Lecture Notes

Pathways of Heat Exchange

  1. Every calorie of heat lost must be replaced metabolically.

  2. Sources of heat loss include conduction, convection, radiation and evaporation.

  3. Conduction is the transfer of energy between two bodies.

    \[Q = kA\frac{T_b - T_a}{d}\]

    1. Where k is thermal conductivity of the material of the body, A is surface area of barrier or surface, d is distance from core to ambient area or width of barrier.

    2. Q is in watts. 1 J/s = .24 Calories/second

  4. Convection is similar to conduction, but involves a boundary layer(this is the layer of air just above the body such that the temperature is close to that of the body). It is the movement of heat from a body to a medium. Can be heat lost through wind blowing on skin in animals.The greater the wind or flow surrounding you, the thinner your boundary layer. Boundary layer is affected by shape, texture and speed of flow. If ther boundary layer absorbs radiative heat, it will emit some of that heat back to you.

    \[Q = kA\frac{T_b - T_a}{d}\]

    1. Where \(A\) is the area of the flat surface over which convection is happening. \(k\) is the thermal conductivity of the boundary layer, and \(d\) is the thickness of the boundary layer.
  5. Radiation can be tracked with the Stefan-Boltzmann equation. Doesn’t depend on temperature gradient:

    \[Q_r = \epsilon \sigma T^4\]

    1. Where \(\epsilon\) is the emissivity, \(\sigma\) is the Steffan-Boltzman constant and T is absolute temperature.

    2. In the winter you radiate less heat because your surfaces are at a lower temperature.

    3. On cloudy nights, heat is reradiated back to earth from clouds, whereas on clear nights it is lost to space.

  6. Simplified energy balance equation. Ignores radiation. Which can be done because endotherms maintain constant body temperature so radiative heat loss is constant for all seasons. Species modify these variables to minimize the heat they must produce.:

    \[M = kA\frac{T_b - T_a}{d}\]

    1. Increasing d or decreasing k is an increase in insulation.

    2. Reducing A is reducing surface area.

    3. Reducing \(T_b - T_a\) is reducing temperature gradient.

    4. Huddling reduces surface area and temperature gradient.

Physical Barriers to Heat Loss

  1. Huddling reduces surface area and decreases surface area to volume ratio.

  2. Den building increases depth of the boundary layer.

  3. Heat can be generated by shivering. Thermal imaging studies of bees show the inside of balls can be quite high in temperature due to shivering in the thorax. Honeybees use stored honey to power this shivering. Honeybees are a non-native species.

    1. Sometimes honeybees can’t move the ball to their food because it uses too much energy.

    2. Study of russian and southern bee colony overwintering survival found significant effect of colony weight and weak or no efffect of region of origin. Colony size is most important predictor of survival.

    3. Thorax is warmer than abdomen and head regardless of location in ball. Insects below 10 degrees C typically lose metabolic activity.

Lower Critical and Thermoneutral Zones

  1. Animals can change posture, insulative capacity of feathers or fur to adjust heat loss.

  2. The Basal Metabolic Rate (BMR) is rate of energy expenditure at rest (not metabolizing more than normal).

  3. The Thermoneutral Zone is the range of atmospheric temperature over which an organism doesn’t need to increase evaporation by sweating to cool down or increase metabolism above BMR to maintain body temperature.

  4. Below the lower critical temperature, more metabolic energy must be expended to produce enough heat to avoid cellular damage. This can be done by Non-shivering thermogenesis (physiological) or shivering thermogenesis (physical).

    1. When comparing lower critical temperatures you should normailize for body size.

    2. Lower critical temperatures are lower for arctic animals.

    3. NST uncouples flow of protons in mitochondria from synthesis of ATP due to uncoupling protein in brown fat, the excess energy is now released as heat.

    4. ST results in muscles expending energy, which releases heat.

  5. Reduce peripheral temperatures: Reduces loss to the environment and incraeses insulation.

  6. You can withdraw limbs to reduce surface area. This is physical thermoregulation. Waterfowl cannot do this because they must remain bouyant, so they use countercurrent exchange. Arteries are wrapped with veins so that heat is transferred to blood coming in rather than to the skin to leave.

    1. As the blood moves down the artery it cools, but always remains hotter than cooler venous blood.
  7. Torpor is a way of reducing heat loss by letting body temperature cool. Bats do this. This reduces energetic cost. Torpid organisms at night use close to the basal metabolic rate of energy. Torpid individuals are still regulating temperature.

  8. Winter in Sciurids

    1. Body size has a lot to do with thermal regulation. If you have a large body, you can store lots of fat. Howeve, it’s expensive to heat your body from torpor.

    2. Chipmunks, Grey squirrels, Red squirrels, flying squirrels, Groundhog, arctic squirrels.

    3. Groundhogs hibernate but don’t store food, their fat gets them by due to their size. Arctic squirrels are the only other sciurid that hibernates.

    4. Chipmunks go into short torpors and wake to feed on stored food.

    5. The other squirrels cache food for the winter as well.

    6. Flying squirrels huddle.

    7. Has white fat and skeletal fat to burn in addition to brown fat. Can also supercool its body fluids to go 7 degrees below freezing.

Biogeographic Rules

  1. Bergmann’s Rule: Northern species are larger

    1. Idea is that they are reducing per gram metabolic costs due to surface to volume ratio decrease.

    2. Criticism: total energy cost is still too high to compensate.

    3. Alternative explanations: competition and resources.
    4. Competition: in the tropics, competition reduces body size because there are larger competitors. Jaguar and Puma example: outside the tropics pumas are larger. Inside them they’re smaller.

    5. In the absence of jaguars, puma eat much larger prey than when in same habitat as them.

  2. Gloger’s Rule: less pigment in species further north.

    1. Camouflage

    2. Reduced emissivity/ radiative heat loss.

    3. Hollow hair/feathers provide better insulation.

      1. Reason is it holds more air.
    4. Tropical side - black pigment is more resistant to bacterial decay.

      1. Antibacterial feathers
  3. Allen’s Rule: Shorter limb length at colder winters/latitudes.

    1. Seen in seabirds. Leg length decreased with higher latitude. Residual graph lets you see how much each bird has longer or shorter legs than predicted by a regression.