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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 have rarely been tested on parasites. A recent study showed that Philopterid and Menoponid lice comply with it, while Ricinid lice exhibit a reversed pattern.[7]


  1. ^ Rensch, B. (1950). Die Abhängigkeit der relativen Sexualdifferenz von der Körpergrösse. Bonner Zoologische Beiträge 1:58-69.
  2. ^ a b Dale, James; Dunn, Peter O.; Figuerola, Jordi; Lislevand, Terje; Székely, Tamás; Whittingham, Linda A. (2007-12-07). "Sexual selection explains Rensch's rule of allometry for sexual size dimorphism". Proceedings of the Royal Society of London B: Biological Sciences. 274 (1628): 2971–2979. doi:10.1098/rspb.2007.1043. ISSN 0962-8452. PMC 2211517. PMID 17878139.
  3. ^ Blanckenhorn, Wolf; Dixon, A. "Proximate Causes of Rensch's Rule: Does Sexual Size Dimorphism in Arthropods Result from Differences in Development Time?". University of Zurich. University of Zurich.
  4. ^ Székely, Tamás; Freckleton, Robert P.; Reynolds, John D. (2004-08-17). "Sexual selection explains Rensch's rule of size dimorphism in shorebirds". Proceedings of the National Academy of Sciences of the United States of America. 101 (33): 12224–12227. doi:10.1073/pnas.0404503101. ISSN 0027-8424. PMC 514460. PMID 15304645.
  5. ^ Lengkeek, W.; Didderen, K.; Côté, I. M.; van der Zee, E. M.; Snoek, R. C.; Reynolds, J. D. (2008-10-01). "Plasticity in sexual size dimorphism and Rensch's rule in Mediterranean blennies (Blenniidae)". Canadian Journal of Zoology. 86 (10): 1173–1178. doi:10.1139/z08-103. ISSN 0008-4301.
  6. ^ Fairbairn, D.J. (1997). "Allometry for Sexual Size Dimorphism: Pattern and Process in the Coevolution of Body Size in Males and Females". Annu. Rev. Ecol. Syst. 28 (1): 659–687. doi:10.1146/annurev.ecolsys.28.1.659.
  7. ^ Piross, I.S.; Harnos, A.; Rozsa, L. (2019). "Rensch's rule in avian lice: contradictory allometric trends for sexual size dimorphism". Scientific Reports. 9: 7908. doi:10.1038/s41598-019-44370-5.