The size and shape of our bodies affect how warm we stay when we’re exposed to the cold. There are two interesting ecogeographical rules that concern body shape/size and heat dissipation.
The first is Bergmann’s Rule, proposed in 1847 by Carl Bergmann, which observes that for closely related animal species that maintain a constant body temperature, such animals tend to be larger near the poles and smaller near the equator. This is because an animal’s surface area is the major determinant of heat loss and its body volume is a primary factor in its heat production. Because volume scales faster than surface area as an animal gets bigger, there will be less surface area as compared to volume as animals get larger.* Thus, it makes sense for species of animals to be larger in colder environments and to be smaller in warmer environments. For example, “the average weight of an adult male white-tailed deer in Florida is about 125 pounds (57 kilograms), while a mature buck in Montana might weigh 250-275 pounds (114-125 kg).”
The second is Allen’s Rule from 1877 proposed by Joel Asaph Allen which provides that body shapes are more round and have shorter extremities towards the poles and that bodies are more linear and have longer and thinner extremities towards the equator. Just like with Bergmann’s Rule, the body shape observations of Allen’s Rule also result in more or less surface area as compared to volume.
Human races have been found to conform to both Bergmann’s and Allen’s Rules.
What About Muscle and Fat?
Both muscle and fat play a role in keeping us warm in the cold.
Muscle keeps us warm by burning energy and also triggering shivering. According to a researcher at Ohio State, ” a protein called sarcolipin helps muscle cells keep the body warm by burning energy, almost like an idling motor car, even if the muscles do not contract.” The muscle twitches that occur when shivering produce heat. Interestingly, a recent study found that muscle mass actually has an effect on how quickly we lose heat from our hands suggesting that “people with more muscle mass are less susceptible to heat loss and
Fat, especially brown fat, keeps us warm by acting as insulation. Research has confirmed that low-fat individuals take longer to warm up after being immersed in cold water. Additionally, as noted above on body shape, fatter people are bigger than their lean counterparts which results in higher body volume as compared with surface area which leads to less heat dissipation.
However, people with greater levels of fat may actually feel cooler at on their skin as “subcutaneous fat traps heat, an obese person’s core will tend to remain warm while his or her skin cools down,” according to Catherine O’Brien, a research physiologist. Dr. O’Brien also helpfully summarized all the factors discussed in this IFOD as follows:
“We have a joke around here that the person who’s best-suited for cold is fit and fat.”
*Here’s a great explanation from the outstanding book Scale by Geoffrey West of why volumes increase faster than surface area:
To begin, consider one of the simplest possible geometrical objects, namely, a floor tile in the shape of a square, and imagine scaling it up to a larger size; see Figure 5. To be specific let’s take the length of its sides to be 1 ft. so that its area, obtained by multiplying the length of two adjacent sides together, is 1 ft. × 1 ft. = 1 sq. ft. Now, suppose we double the length of all of its sides from 1 to 2 ft., then its area increases to 2 ft. × 2 ft. = 4 sq. ft. Similarly, if we were to triple the lengths to 3 ft., then its area would increase to 9 sq. ft., and so on. The generalization is clear: the area increases with the square of the lengths. This relationship remains valid for any two-dimensional geometric shape, and not just for squares, provided that the shape is kept fixed when all of its linear dimensions are increased by the same factor. A simple example is a circle: if its radius is doubled, for instance, then its area increases by a factor of 2 × 2 = 4. A more general example is that of doubling the dimensions of every length in your house while keeping its shape and structural layout the same, in which case the area of all of its surfaces, such as its walls and floors, would increase by a factor of four.
This argument can be straightforwardly extended from areas to volumes. Consider first a simple cube: if the lengths of its sides are increased by a factor of two from, say, 1 ft. to 2 ft., then its volume increases from 1 cubic foot to 2 × 2 × 2 = 8 cubic. Similarly, if the lengths are increased by a factor of three, the volume increases by a factor of 3 × 3 × 3 = 27. As with areas, this can straightforwardly be generalized to any object, regardless of its shape, provided we keep it fixed, to conclude that if we scale it up, its volume increases with the cube of its linear dimensions.
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