Teacher: The jackrabbit’s big ears provide a large surface area to increase the animal’s rate of heat loss to its surroundings.

Student: But doesn’t it work both ways? That is, the large surface area will also increase the jackrabbit’s rate of heat gain from its surroundings when the surrounding is hot?

Teacher: Err…

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At primary school level, students learn the following concepts about heat.

  • It always flows from a place of higher temperature to a place of lower temperature.
  • Heat transfer happens by conduction.

Now, the average body temperature of a jackrabbit hovers between 38 and 39 degrees Celsius. In summer, the temperature at a jackrabbit’s desert habitat can reach a blistering 46 degrees Celsius. So won’t the jackrabbit gain heat more quickly from its surroundings (because heat flows from a hotter to cooler place) by virtue of its big ears compared to a bunny with regular ears? At a time when it most needs to get rid of excess heat too!

The answer lies in another mode of heat transfer, which is not covered in Primary School syllabus – radiation (convection is yet another mode of heat transfer, but it will not be covered in this discussion).

Thermal radiation is the transfer of heat through electromagnetic (EM) waves. Visible light, X-rays, microwaves and radio waves are all examples of EM waves. Thermal radiation is how heat from the sun reaches Earth.

Fact #1: An object will radiate heat to its surroundings as long as its temperature is above absolute zero (-273.15 degrees Celsius).

Fact #2: The amount of heat radiated is proportional to the surface area of the body producing it (Stefan-Boltzmann law).

It’s only when you marry fact #1 with fact #2 that the surface area explanation begins to make sense.

In fact, radiation is the dominant mode of heat loss in organisms. It is estimated that in a surrounding temperature of 45 degrees Celsius, a human (whose average temperature is 37 degrees Celsius) will lose 109 watts by radiation compared to 8 watts by conduction. Believe it or not, 109 watts is enough energy to power a laptop. As we speak, scientists are working on building contraptions to harvest heat energy produced by the body. Indeed, it seems our bodies will be the batteries of the future!

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There you have it. The jackrabbit’s big ears provide a larger surface area for more heat radiation to occur, helping the animal cool down more efficiently. But since radiation is not covered in Primary School syllabus, it’s not wrong to say that the larger surface area helps the animal lose heat to its surroundings more easily.

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