Rensch's rule

Rensch's rule is a biological rule on allometrics, concerning the relationship between the extent of sexual size dimorphism and which sex is larger. Across species within a lineage, size dimorphism increases with increasing body size when the male is the larger sex, and decreases with increasing average body size when the female is the larger sex. The rule was proposed by the evolutionary biologist Bernhard Rensch in 1950.^[1]

After controlling for confounding factors such as evolutionary history, an increase in average body size makes the difference in body size larger if the species has larger males, and smaller if it has larger females.^[2] Some studies propose that this is due to sexual bimaturism, which causes male traits to diverge faster and develop for a longer period of time.^[3] The correlation between sexual size dimorphism and body size is hypothesized to be a result of an increase in male-male competition in larger species,^[4] a result of limited environmental resources, fuelling aggression between males over access to breeding territories^[5] and mating partners.^[2]

Phylogenetic lineages that appear to follow this rule include primates, pinnipeds, and artiodactyls.^[6]

This rule has rarely been tested on parasites. A 2019 study showed that ectoparasitic philopterid and menoponid lice comply with it, while ricinid lice exhibit a reversed pattern.^[7]

References

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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