolism & Insulation
1) Factors that affect insulation (from heat or cold)
E) Fur thickness & insulation Quality/Value
• Positive correlation between fur thickness & insulation quality of fur
Highest value
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Endothermy & Thermoregulation
• Metabolism & Insulation
1) Factors that affect insulation (from heat or cold)
E) Insulation - Blubber
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• Metabolism & Insulation
1) 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