I thought that by being larger an animal would have certain advantages in the cold but honestly it was hard to put my finger on exactly why so I did some research and I thought I would share my findings if anyone is interested. The following might also help explain how a large animal evolved in a warm environment might more easily adapt to a colder enviroment than smaller animals. There are numerous examples of animals growing larger as they moved into colder environments (e.g. certain lemmings, tundra wolves, polar bears, Caribou etc…).

Being larger has certain advantages in surviving the cold. It also has some disadvantages, such as the need to find more food in a typically sparse environment. There is always a balancing act.

In adapting to cold environments, animals typically will try to decrease surface area to mass ratio by becoming fatter and/or reducing extremities like ears, fingers, legs, etc. but size is important too. To understand why, you need to consider the various modes of energy transfer and how they are important to animals. I bolded important items. The modes are as follows

1. Conduction: Energy transfer through molecular collision. There must be contact between media involved in heat transfer (e.g., skin and air, feet and ground) therefore dependant on surface area of exposure, efficiency of heat transfer through conducting media (high density materials have high thermal conductivity; (because layer of fur or feathers is not solid, there is little thermal conductance). The greater the temperature difference, the greater the conductance.

2. Convection: Transfer of energy via a moving fluid, especially air or water; both total area exposed to the moving fluid, and temperature difference between surface of object and fluid are important in determining total heat loss. Also, rate of heat transfer by convection is influenced by the thickness of boundary layer enveloping object, which is influenced by size, the shape of object, surface roughness, and wind speed. Obviously a Bigfoot (or a bear) would have a much thicker insulating layer than a mouse even though the mouse had the same fur thickness relative to its size. This is an important factor to consider. The heat loss in your house will decrease dramatically with added thicknesses of insulation (generally).

3. Radiation: Propagation: of energy through space. Two properties that determine amount of energy radiated: Temperature (independent of ambient temp) and characteristic efficiency of radiant energy transfer, termed emissivity. Most natural objects emit radiant energy at an efficiency of between 90-99% of the maximum possible for their temperature.

4. Latent heat exchange: Amount of heat energy either tied up or liberated with phase changes of water (e.g. ice to water, water to vapor) when reverse process occurs, heat is liberated (e.g., in winter, need to reduce evaporative heat loss and heat loss associated with respiration).

There are several ways an animal can maintain a constant metabolic rate as air temperature drops such as: 1. by decreasing thermal conductivity, 2. decreasing surface area exposed; (e.g., curling up and retracting extremities), 3. huddling with other animals, 4. increasing thickness of insulating layer (e.g., erection of hairs or fluffing of feathers (trapping air)).

The animal can also try and elevate ambient air temperature through nest building or going below ground. Reducing body temperature also has same results (e.g. a tiny humingbird's temperature drops dramatically while asleep to prevent it from burning too many calories). Many small mammals that are solitary during summer become social in winter, constructing communal nests under snow pack.

When air temperatures become low enough, superficial veins in extremities constrict, directing more blood through deeper veins lying close to arteries. This has the effect of lowering the temperature of the skin, thus reducing the heat lost to the environment.

Great post.

You'll likely enjoy this:

http://www.rfthomas.clara.net/papers/gill.html

Note the data associated with Bergmann's Rule and size variance from south to north.

Yes I found it very interesting and informative, but either I am missing something or they stated Bergman's rule incorrectly. Maybe I am being picky, I don't know, but they need to be more careful in their assumptions assuming I am not totally off on this one. According to Bergman's rule: The larger the animal, the better it is at retaining heat (I found this on another site looking up "Bergman's rule" on Yahoo (I have no problem with how it is stated there)). What the bigfoot site said; however, was that a larger animal has less surface area per unit volume. This is not true unless the models in my mind are flawed somehow. I picture two cubes of different sizes. Take a proportionate piece out of each sphere equal to lets say one 27th of the volume. The ratio of the surface area to volume of both would be the same. They have to be because the formulas and shapes are the same. Only changing shape can change the surface area to weight (volume) ratio. This model should be consistent with other objects of equal shape (again, unless I am missing something). I guess I am too lazy right now to actually get a pencil and draw it out and be sure.

By the way, it was very interesting that the foot size averaged larger toward the north.

I have a couple questions...

Given the information provided, would being "big and round" be the most beneficial to conserving heat? Also, I know that most arctic animals are colored white, but do you think being dark-colored would help absorb heat during the winter months? Of course it may not make much of a difference if you're constantly under a forest canopy, or in a region where the sun hardly shines.

Jim, I can't help but tweek you a bit on your math. Of course surface area per unit volume decreases as we increase the size of a cube. Think about it--surface area increases only geometrically while volume increases exponentially. Accordingly, if we consider cubes with sides 1, 2, 3, and 4 units long, the surface area per unit volume is, respectively, 6, 3, 2, and 1.5.

We really needn't construct a sophisticated heat transfer model. The important point to remember is that heat transfer can occur only at the interface of the temperature gradients--the skin for a biological entity. So yes, the big advantage is having less skin per unit volume. Of course, biological entities don't occur in neat geometric shapes. But cold adaptations such as you point out--massive build, short atms and legs, stubby digits--have the net effect of creating a package with less surface area per unit volume.

-Vern

Varq--

Yup, a sphere has less surface area per unit volume than any other shape of the same volume. But don't you think it would be a tad hard to function if you were shaped like a bowling ball and had no arms or legs? We do see the idea reflected in the relatively massive bodies of many cold adapted animals, though.

The white coloration of arctic species (which many have only in winter) doesn't relate to heat conservation but to camouflage. I don't know whether prey or predator species developed it first, but it is part of the old arms race--once one side had it, the other needed to acquire it.

-Vern

Bears are awfully big too. Being big with thick hair, skin and lots of fat can keep you warm. Being big can keep things from attaking you hehe.

Much of the same logic that has been put forward in this thread can be backed up by historical facts in the develepment of homo sapiens.

I know there is a thread somewhere that discussed this before about the physical characteristics of a neaderthal compared to other species of man.

The theory is that because of the northern climates they were more stockily built than their southern cousins. Lends itself somewhat to the sphere idea put forward here.

I'm going to try to find that thread on neaderthals.

WIS

Hehe, very true... it seems as though that shape may only be advantageous to animals that primarily depend on water for their protection, and who don't have to do much travelling through hilly, forested areas.



Ah yes, good old camouflage ... it would be interesting to know if BF hair would change color depending on their habitat, climate, and season. Maybe only if there were arctic BF