Have you ever wondered why moose in Alaska are larger than their southern relatives? Have you considered why many Arctic mammals appear bulkier than species living closer to the equator? There’s a scientific reason for this, also known as Bergmann’s Rule.

Bergmann’s rule explains the size differences among animals in different regions. The climate-driven pattern reveals how evolution shapes body size to conserve heat. Keep reading to learn more about Bergmann’s rule and what it might mean in a warming world. 

What Is Bergmann’s Rule?

Coined by biologist Carl Bergmann in the 1800s, Bergmann’s rule describes a climate-driven pattern among wildlife. The rule explains why animals in colder regions tend to have larger bodies than those in warmer ones.

“Large animals can conserve heat more efficiently than smaller animals in cold climates,” explains Bob Mulvihill, Lead Ornithologist for the National Aviary. “Just think about this—dogs are all the same species, but we can probably agree that a Chihuahua might be miserable outdoors in Alaska, but comfortable in Mexico! The allometric relationship between body size (i.e., volume) and surface area is the reason why.”

This means that smaller-bodied animals are better suited to warmer environments because they are less likely to overheat, Mulvihill adds.

“To use the dog analogy again, a Saint Bernard would be miserable in Mexico, yet comfortable in Alaska!” he explains. “This north-south (in the Northern Hemisphere) trend in body size has been observed both between related species and within species having a wide latitudinal range.”

How Bergmann’s Rule Developed

As stated earlier, German biologist Carl Bergmann first coined Bergmann’s rule in 1847. He observed that endothermic animals living in colder climates tended to be larger than those in warmer climates.

“The biological explanation for the rule is the simple allometric relationship between body size and surface area,” Mulvihill says. “Surface area does not increase 1:1 with the volume of a body. Rather, it increases 0.66:1 with it.” 

In simpler terms, “larger animals have a proportionately smaller surface area compared to smaller animals,” Mulvihill explains. “Because all warm-blooded animals lose the internal heat generated by their metabolism to their environment through their body surface, larger-bodied animals lose heat at a lower rate compared to small animals.”

Are There Exceptions to Bergmann’s Rule?

Now that we understand Bergmann’s rule and its origin, let’s explore some of its exceptions. 

Many animals prove Bergmann’s rule to be true. However, some choose their own “microclimates” within the larger environment, says Mulvihill. 

For example, consider small rodents like deer mice. While some live in cold regions, they do not necessarily grow larger than those in warmer regions. Instead, they create microclimates by burrowing or nesting.

“In addition, seasonal changes in pelage (i.e., a thicker fur coat) and plumage (i.e., a denser feather coat) in animals can mitigate heat loss in cold climates,” Mulvihill explains. “Furthermore, behavioral adjustments like huddling (effectively creating [a] larger size and smaller exposed surface area out of a conglomerate of smaller animals), and physiological adjustments like torpor (a facultative lowering of heart and respiration rate and body temperature) can enable smaller animals to survive in very cold environments.” 

“One bird that many people know, the Black-capped Chickadee, weighs less than a half an ounce but is nevertheless among the most cold-hardy birds!” he adds.

What This Pattern Reveals About Evolution and Climate Change

Bergmann’s rule is considered a climate-driven pattern. Since evolution often responds to selective pressures like temperature, it’s also impacted by climate change.

“Given the rapid change in distribution of temperatures across the planet, it might be hypothesized that animals that evolved under a different prevailing pattern of temperature should be under natural selection for a change in body size,” Mulvihill points out. “As northern areas get progressively warmer, the selective advantages accruing to animals having [a] larger size may be reduced or even reversed.”

He acknowledges that studies have not been consistent in their assessments of Bergmann’s rule as it relates to global warming. However, he highlights the “indisputable” challenges animals face in a rapidly changing environment.

“Nature has a phenomenal track record for accommodating huge environmental changes across vast time periods, always with a long list of losers and winners in the game for survival,” he says. “But ongoing planet-scale changes due solely to the collective actions of our species have been occurring at a very fast rate that may outpace opportunities for evolutionary change and survival of many species, such as polar bears, that we know and love today.”