Bergmann's rule is one of the best-known generalizations in zoology. It is generally defined as a within-species tendency in homeothermic (warm-blooded) animals to have increasing body size with increasing latitude and decreasing ambient temperature. That is, Bergmann's rule states that among mammals and birds, individuals of a particular species in colder areas tend to have greater body mass than individuals in warmer areas. For instance, white-tailed deer are larger in Canada than in the Florida Keys, and the body size of wood rat populations are inversely correlated with ambient temperature. This principle is named after a nineteenth-century German biologist, Karl Bergmann, who published observations along these lines in 1847.
This well-known ecogeographic pattern is sometimes considered a trend for animal species in general, not just homeothermic animals. For example, recent studies have indicated that turtles and salamanders also tend to follow Bergmann's rule, with exceptions concentrated within lizards and snakes (Queiroz and Ashton 2004). Historically, Bergmann's rule has also referred to a pattern of geographic distributions beyond the level of a species, such as stating that larger mammal and bird species tend to be in the more northern latitudes and smaller species farther south.
The generality of Bergmann's rule remains a question, as many exceptions to the rule are known. This reflects both the uncertainty in science and the diversity of nature.
The general explanation for the phenomenon is attributed to the importance of the surface area to volume ratio, with warm-blooded animals in colder climates having an advantage in the reduction of the surface area to volume ratio, in order to reduce heat loss. For example, polar bears have large, compact bodies, relative to bears in warmer climates, thus offering a smaller relative surface area (relative to volume) for losing heat.
Bergmann's rule has also been applied to populations of humans. Pygmies are found only in the tropics, and other tropical races tend to be shorter than those in temperate regions. Also, in Europe, Southern Europeans, such as Italians, tend to be shorter on average than Northern Europeans, such as Swedes. The same goes for Asians, as northern Asians are on average larger than their Southeast Asian counterparts. The Eskimos of Alaska and northern Canada are known for their accumulation of fat as acclimatization to severe cold. However, there are exceptions as well.
A corollary of Bergmann's rule is Allen's rule, which indicates a trend in terms of appendages of warm-blooded animals according to latitude. Allen's rule holds that individuals in populations of the same species have a tendency to have shorter limbs than the corresponding animals in warmer climates.
Karl Bergmann was a biologist, who in 1847 wrote that generally "larger species live farther north and the smaller ones farther south," and that "if we could find two species of animals which would only differ with respect to size... the geographic distribution of the two species would have to be determined by their size.... If there are genera in which the species differ only in size, the smaller species would demand a warmer climate" (Blackburn and Hawkins 2004). This led to the rule that individuals in populations of the same species in warm climates tended to be less massive (smaller and lighter) than individuals found farther from the equator in colder regions.
The most frequently cited reason for Bergmann's rule attributes this phenomenon to possible climatic adaptations relating to the surface/volume ratio. Larger animals have a lower surface area to volume ratio than smaller animals, so they radiate less body heat, and stay warmer in cold climates. Conversely, smaller animals in warmer climates have the opposite problem in that body heat generated by metabolism needs to be dissipated, and a higher surface area to volume ratio facilitates heat loss through the skin.
The surface to volume ratio is a central concept in biology and evolution. As described in the 1630's by Galileo, if an object grows larger, but its shape remains the same, it will decrease in surface area relative to volume. This is because surface area increases by the square of its length, while volume increases by the cube of length. Thus, volume grows much more rapidly than surface area. Functions that depend on surfaces—such as heat dissipation, oxygen respiration, flight, and so forth—must serve the entire volume. Small species can get by without many things that larger organisms require. Thus, insects and long, but thin, tapeworms do not need lungs to increase surface area for respiration. For larger animals, systems are needed to bring food and oxygen from the surface to the interior, and to remove wastes and dissipate heat.
For the same reason, the science fiction techniques of greatly increasing or decreasing the size of animals or humans generally fail to capture the reality embodied in the concept of the surface area to volume ratio (Gould 1977). Insects can walk up walls and on surfaces of ponds because the small gravitational force exerted by their small mass can be countered by the stronger surface adhesion—something the Spiderman of the movies could never experience at his mass. A flying insect increased to mammoth size, but retaining the same shape, could not fly because its weight would increase much more rapidly than the surface areas of its wings, and its legs would collapse under the weight if not much broader.
Some researchers have cast doubt on the view that thermoregulatory capacity (conservation of metabolic heat) is the main factor in explaining Bergmann's Rule. Contributing to this view is the finding of Bergmann's rule in pokilotherms, organisms with varying internal temperatures, and ones often reflecting ambient temperatures, and ectotherms, those organisms using external, not internal, means to control temperature. Among other explanations proposed for Bergmann's rule is that the effect correlates with resource availability, since fat reserves increase more rapidly with body size than does metabolic rate, or with the change in size of prey species.
Joel Asaph Allen observed in 1877 that the length of appendages (arms, legs, etc.) in warm-blooded animals also corresponds to latitude and environmental temperature. Individuals in populations of the same species located in warm climates near the equator tend to have longer limbs than individuals in populations located in colder climates further from the equator. For example, the Inuit people, which live and hunt in northern climates, tend to have a more stocky body with shorter appendages than the Masai people of Kenya and North Tanzania, which have a taller, slender body shape, with long limbs.
This rule, called Allen's rule, is considered a corollary of Bergmann's rule, and is attributed to the same factor of heat conservation. That is, longer appendages offer more surface area, and thus greater opportunity to dissipate heat, while shorter appendages offer less surface area and are more effective in maintaining body heat.
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