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HOW SIZE MATTERS IN REGULATING BODY TEMPERATURE

Size affects everything. Here, though, I will discuss specifically how size matters to regulating body temperature.

Concept: Small-bodied animals or plant parts (e.g., leaves) heat up and cool down faster; bigger and/or thicker bodies heat up and cool down slower.

Explanation: smaller/thinner bodies have a larger surface area to volume ratio (see examples below). Bodies gain and lose heat out of the surface of their body; more surface area means greater gains and losses. Bodies retain heat within their bodies; more volume means more heat retention. When the surface area is large compared to the volume (small/thin things), heat is gained and lost quickly because there is lots of surface area to gain and lose heat and relatively little volume to retain heat.

Advantages and Disadvantages to being Large:

• Heat is gained and lost more slowly so, for example, on a hot summer day, the a large animal may never reach lethal temperatures by the time the sun sets.
• Because heat is lost more slowly, the animal doesn't have to replace lost heat as quickly; therefore, the animal doesn't have to eat as much compared to its body weight (e.g., only has to eat 1/4 its body weight). However, it usually does have to eat more total food than a smaller animal.

Advantages and Disadvantages to being Small:

• Heat is gained and lost faster so, for example, on a hot summer day, a small leaf (less than about 1 square centimeter) will shed heat as fast as it acquires heat; therefore the leaf will never reach temperatures higher than air temperature (compared to a large, thick leaf that acquires a heat load and can reach very high, lethal temperatures).
• Because heat is lost faster, the animal has to eat faster to replace the lost energy (e.g., very small mammals may eat up to 4 times their body weight each day).

Summary Example : A plant can either have very small leaves that never heat up higher than air temperature or very large leaves that heat up so slowly that they never reach lethal temperatures, but a plant doesn't want to have leaves in the middle sizes if the temperatures are going to be extremely hot or cold.

Examples illustrating surface area to volume ratio:

1. Small cookies cool down faster than larger cookies after coming out of the oven. Also small cookies burn faster.
2. Your hand has the same volume whether it is balled up (fat) or spread out (thin). On a cold day, your hand will get cold faster when spread out because balling up you hand into a fist effectively reduces surface area because now the part of your hand within your fist is no longer "surface".

3. Consider two individuals exactly alike except in size. The smaller individual is 1' by 1' by 1' in size and the larger individual is 2' by 2' by 2' in size. The small individual has less surface area (length times width times number of sides = 1' x 1' x 6 sides = 6 square feet) than the large individual (24 square feet). The small individual has less volume (length times width times height = 1' x 1' x 1' = 1 cubic foot) than the large individual (8 cubic feet). But the small individual has twice as high a surface area to volume ratio (surface area divided by volume = 6 divided by 1 = 6) as the large individual (24 divided by 8 = 3)!