Bergmann’s rule is an ecological pattern stating that warm-blooded animals tend to be larger in colder climates and smaller in warmer ones. First described by German biologist Carl Bergmann in the mid-1800s, the rule is grounded in a simple physical relationship: as a body gets bigger, its volume increases faster than its surface area, which means large animals lose heat more slowly. This makes bigger bodies advantageous in cold environments and smaller bodies better suited to hot ones.
The Physics Behind the Rule
Every warm-blooded animal generates heat internally through its metabolism, and that heat production is proportional to the animal’s volume. Heat loss, on the other hand, happens through the skin, which means it depends on surface area. Here’s the key insight: when an animal grows larger, its volume (and therefore heat production) increases much faster than its surface area (and therefore heat loss). A large animal has a lower surface-area-to-volume ratio, so it retains heat more efficiently.
In cold climates, that’s a survival advantage. A stocky, heavy-bodied animal holds onto metabolic heat better than a small, lean one. In warm climates, the opposite is true. A smaller body with relatively more surface area can shed excess heat more easily, reducing the risk of overheating. Bergmann recognized that these relationships follow basic laws of physics and concluded that there must be a minimum possible size for warm-blooded animals, because below a certain point, the body loses heat faster than it can produce it.
Bears as a Textbook Example
Bears are one of the clearest illustrations of Bergmann’s rule in action. A 2024 study published in Nature Communications confirmed a strong statistical relationship between body mass and latitude across eight bear species. Latitude alone explained 75% of the variation in body mass across the bear family, with a 1-degree increase in latitude corresponding to roughly a 25% relative increase in expected body mass. Polar bears, living in Arctic conditions, are the largest. Sun bears, found in tropical Southeast Asia, are the smallest, typically weighing less than a fifth of what a polar bear weighs.
You can see similar patterns in other groups. White-tailed deer in Canada are noticeably bulkier than those in Florida. Wolves in the Arctic outweigh wolves in Mexico. The trend isn’t universal across every species, but it’s common enough to qualify as a genuine ecological pattern.
How Well the Rule Holds Up
No biological “rule” applies without exceptions, so researchers have tested Bergmann’s rule extensively. A meta-analysis published in The American Naturalist found that about 71% of mammal species (78 out of 110) showed the expected positive correlation between body size and latitude. When the relationship was tested against temperature directly rather than latitude, 75% of species (48 out of 64) followed the predicted pattern of larger bodies in colder environments.
Those numbers are well above what you’d expect by chance, which means the rule captures something real. But the roughly 25% of species that don’t follow it remind us that body size is shaped by many pressures beyond temperature: food availability, predation, competition, and migration patterns all play a role.
Bergmann’s Rule in Humans
The pattern shows up in human populations too, though it’s more nuanced. Indigenous peoples who evolved in cold, high-latitude environments tend to have broader, stockier builds. Inuit populations in Alaska and indigenous peoples of northern Europe, for example, have characteristically wide pelvises and compact frames. Studies of Alaskan Inupiat and Finnish adults confirm that both populations are notably broad-bodied, with the Finnish sample also being very tall. Finnish men have particularly wide shoulders relative to their hips.
Populations from equatorial regions, by contrast, tend to have longer limbs and narrower builds, increasing their surface-area-to-volume ratio. This pattern is visible even in fossil evidence from the late Pleistocene, where skeletal specimens of early humans from higher latitudes show the same broad, heavy-bodied proportions seen in modern cold-climate populations. That said, some researchers have argued that calling this “Bergmann’s rule” oversimplifies the story, since human body shape is also influenced by diet, activity patterns, and genetic drift.
Allen’s Rule: The Companion Pattern
Bergmann’s rule has a well-known companion called Allen’s rule, which focuses not on overall body size but on limb proportions. Allen’s rule states that animals in cold climates tend to have shorter extremities (legs, ears, tails, snouts), while those in warm climates have longer ones. The logic is the same: shorter limbs reduce the surface area available for heat loss, while longer limbs increase it.
Both rules invoke the same underlying principle of surface-area-to-volume ratio, but they describe different ways animals can adjust that ratio. A species might evolve a larger body (Bergmann’s), shorter limbs (Allen’s), or both. Arctic foxes, for instance, are compact and round-bodied with short ears and legs. Desert-dwelling fennec foxes are small with enormous ears that act as radiators. Research on birds has found that species sometimes show trade-offs between the two strategies, relying more heavily on body size changes or limb length changes depending on other constraints on their anatomy.
Cold-Blooded Animals Flip the Script
Bergmann originally described his rule for warm-blooded animals (birds and mammals), and applying it to cold-blooded species like insects, fish, and reptiles is more complicated. Cold-blooded animals don’t generate their own body heat, so the thermoregulation logic doesn’t apply the same way. Still, researchers have found that many ectotherms do grow larger at lower temperatures, though for different reasons.
In a review of 61 studies on aquatic cold-blooded organisms, about 90% grew to a smaller size when reared at higher temperatures. Only six species bucked that trend, including one type of diatom, a parasitic crustacean, and four mayfly species. The mechanisms behind these exceptions likely involve oxygen availability, predation pressure, and life-cycle timing rather than heat retention. For cold-blooded animals, colder water holds more dissolved oxygen, which may support larger body sizes. The pattern looks like Bergmann’s rule on the surface, but the underlying biology is different.
Climate Change and Shrinking Animals
If Bergmann’s rule reflects a real link between temperature and body size, rising global temperatures should push animal sizes downward. That’s exactly what recent research suggests. A large-scale study analyzing over 119,000 bird and 183,000 mammal body mass records found that smaller body sizes are consistently associated with conditions closer to a species’ upper heat tolerance limits. Both birds and mammals showed statistically significant reductions in body mass as temperatures approached the warm end of their tolerable range.
This pattern suggests that animals are either adapting across generations or responding within their lifetimes to warming conditions by getting smaller. The relationship also extends to aridity: species experiencing increasingly dry conditions tend to be smaller as well. If warming and drying trends continue, the research predicts that body sizes across bird and mammal species will likely continue to decline. Some biologists have already documented measurable shrinking in songbird species over just the past few decades, consistent with what Bergmann’s rule would predict in a warming world.