Allen's rule
Encyclopedia
Allen's rule is a biological
rule posited by Joel Asaph Allen
in 1877. It states that endotherm
s from colder climates usually have shorter limbs (or appendages) than the equivalent animals from warmer climates.
s, which will aid or impede their temperature regulation.
Consider eight cubical boxes of unit volume and unit area of a side. A rectilateral stack two boxes wide, one long and four tall will have a volume of 8 units and a surface area of 28 units. A cubical stack two boxes wide, two long and two high will have the same volume of 8 units but a surface area of only 24 units.
In cold climates, the greater the exposed surface area, the greater the loss of heat and therefore energy. Animals in cold climates need to conserve as much energy as possible. A low surface area to volume ratio
helps to conserve heat as there is a smaller surface area for the heat to pass through.
In warm climates, the opposite is true. An animal will overheat quickly if it has a low surface area to volume ratio. Therefore, animals in warm climates will have high surface area to volume ratios so as to help them lose heat.
R.L. Nudds and S.A. Oswald (2007) of the Institute of Integrative and Comparative Biology at the University of Leeds claimed that there is poor empirical support for Allen's rule despite being an "established ecological tenet". They claim the support for Allen's rule mainly draws from single-species studies, since multiple species studies are "confounded" by the scaling effects of "Bergmann's rule" and alternative adaptions that go against the predictions of Allen's rule.
According to J.S. Alho et al. of the Ecological Research Unit of the University of Helsinki, Finland, although Allen's rule was originally formulated for endotherms, it can be applied to ectotherms which derive body temperature from the environment. The reason for this is that ectotherms with less surface to volume would heat up slower and cool down slower which they claim might be adaptive in "thermally heterogeneous environments".
According to J.S. Alho et al. of the Ecological Research Unit of the University of Helsinki, Finland, there has been a renewed interest in Allen's rule due to global warming and the "microevolutionary changes" it predicts.
is partly dependent on temperature. "Researchers at Pennsylvania State University
have shown that temperature can directly affect cartilage growth, providing a biological explanation for this rule. It was a simple experiment. They raised mice
either at 7 degrees, 21 degrees or 27 degrees Celsius and then measured their tails and ears. They found that they were significantly shorter in the mice raised in the cold, compared with the mice raised at warmer temperatures, even though their overall bodyweights were the same. They found that the mice raised in the cold had less blood flow in their extremities. And when they tried growing bone samples at different temperatures, the researchers found that the samples grown in warmer temperatures had significantly more cartilage growth than those grown in colder temperatures."
" face. Steegman did an experiment that involved the survival of rats in the cold. Steegman found the rats with narrow nasal passages, broader faces, shorter tails and shorter legs survived the best in the cold. Steegman paralleled his findings with the "Arctic Mongoloids", particularly the "Eskimo" and "Aleut," by claiming these "Arctic Mongoloids" have similar features in accordance with Allen's rule: a narrow nasal passage, relatively large heads, long to round heads, large jaws, relatively large bodies, and short limbs. Kenneth L. Beals of the Department of Anthropology at Oregan State University claimed that human head length should theoretically follow "Allen's rule" with people indigenous to colder climates having heads more like a "sphere". In a study that measured the cephalic index of multiple human populations around the globe, Beals confirmed his hypothesis and concluded that the "value of the cephalic index is positively correlated with the intensity of cold stress and is negatively correlated to the intensity of heat stress." Beals notes that the indigenous people of the Americas are an exception to this rule, since the indigenous people of the hot climates of North and South America have cold-adapted, high cephalic indexes. Beals explanation is that these peoples have not yet evolved the appropriate cephalic index for their climate, being, comparatively, only recently descended from the cold-adapted "Arctic Mongoloid".
, Ursus maritimus; this species exhibits stocky limbs and very short ears. The limbs are predictable from a ratio of surface area to body mass, while the outer ear architecture reflects the need to minimize surface area of heat radiating elements. Populations of the same species from different latitudes may also follow Allen's rule. R.L. Nudds and S.A. Oswald (2007) of the Institute of Integrative and Comparative Biology at the University of Leeds performed a study on seabird exposed length that found exposed leg length was negatively correlated with maximum environmental temperature, supporting the predictions of Allen's rule. According to a study done by J.S. Alho et al. of the Ecological Research Unit of the University of Helsinki, Finland, on the Common Frog
, tibia and femur length peaked in populations indigenous to the mid latitudes, consistent with the predictions of Allen's rule for ectothermic organisms.
Biology
Biology is a natural science concerned with the study of life and living organisms, including their structure, function, growth, origin, evolution, distribution, and taxonomy. Biology is a vast subject containing many subdivisions, topics, and disciplines...
rule posited by Joel Asaph Allen
Joel Asaph Allen
Joel Asaph Allen was an American zoologist and ornithologist, born in Springfield, Massachusetts.He studied at Harvard University under Louis Agassiz...
in 1877. It states that endotherm
Warm-blooded
The term warm-blooded is a colloquial term to describe animal species which have a relatively higher blood temperature, and maintain thermal homeostasis primarily through internal metabolic processes...
s from colder climates usually have shorter limbs (or appendages) than the equivalent animals from warmer climates.
Theory
The theory behind Allen's rule is that endothermic animals with the same volume may have differing surface areaSurface area
Surface area is the measure of how much exposed area a solid object has, expressed in square units. Mathematical description of the surface area is considerably more involved than the definition of arc length of a curve. For polyhedra the surface area is the sum of the areas of its faces...
s, which will aid or impede their temperature regulation.
Consider eight cubical boxes of unit volume and unit area of a side. A rectilateral stack two boxes wide, one long and four tall will have a volume of 8 units and a surface area of 28 units. A cubical stack two boxes wide, two long and two high will have the same volume of 8 units but a surface area of only 24 units.
In cold climates, the greater the exposed surface area, the greater the loss of heat and therefore energy. Animals in cold climates need to conserve as much energy as possible. A low surface area to volume ratio
Surface area to volume ratio
The surface-area-to-volume ratio also called the surface-to-volume ratio and variously denoted sa/vol or SA:V, is the amount of surface area per unit volume of an object or collection of objects. The surface-area-to-volume ratio is measured in units of inverse distance. A cube with sides of...
helps to conserve heat as there is a smaller surface area for the heat to pass through.
In warm climates, the opposite is true. An animal will overheat quickly if it has a low surface area to volume ratio. Therefore, animals in warm climates will have high surface area to volume ratios so as to help them lose heat.
R.L. Nudds and S.A. Oswald (2007) of the Institute of Integrative and Comparative Biology at the University of Leeds claimed that there is poor empirical support for Allen's rule despite being an "established ecological tenet". They claim the support for Allen's rule mainly draws from single-species studies, since multiple species studies are "confounded" by the scaling effects of "Bergmann's rule" and alternative adaptions that go against the predictions of Allen's rule.
According to J.S. Alho et al. of the Ecological Research Unit of the University of Helsinki, Finland, although Allen's rule was originally formulated for endotherms, it can be applied to ectotherms which derive body temperature from the environment. The reason for this is that ectotherms with less surface to volume would heat up slower and cool down slower which they claim might be adaptive in "thermally heterogeneous environments".
According to J.S. Alho et al. of the Ecological Research Unit of the University of Helsinki, Finland, there has been a renewed interest in Allen's rule due to global warming and the "microevolutionary changes" it predicts.
Mechanism
A contributing factor to Allen's Rule may be that the growth of cartilageCartilage
Cartilage is a flexible connective tissue found in many areas in the bodies of humans and other animals, including the joints between bones, the rib cage, the ear, the nose, the elbow, the knee, the ankle, the bronchial tubes and the intervertebral discs...
is partly dependent on temperature. "Researchers at Pennsylvania State University
Pennsylvania State University
The Pennsylvania State University, commonly referred to as Penn State or PSU, is a public research university with campuses and facilities throughout the state of Pennsylvania, United States. Founded in 1855, the university has a threefold mission of teaching, research, and public service...
have shown that temperature can directly affect cartilage growth, providing a biological explanation for this rule. It was a simple experiment. They raised mice
MICE
-Fiction:*Mice , alien species in The Hitchhiker's Guide to the Galaxy*The Mice -Acronyms:* "Meetings, Incentives, Conferencing, Exhibitions", facilities terminology for events...
either at 7 degrees, 21 degrees or 27 degrees Celsius and then measured their tails and ears. They found that they were significantly shorter in the mice raised in the cold, compared with the mice raised at warmer temperatures, even though their overall bodyweights were the same. They found that the mice raised in the cold had less blood flow in their extremities. And when they tried growing bone samples at different temperatures, the researchers found that the samples grown in warmer temperatures had significantly more cartilage growth than those grown in colder temperatures."
In humans
According to William R. Leonard of the Department of Anthropology at the University of Florida, human populations follow Allen's rule. As evidence, Leonard cites a study by D.F. Roberts of the Anthropology Laboratory at Oxford University that showed human populations follow Allen's rule. A.T. Steegman of the Department of Anthropology at State University of New York investigated the assumption that Allen's rule caused the structural configuration of the "Arctic MongoloidMongoloid race
Mongoloid is a term sometimes used by forensic anthropologists and physical anthropologists to refer to populations that share certain phenotypic traits such as epicanthic fold and shovel-shaped incisors and other physical traits common in East Asia, the Americas and the Arctic...
" face. Steegman did an experiment that involved the survival of rats in the cold. Steegman found the rats with narrow nasal passages, broader faces, shorter tails and shorter legs survived the best in the cold. Steegman paralleled his findings with the "Arctic Mongoloids", particularly the "Eskimo" and "Aleut," by claiming these "Arctic Mongoloids" have similar features in accordance with Allen's rule: a narrow nasal passage, relatively large heads, long to round heads, large jaws, relatively large bodies, and short limbs. Kenneth L. Beals of the Department of Anthropology at Oregan State University claimed that human head length should theoretically follow "Allen's rule" with people indigenous to colder climates having heads more like a "sphere". In a study that measured the cephalic index of multiple human populations around the globe, Beals confirmed his hypothesis and concluded that the "value of the cephalic index is positively correlated with the intensity of cold stress and is negatively correlated to the intensity of heat stress." Beals notes that the indigenous people of the Americas are an exception to this rule, since the indigenous people of the hot climates of North and South America have cold-adapted, high cephalic indexes. Beals explanation is that these peoples have not yet evolved the appropriate cephalic index for their climate, being, comparatively, only recently descended from the cold-adapted "Arctic Mongoloid".
Examples in the animal kingdom
An example of this rule among mammals is the Polar bearPolar Bear
The polar bear is a bear native largely within the Arctic Circle encompassing the Arctic Ocean, its surrounding seas and surrounding land masses. It is the world's largest land carnivore and also the largest bear, together with the omnivorous Kodiak Bear, which is approximately the same size...
, Ursus maritimus; this species exhibits stocky limbs and very short ears. The limbs are predictable from a ratio of surface area to body mass, while the outer ear architecture reflects the need to minimize surface area of heat radiating elements. Populations of the same species from different latitudes may also follow Allen's rule. R.L. Nudds and S.A. Oswald (2007) of the Institute of Integrative and Comparative Biology at the University of Leeds performed a study on seabird exposed length that found exposed leg length was negatively correlated with maximum environmental temperature, supporting the predictions of Allen's rule. According to a study done by J.S. Alho et al. of the Ecological Research Unit of the University of Helsinki, Finland, on the Common Frog
Common Frog
The Common Frog, Rana temporaria also known as the European Common Frog or European Common Brown Frog is found throughout much of Europe as far north as well north of the Arctic Circle in Scandinavia and as far east as the Urals, except for most of Iberia, southern Italy, and the southern Balkans...
, tibia and femur length peaked in populations indigenous to the mid latitudes, consistent with the predictions of Allen's rule for ectothermic organisms.
Other Factors
Exposure of exterior surface area is only one of many mechanisms by which animals retain or dispose of body heat. The following mechanisms are also important:- SweatingSweatingPerspiration is the production of a fluid consisting primarily of water as well as various dissolved solids , that is excreted by the sweat glands in the skin of mammals...
- ShiveringShiveringShivering is a bodily function in response to early hypothermia in warm-blooded animals. When the core body temperature drops, the shivering reflex is triggered to maintain homeostasis. Muscle groups around the vital organs begin to shake in small movements in an attempt to create warmth by...
- skin colorHuman skin colorHuman skin color is primarily due to the presence of melanin in the skin. Skin color ranges from almost black to white with a pinkish tinge due to blood vessels underneath. Variation in natural skin color is mainly due to genetics, although the evolutionary causes are not completely certain...
- BehaviorBehaviorBehavior or behaviour refers to the actions and mannerisms made by organisms, systems, or artificial entities in conjunction with its environment, which includes the other systems or organisms around as well as the physical environment...
(e.g., gathering insulation when it is cold, moving into shade when it is hot)
See also
- Bergmann's RuleBergmann's RuleBergmann's rule is an ecogeographic principle that states that within a broadly distributed genus, 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...
- that correlates latitude with body mass in animals. - Gloger's ruleGloger's ruleGloger's Rule is a zoological rule which states that within a species of endotherms, more heavily pigmented forms tend to be found in more humid environments, e.g. near the equator. It was named after the zoologist Constantin Wilhelm Lambert Gloger, who first remarked upon this phenomenon in 1833...
- that correlates humidity with pigmentation in animals. - Hesse's Rule