Zoology - Thermoregulation

Zoology: Thermoregulation

Learning Outcomes

• Understand why temperature is important for organisms.
• Understand the principles of heat transfer.
• Define and apply the concept of operative temperatures.
• Understand and explain behavioral thermoregulation.

The Effect of Temperature on Organisms

• Performance: Impacted by body temperature ($T_b$).
• Thermodynamics:
  • Reaction rates generally decrease with lower temperatures.
  • Capacity limitations may occur due to temperature constraints.
• Damage:
  • Proteins and membranes can be damaged by extreme temperatures.
  • Critical temperatures can lead to death.
• Thermal Performance Curve: Organisms exhibit an optimal temperature range for performance.

Responses to Temperature Variation

• $T_b$ = Environmental Temperature
• Acclimation:
  • Organisms regulate cellular and biochemical capacities.
  • Optimal conditions may differ across varied environments.

Regulating Body Temperature

• Physiologically:
  • Internal heat production can be regulated.
  • Biochemical processes are optimized at a specific $T_b$.
• Cardiovascular Changes:
  • Changes in blood flow affect heat transfer rates.
• Behaviorally:
  • Response to the high heterogeneity in the environment.
  • Microhabitat selection allows for thermoregulation.

Heat Transfer

Mechanisms of Heat Transfer

• Radiation
  • Short wave solar radiation
  • Long wave thermal radiation
• Conduction
  • Heat exchange within a solid or between two solids
• Convection
  • Heat exchange between a fluid and a solid

Radiation

• Solar radiation is defined by the equation: $q = aAQ$ where:
  • $a$ = absorptivity
  • $A$ = area
  • $Q$ = wave energy
• Different surfaces absorb at different rates.
• Wave energy varies with wavelength.

Conduction

• Defined by the equation: $q = k binom{A}{l} (T<em>1 - T</em>2)$
  • $k$ = conductivity
  • $A$ = area
  • $l$ = thickness
  • $T$ = temperature
• Temperature differential exists within a solid.
• Molecules have different energetic states.
• Energy is exchanged over time.
• Conductivity represents the rate at which energy is exchanged.

Convection

• Heat exchange between a fluid and a solid
  • Defined by the equation: $q = hA(T<em>s - T</em>{\infty})$ where:
    • $h$ = convection coefficient
    • $A$ = area
    • $T_s$ = Temperature of the solid surface
    • $T_{\infty}$ = Temperature of the fluid far from the surface
• Temperature differential exists between the fluid and solid.
• A boundary layer's properties depend on the flow.
• The convection coefficient describes the rate at which energy is exchanged within the boundary layer.

Thermal Environment

• Heat exchange involves radiation, evaporation, convection (in air), and conduction.

Operative Temperature

• Describes the thermal environment of an organism.
• It's a single temperature that integrates all heat exchange processes at the animal's surface.
• Conceptually, it represents an 'average' surface temperature.
• Calculated from heat transfer equations, considering the energy balance at the surface.

Heat Energy Balance

• $E<em>{in} = E</em>{out}$
• $Q<em>{rad} = q</em>{convection} + q_{conduction}$
• $aAQ = hA(T - T<em>a) + binom{k}{l} A(T - T</em>g)$
• Solving for $T$ gives the operative temperature $T_e$:
  • $T<em>e = binom{aQ + hAT</em>a + binom{k}{l} AT_g}{hA + binom{k}{l} A}$

Behaviour: Reptiles

• Observed body temperature patterns may not be random.
• Reptiles actively regulate their $T_b$.

Thermoregulation: Analysing Temperatures

• Body temperature:
  • Is it random?
  • What is the target temperature?
  • What mechanisms are involved?
• Frequency distributions:
  • Take $T_b$ measurements from individuals.
  • Plot the frequency of observations.
  • Compare means and variation.

Beer Can $T_b$

• Analogy to lizard $T_b$ to demonstrate environmental temperature distribution.

Showing Thermoregulation

• Observe thermoregulatory behaviour directly.
• Use a control (null-model):
  • $T_b$ of an animal that moves/behaves randomly.
• Compare measured $T_b$ of a real animal to the null-model.

Behavioural Thermoregulation

Effectiveness of Thermoregulation

1. Determine "selected" $T_b$:
   • Ideal $T_b$ with no constraints.
   • Establish a thermal gradient in the lab.
2. Measure $T_b$ of animals in the field.
3. Measure null-distributions:
   • Random distribution of operative temperatures.
4. Determine $T<em>b - T</em>s = db$, and $T<em>e - T</em>s = de$
   • Index of thermoregulatory efficiency: $E = 1 - binom{db}{de}$
     • $E = 0$: no regulation.
     • $E = 1$: perfect regulation.

• Compare $T_b$ to a theoretical animal that is not thermoregulating (random operative temperature distributions) to provide a control.

Species Distribution

• Does thermoregulation limit distributions?
  • Do species thermoregulate to the same $T_b$?
  • Is the same thermoregulatory efficiency required across a species range?
• Sample across latitudes.
• Measure $T<em>e$, $T</em>b$, $db$, $de$.
• Calculate efficiency $E$.

Observations

• Mean $T_b$ differs between sites (latitudes).
• Thermoregulatory efficiency differs between latitudes.
  • Different thermoregulatory efforts are observed.
  • Species distribution is tied to thermoregulatory ability.

Predicting Habitat Suitability

• Use physiological data to predict:
  • Habitat suitability.
  • Altered distributions under climate change.

Conservation Physiology

• Physiology acts as a filter between environment and animal responses.
• It provides a causal dimension of ecology related to movement, biogeography, and biodiversity.