Allen's rule

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Two cubic prisms with the same volume and different surface areas

Allen's Rule is a biological principle proposed by Joel Asaph Allen in 1877. It suggests that endothermic (warm-blooded) animals from colder regions typically have shorter limbs or appendages compared to similar animals from warmer regions. This rule is a reflection of how the morphology of organisms can adapt over time to their environment, specifically in relation to thermoregulation.

Background[edit | edit source]

Endothermic animals possess the ability to maintain their body temperature through internal means, primarily metabolic processes. Their surface area and volume play a critical role in this thermoregulatory ability, influencing heat conservation or dissipation.

Theory Behind Allen's Rule[edit | edit source]

The principle supporting Allen's Rule revolves around the relationship between an animal's volume and its surface area, and how this relationship affects temperature regulation.

For instance, consider eight cubical boxes, each having a volume of one unit and an area of one side unit. When these boxes are arranged in a stack that is two boxes wide, one long, and four tall, the entire structure has a volume of eight units and a surface area of 28 units. Conversely, a cube-shaped stack that is two boxes in each dimension (width, length, and height) will still have a volume of eight units, but its surface area is reduced to 24 units.

Cold climates: A larger surface area in relation to volume means more heat loss. Hence, in colder regions, animals need to minimize their exposed surface area to conserve heat and energy. A reduced surface area to volume ratio aids this conservation.
Warm climates: In hotter regions, animals must dissipate heat to prevent overheating. A larger surface area to volume ratio facilitates faster heat loss, preventing overheating.

Other Thermoregulatory Factors[edit | edit source]

While Allen's Rule focuses on the morphological adaptations of animals in relation to temperature regulation, several other mechanisms play significant roles:

Sweating: Helps in cooling the body by evaporative cooling.
Shivering: Generates heat through rapid muscle contraction.
Skin color: Darker pigments can absorb more sunlight and heat, whereas lighter pigments reflect more sunlight, reducing heat absorption.
Behavior: Animals adapt behaviors to cope with temperature changes, such as seeking insulation during cold periods or shade during hot times.

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v t e Biological rules
Rules	Allen's rule Shorter appendages in colder climates Bateson's rule Extra limbs mirror their neighbours Bergmann's rule Larger bodies in colder climates Cope's rule Bodies get larger over time Deep-sea gigantism Larger bodies in deep-sea animals Dollo's law Loss of complex traits is irreversible Eichler's rule Parasites co-vary with their hosts Emery's rule Insect social parasites are often in same genus as their hosts Fahrenholz's rule Host and parasite phylogenies become congruent Foster's rule (Insular gigantism, Insular dwarfism) Small species get larger, large species smaller, after colonizing islands Gause's law Complete competitors cannot coexist Gloger's rule Lighter coloration in colder, drier climates Haldane's rule Hybrid sexes that are absent, rare, or sterile, are heterogamic Harrison's rule Parasites co-vary in size with their hosts Hamilton's rule Genes increase in frequency when relatedness of recipient to actor times benefit to recipient exceeds reproductive cost to actor Kleiber's law An animals metabolic rate decreases with its size Hennig's progression rule In cladistics, the most primitive species are found in earliest, central, part of group's area Jarman–Bell principle The correlation between the size of an animal and its diet quality; larger animals can consume lower quality diet Jordan's rule Inverse relationship between water temperature and no. of fin rays, vertebrae Lack's principle Birds lay only as many eggs as they can provide food for Rapoport's rule Latitudinal range increases with latitude Rensch's rule Sexual size dimorphism increases with size when males are larger, decreases with size when females are larger Rosa's rule Groups evolve from character variation in primitive species to a fixed character state in advanced ones Schmalhausen's law A population at limit of tolerance in one aspect is vulnerable to small differences in any other aspect Thorson's rule No. of eggs of benthic marine invertebrates decreases with latitude Van Valen's law Probability of extinction of a group is constant over time von Baer's laws Embryos start from a common form and develop into increasingly specialised forms Williston's law Parts in an organism become reduced in number and specialized in function
Related	Countergradient variation Where genetics opposes environment as a factor Gigantothermy Large ectothermic animals more easily maintain constant body temperature

Allen's rule

Background[edit | edit source]

Theory Behind Allen's Rule[edit | edit source]

Other Thermoregulatory Factors[edit | edit source]

See also[edit | edit source]

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