Friday Field Notes: On Bergmann's Rule, Ratios, and the Art of Staying Warm in Northern Climates

When MNHC's distinguished naturalists enter elementary school classrooms throughout western Montana this February for their monthly visit/natural history lesson, they will be discussing a fairly important ecological principle with 4th graders: Bergmann's rule. First articulated by German biologist Christian Bergmann (hence the name), the principle states that within a broadly distributed species or taxonomic group, an organism's body mass tends to increase with an increase in latitude (and the corresponding colder climate). While this principle has been most widely applied to mammals and birds, there are also examples of cold-blooded species that conform to the rule.

Why is such a principle observed in nature, you ask? Fundamentally, it is a function of larger animals exhibiting a decreased surface-area-to-volume ratio. Thus, larger-bodied animals lose less heat per unit of body mass, a characteristic that becomes vitally important in places like Montana where winters are long and cold. The following graphic helps better display this relationship:

Graphic displaying the mathematics underlying Bergmann's rule.

The important thing to note is the red text: While the larger cube has a considerably higher surface area, its volume has increased proportionately even more, thus lowering the surface-area-to-volume ratio.

Now, I know what you're thinking: Surface-area-to-volume ratios are a bit much for 4th graders. Most of us hadn't even heard of Bergmann's rule until our first college-level ecology course. But the science and mathematics underlying this stalwart of ecology can be easily explained and visualized. Consider this simple experiment, best suited for a brisk (32 degrees F or less) Montana winter day:

• Simultaneously fill up two cans of different sizes (i.e., a coffee can and a soup can) with near-boiling water. Place a thermometer in each can, and record the initial temperature.
• Place both cans outside. Record the temperature of each can every minute for approximately ten minutes.
• Afterwards, compare the change in temperature of the cans. The larger can should be significantly warmer than the smaller one, a result of its lower surface-area-to-volume ratio.

This simple experiment is a great way to test and conceptualize Bergmann's rule, and sheds light onto why being bigger is better when you live in northern latitudes. This fundamental relationship between surface area and volume explains why animals such as deer, elk, moose, and bears get larger as you move further north within their range. In some cases, the size difference can be quite dramatic. Male grizzly bears, for example, whose average weight is in the range of 500-1000 pounds in the Interior West, can reach up to 1,500 pounds in Alaska! In white-tailed deer, an incredibly far-ranging species, a similarly dramatic change is observed:

Bergmann's rule exhibited in white-tailed deer

So, if you're feeling guilty about those extra pounds you're still carrying around from holiday feasts or winter vacations, look on the bright side: You're going to stay much warmer this winter than you would without them!