Endothermy & Thermoregulation

Endothermy: maintain appreciable difference between body temp (TB) & ambient temp (TA); due to cellular respiration

Homeothermy: maintain relatively stable TB

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

Evolution of Endothermy

2 Late Permian fossil lineages of synapsids = have respiratory turbinates, whereasDinosaurs did not have them

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

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

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

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

THE CONCEPT OF METABOLISM

• Maintenance Metabolism

Field Metabolism (FMR)– sum of resting metabolism and energy

used for all other activities

ENERGY REQUIREMENTS DEPEND ON BODY SIZE

Deviations in Energy Required for Maintenance– aquatic mammals: higher-than-expected RMR

ENERGY REQUIREMENTS DEPEND ON BODY SIZE

Deviations in Energy Required for Maintenance– aquatic mammals: higher-than-expected RMRs– sloths: lower-than-expected RMRs

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

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

THE BRAIN AS AN ENERGY DRAIN

Brain size is determined by:– body size– energy expended

for brain support

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)

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

THE BRAIN AS AN ENERGY DRAIN

Why Do We Sleep?– Restore glycogen...?– Brain development…?– Memory consolidation…?– Conserve energy…?

• 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

• 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

– 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)

Endothermy & Thermoregulation

• Temperatures above thermoneutrality

– Approaching TUC

– Lose excess heat via evaporative cooling• e.g., sweating,

panting, spread saliva on fur

• 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

• Temperatures above thermoneutrality

– Daily cycling of TB relative to availability of water• e.g., common in camels

• Temperatures below thermoneutrality

– Approaching TLC

– Increase basal metabolic rate

– TLC = point where increasing insulation fails to prevent drop in TB; must increase metabolic rate

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

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

• 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)

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

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

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

• 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

Endothermy & Thermoregulation

• Metabolism & Insulation

1) Factors that affect insulation (from heat or cold)

E) Insulation - Blubber

• 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