Lecture 13 - Endothermy

What are some thermoregulations when temperatures are too high (above TNZ)?

• Evaporative cooling

  • Sweating

  • Panting

  • Gular fluttering

• Allow hypothermia

What are some thermoregulations when temperatures are too low (below TNZ)?

• Shivering thermogenesis

  • Muscle contractions produce heat

• Non-shivering thermogenesis (NST)

  • Heat produced w/out muscle activity

  • Uses uncoupling proteins in brown fat

  • Proton leak produces heat instead of ATP

Brown fat

A specialized type of body fat that burns calories to generate heat (thermogenesis) rather than storing energy, helping maintain body temperature and improve metabolism.

How can animals regulate their metabolism to thermoregulate during the winter?

Increase metabolic rate

Biochemical benefits of endothermy

• Stable, higher body temperatures

• Improves enzyme activity and biochemical reactions

Uncoupling proteins

Mitochondrial inner membrane proteins that dissipate the proton gradient generated by the electron transport chain, releasing energy as heat (thermogenesis) rather than producing ATP.

• Typically found in brown fat

Physiological benefits of endothermy

• More effective thermoregulation

• Higher aerobic metabolic capacity

Life-history benefits of endothermy

• Higher reproductive rates

• Greater movement capacity

  • Migration

  • Searching for food

• Less time spent behavioral thermoregulation

What are some hypotheses for the evolution of endothermy?

1. Warmer bodies improve physiological performance

2. Maxithermy hypothesis:

• higher metabolic rates → increased competitive ability and reproductive success

3. Selection for higher aerobic capacity led to endothermy

Aerobic scope hypothesis

The hypothesis that selection for higher aerobic capacity led to evolution of endothermy as a by-product

How could we test the aerobic scope hypothesis?

Select for enhanced aerobic capacity and see whether BMR increases.

Also examine whether shifts in metabolic capacity (e.g., larger organ sizes) occur.

Absolute aerobic scope

Maximal metabolic rate (MMR) minus resting metabolic rate (RMR)

• Endotherms have greater absolute aerobic scope, meaning they can sustain much higher levels of aerobic activity

• Endotherms have much higher RMR’s than ectotherms

Factorial aerobic scope (FAS)

Maximal metabolic rate (MMR) / resting metabolic rate (RMR)

• Roughly similar between reptiles and mammals (~10).

• Suggests mechanistic linkage (through organ size?)

Reptilians can produce very short bursts of high power, allowing them to escape from or capture endothermic animals. What are some limitations/consequences of this?

• Low capacity for sustained aerobic activity

  • Fatigue more quickly as they rely more on anaerobic metabolism

• Increasing MMR = increasing RMR

Is BMR and MMR heritable or unheritable?

Heritable

Benefits of insulation

• Insulation helps retain metabolic heat.

• Leads to:

  • Higher body temperature

  • Increased metabolic rates

  • Greater aerobic scope

Torpor

A temporary state where an animal lowers its body temperature and metabolic rate to save energy, usually when food is limited or temperatures are cold.

Define thermoregulatory scope (TS). What does a high or low TS indicate?

Range of body temperature variation.

Species with greater TS tend to be:

• Small-bodied

• Living at high latitudes

• Non-hoarding species

• High TS: animals allow large body temperature fluctuations (often use torpor).

• Low TS: animals maintain stable body temperature through strong thermoregulation.

What are factors that affect torpor?

Reduced Insulation

• Example: fur shaving

• Leads to more frequent torpor.

Increased Food Supply

• Unlimited food reduces or eliminates torpor.

Environmental Temperature

• Colder environments increase energy demands

The plots depict how body mass, latitude, and food hoarding behavior alter the amount of body temperature variation (increases in TS, thermoregulatory scope= more heterothermy) in mammals.

First state hypotheses for how the listed factors will alter TS

• Body mass

• Latitude

• Food hoarding

Body mass: Smaller mammals will have higher TS

• Due to higher surface area-to-volume ratio, so they benefit more from torpor

Latitude: Mammals at higher latitudes will have higher TS

• Colder environments favor greater use of torpor or body temp reduction to save energy

Hoarding: Mammals that don’t hoard will have a higher TS

• Non-hoarders rely more on torpor and body temperature variation to conserve energy.

How do you think 1) fur shaving and 2) food supplements will change the incidence of torpor in shrews?

1. Fur shaving - Will increase torpor due to less insulation and heat retainment

2. Food supplements - Will decrease torpor due to higher food supply and therefore, higher metabolic rates

Draw or state your expectations for how the frequency and duration of torpor as well as body temperature during torpor will respond to food intake in opossums?

Frequency - Decreases with more food intake

Duration - Decreases with more food intake

Body temp - Increases with more food intake