Bergmann's rule is an ecogeographical rule that states that within a broadly distributed taxonomic clade, populations and species of larger size are found in colder environments, and species of smaller size are found in warmer regions. Although originally formulated in terms of species within a genus, it has often been recast in terms of populations within a species. It is also often cast in terms of latitude.
The rule is named after nineteenth century German biologist Carl Bergmann, who described the pattern in 1847, although he was not the first to notice it. Bergmann's rule is most often applied to mammals and birds which are endotherms, but some researchers have also found evidence for the rule in studies of ectothermic species such as the ant Leptothorax acervorum. While Bergmann's rule appears to hold true for many mammals and birds, there are exceptions.
There seems to be a tendency for larger-bodied animals to conform more closely than smaller-bodied animals, at least up to certain latitudes, perhaps reflecting a reduced ability to avoid stressful environments by burrowing or other means. In addition to being a general pattern across space, Bergmann’s rule has been reported in populations over historical and evolutionary time when exposed to varying thermal regimes. In particular, reversible dwarfing of mammals has been noted during two relatively brief upward excursions in temperature during the Paleogene: the Paleocene-Eocene thermal maximum and the Eocene Thermal Maximum 2.
Human populations who live near the poles, including the Inuit, Aleut, and Sami people, are on average heavier than populations from mid-latitudes, consistent with Bergmann's rule. They also tend to have shorter limbs and broader trunks, consistent with Allen's rule.
According to Marshall T. Newman in a 1953 article for the Journal of the American Anthropologist, Native American populations are generally consistent with Bergmann's rule although the cold climate and small body size combination of the Eastern Eskimo, Canoe Indians, Yuki people, Andes natives and Harrison Lake Lillouet runs contrary to the expectations of Bergmann's rule. Newman contends that Bergmann's rule holds for the populations of Eurasia, but it does not hold for those of sub-Saharan Africa.
The earliest explanation, given by Bergmann when originally formulating the rule, is that larger animals have a lower surface area to volume ratio than smaller animals, so they radiate less body heat per unit of mass, and therefore stay warmer in cold climates. Warmer climates impose the opposite problem: body heat generated by metabolism needs to be dissipated quickly rather than stored within.
Thus, the higher surface area-to-volume ratio of smaller animals in hot and dry climates facilitates heat loss through the skin and helps cool the body. It is important to note that when analyzing Bergmann's Rule in the field that groups of populations being studied are of different thermal environments, and also have been separated long enough to genetically differentiate in response to these thermal conditions.
In marine crustaceans, it has been proposed that an increase in size with latitude is observed because decreasing temperature results in increased cell size and increased life span, both of which lead to an increase in maximum body size (continued growth throughout life is characteristic of crustaceans). The size trend has been observed in hyperiid and gammarid amphipods, copepods, stomatopods, mysids, and planktonic euphausiids, both in comparisons of related species as well as within widely distributed species. Deep-sea gigantism is observed in some of the same groups, probably for the same reasons.
In 1937 German zoologist and ecologist Richard Hesse proposed an extension of Bergmann's rule. Hesse's rule, also known as the heart–weight rule, states that species inhabiting colder climates have a larger heart in relation to body weight than closely related species inhabiting warmer climates.
According to a study by Valerius Geist in 1986, Bergmann's rule is false: the correlation with temperature is spurious; instead, Geist found that body size is proportional to the duration of the annual productivity pulse, or food availability per animal during the growing season.
Because many factors can affect body size, there are many critics of Bergmann’s Rule. Some believe that latitude itself is a poor predictor of body mass. Examples of other selective factors that may contribute to body mass changes are the size of food items available, effects of body size on success as a predator, effects of body size on vulnerability to predation, and resource availability. For example, if an organism is adapted to tolerate cold temperatures, it may also tolerate periods of food shortage, due to correlation between cold temperature and food scarcity. A larger organism can rely on its greater fat stores to provide the energy needed for survival as well being able to procreate for longer periods.
Resource availability is a major constraint on the overall success of many organisms. Resource scarcity can limit the total number of organisms in a habitat, and over time can also cause organisms to adapt by becoming smaller in body size. Resource availability thus becomes a modifying restraint on Bergmann’s Rule.
- Allen's rule – individuals have shorter appendages in colder climates
- Animal migration
- Gene flow
- Gigantothermy – large ectothermic animals more easily maintain constant body temperature
- Insular dwarfism – small populations promote reduction in size of individuals
- Island gigantism
- Sand, Håkan K.; Cederlund, Göran R.; Danell, Kjell (June 1995). "Geographical and latitudinal variation in growth patterns and adult body size of Swedish moose (Alces alces)". Oecologia. 102 (4): 433–442. doi:10.1007/BF00341355. (subscription required (. ))
- Olalla-Tárraga, Miguel Á.; Rodríguez, Miguel Á.; Hawkins, Bradford A. (2006). "Broad-scale patterns of body size in squamate reptiles of Europe and North America". Journal of Biogeography. 33 (5): 781–793. doi:10.1111/j.1365-2699.2006.01435.x.
- Timofeev, S. F. (2001). "Bergmann's Principle and Deep-Water Gigantism in Marine Crustaceans". Biology Bulletin (Russian version, Izvestiya Akademii Nauk, Seriya Biologicheskaya). 28 (6): 646–650 (Russian version, 764–768). doi:10.1023/A:1012336823275. Retrieved 2012-02-08. (subscription required (. ))
- Meiri, S.; Dayan, T. (2003-03-20). "On the validity of Bergmann's rule". Journal of Biogeography. 30 (3): 331–351. doi:10.1046/j.1365-2699.2003.00837.x. (subscription required (. ))
- Ashton, Kyle G.; Tracy, Mark C.; Queiroz, Alan de (October 2000). "Is Bergmann's Rule Valid for Mammals?". The American Naturalist. 156 (4): 390–415. doi:10.1086/303400. JSTOR 10.1086/303400.
- Millien, Virginie; Lyons, S. Kathleen; Olson, Link; et al. (May 23, 2006). "Ecotypic variation in the context of global climate change: Revisiting the rules". Ecology Letters. 9 (7): 853–869. doi:10.1111/j.1461-0248.2006.00928.x.
- Freckleton, Robert P.; Harvey, Paul H.; Pagel, Mark (2003). "Bergmann's rule and body size in mammals". The American Naturalist. 161 (5): 821–825. doi:10.1086/374346. JSTOR 10.1086/374346. PMID 12858287.
- Smith, Felia A.; Betancourt, Julio L.; Brown, James H. (December 22, 1995). "Evolution of Body Size in the Woodrat over the Past 25,000 Years of Climate Change". Science. 270 (5244): 2012–2014. Bibcode:1995Sci...270.2012S. doi:10.1126/science.270.5244.2012. (subscription required (. ))
- Huey, Raymond B.; Gilchrist, George W.; Carlson, Margen L.; Berrigan, David; Serra, Luı́s (January 14, 2000). "Rapid Evolution of a Geographic Cline in Size in an Introduced Fly". Science. 287 (5451): 308–309. Bibcode:2000Sci...287..308H. doi:10.1126/science.287.5451.308. PMID 10634786. (subscription required (. ))
- Hunt, Gene; Roy, Kaustuv (January 31, 2006). "Climate change, body size evolution, and Cope's rule in deep-sea ostracodes" (PDF). Proceedings of the National Academy of Sciences of the United States of America. 103 (5): 1347–1352. Bibcode:2006PNAS..103.1347H. doi:10.1073/pnas.0510550103. PMC . PMID 16432187.
- Secord, Ross; Bloch, Jonathan I.; Chester, Stephen G. B.; et al. (February 24, 2012). "Evolution of the Earliest Horses Driven by Climate Change in the Paleocene-Eocene Thermal Maximum". Science. 335 (6071): 959–962. Bibcode:2012Sci...335..959S. doi:10.1126/science.1213859. PMID 22363006. (subscription required (. ))
- Erickson, Jim (November 1, 2013). "Global warming led to dwarfism in mammals — twice". University of Michigan. Retrieved 2013-11-12.
- Holliday, Trenton W.; Hilton, Charles E. (June 2010). "Body proportions of circumpolar peoples as evidenced from skeletal data: Ipiutak and Tigara (Point Hope) versus Kodiak Island Inuit". American Journal of Physical Anthropology. 142: 287–302. doi:10.1002/ajpa.21226. (subscription required (. ))
- Newman, Marshall T. (August 1953). "The Application of Ecological Rules to the Racial Anthropology of the Aboriginal New World". American Anthropologist. 55 (3): 311–327. doi:10.1525/aa.1953.55.3.02a00020. (subscription required (. ))
- Brown, James H.; Lee, Anthony K. (January 1969). "Bergmann's Rule and Climatic Adaptation in Woodrats (Neotoma)". Evolution. 23 (2): 329–338. doi:10.2307/2406795. JSTOR 2406795.
- Baum, Steven (January 20, 1997). "Hesse's rule". Glossary of Oceanography and the Related Geosciences with References. Texas Center for Climate Studies, Texas A&M University. Retrieved 2011-01-09.
- Geist, Valerius (April 1987). "Bergmann's rule is invalid". Canadian Journal of Zoology. 65 (4): 1035–1038. doi:10.1139/z87-164. (subscription required (. ))
- Clauss, Marcus; Dittmann, Marei T.; Müller, Dennis W. H.; et al. (October 2013). "Bergmann′s rule in mammals: A cross-species interspecific pattern". Oikos. 122 (10): 1465–1472. doi:10.1111/j.1600-0706.2013.00463.x. (subscription required (. ))
- Bergmann, Carl (1847). "Über die Verhältnisse der Wärmeökonomie der Thiere zu ihrer Grösse". Göttinger Studien. 3 (1): 595–708.
- Roberts DF (1953). "Body weight, race and climate". American Journal of Physical Anthropology. 11 (4): 533–558. doi:10.1002/ajpa.1330110404. PMID 13124471.
- Roberts DF (1978). Climate and Human Variability (2nd ed.). Menlo Park, CA: Cummings.
- Ruff CB (1994). "Morphological adaptation to climate in modern and fossil hominids". Yearbook of Physical Anthropology. 37: 65–107. doi:10.1002/ajpa.1330370605.
- Schreider E (1950). "Geographical distribution of the body-weight/body-surface ratio". Nature. 165 (4190): 286. Bibcode:1950Natur.165..286S. doi:10.1038/165286b0. PMID 15410342.