Bergmann's Rule

Origins and the heat-loss explanation

In a paper on the heat economy of animals in relation to their size, Carl Bergmann (1847) argued that larger bodies have less surface area relative to volume and so lose less heat, making large size advantageous in cold regions. The American zoologist Joel Allen (1877) used the same logic to propose Allen's rule, under which animals in colder regions have shorter appendages such as ears, tails and limbs. Ernst Mayr (1956) redefined Bergmann's rule as an intraspecific trend in body size across regions and regarded it as evidence of climatic adaptation through natural selection.

Alternative explanations and debates over validity

Scholander (1955) criticized the heat-conservation explanation, arguing that other adaptations such as insulation by fur and feathers and vascular control reduce heat loss far more effectively than body size. James (1970) showed that body size in North American birds is related to humidity as well as temperature, and McNab (1971) argued that size differences can also be explained by ecological factors such as prey size and competition. Geist (1987) went so far as to call the rule invalid, noting that in some mammals body size peaks at intermediate latitudes rather than increasing steadily. Other proposed explanations include resistance to starvation, seasonal productivity and phylogenetic relationships.

Meta-analyses and climate change research

In a meta-analysis of mammal data, Ashton, Tracy and de Queiroz (2000) reported that most species show trends consistent with Bergmann's rule, and Meiri and Dayan (2003) concluded that the rule generally holds in birds and mammals. Blackburn, Gaston and Loder (1999) and Watt, Mitchell and Salewski (2010) pointed out that confusion arises because the rule is used with different meanings, depending on whether it refers to species or populations and whether the cause is assumed to be thermal. A major recent question is whether climate warming is shrinking animal body sizes. Gardner et al. (2011) examined whether declining body size can be seen as a third universal response to warming, and Ryding et al. (2021) reported shape-shifting in which appendages such as bills and ears lengthen in some animals. The topic is read alongside research on body size and physiology such as Kleiber's law.

FAQ

What is Bergmann's rule in simple terms?

Among closely related birds and mammals, those living in colder regions tend to be larger. The classic explanation is that a larger body has less surface relative to its volume and so loses less heat.

Does Bergmann's rule always hold?

No. The trend is found in many birds and mammals, but there are many exceptions, and researchers still debate whether the cause is heat conservation or other factors such as food and seasonality.

Sources reviewed: 2026-10-26

References

1. Bergmann, C. (1847). Über die Verhältnisse der Wärmeökonomie der Thiere zu ihrer Grösse. Göttinger Studien, 3(1), 595–708.
2. Allen, J. A. (1877). The influence of physical conditions in the genesis of species. Radical Review, 1, 108–140.
3. Mayr, E. (1956). Geographical character gradients and climatic adaptation. Evolution, 10(1), 105–108.
4. Scholander, P. F. (1955). Evolution of climatic adaptation in homeotherms. Evolution, 9(1), 15–26.
5. James, F. C. (1970). Geographic size variation in birds and its relationship to climate. Ecology, 51(3), 365–390.
6. McNab, B. K. (1971). On the ecological significance of Bergmann's rule. Ecology, 52(5), 845–854.
7. Geist, V. (1987). Bergmann's rule is invalid. Canadian Journal of Zoology, 65(4), 1035–1038.
8. Ashton, K. G., Tracy, M. C., & de Queiroz, A. (2000). Is Bergmann's rule valid for mammals? American Naturalist, 156(4), 390–415.
9. Meiri, S., & Dayan, T. (2003). On the validity of Bergmann's rule. Journal of Biogeography, 30(3), 331–351.
10. Blackburn, T. M., Gaston, K. J., & Loder, N. (1999). Geographic gradients in body size: A clarification of Bergmann's rule. Diversity and Distributions, 5(4), 165–174.
11. Watt, C., Mitchell, S., & Salewski, V. (2010). Bergmann's rule: A concept cluster? Oikos, 119(1), 89–100.
12. Gardner, J. L., Peters, A., Kearney, M. R., Joseph, L., & Heinsohn, R. (2011). Declining body size: A third universal response to warming? Trends in Ecology & Evolution, 26(6), 285–291.
13. Ryding, S., Klaassen, M., Tattersall, G. J., Gardner, J. L., & Symonds, M. R. E. (2021). Shape-shifting: Changing animal morphologies as a response to climatic warming. Trends in Ecology & Evolution, 36(11), 1036–1048.