Rensch's rule is a

biological rule A biological rule or biological law is a generalized law, principle, or rule of thumb formulated to describe patterns observed in living organisms. Biological rules and laws are often developed as succinct, broadly applicable ways to explain comp ...

on allometrics, concerning the relationship between the extent of

sexual size dimorphism Sexual dimorphism is the condition where the sexes of the same animal and/or plant species exhibit different morphological characteristics, particularly characteristics not directly involved in reproduction. The condition occurs in most ani ...

and which sex is larger. Across species within a

lineage Lineage may refer to: Science * Lineage (anthropology), a group that can demonstrate its common descent from an apical ancestor or a direct line of descent from an ancestor * Lineage (evolution), a temporal sequence of individuals, populati ...

, 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 Evolutionary biology is the subfield of biology that studies the evolutionary processes (natural selection, common descent, speciation) that produced the diversity of life on Earth. It is also defined as the study of the history of life for ...

Bernhard Rensch Bernhard Rensch (21 January 1900 – 4 April 1990) was a German evolutionary biologist and ornithologist who did field work in Indonesia and India. Starting his scientific career with pro-Lamarckian views, he shifted to selectionism and became on ...

in 1950. After controlling for

confounding In statistics, a confounder (also confounding variable, confounding factor, extraneous determinant or lurking variable) is a variable that influences both the dependent variable and independent variable, causing a spurious association. Con ...

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. Some studies propose that this is due to

sexual bimaturism Sexual bimaturism describes a difference in developmental timing between males and females of the same species. Sexual bimaturism can result in sexual dimorphism, but sexual dimorphism could also develop through differential rates of development. I ...

, which causes male traits to diverge faster and develop for a longer period of time. 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, a result of limited environmental resources, fuelling aggression between males over access to breeding territories and mating partners.

Phylogenetic In biology, phylogenetics (; from Greek φυλή/ φῦλον [] "tribe, clan, race", and wikt:γενετικός, γενετικός [] "origin, source, birth") is the study of the evolutionary history and relationships among or within groups o ...

lineages that appear to follow this rule include

primates Primates are a diverse order of mammals. They are divided into the strepsirrhines, which include the lemurs, galagos, and lorisids, and the haplorhines, which include the tarsiers and the simians (monkeys and apes, the latter including huma ...

pinnipeds Pinnipeds (pronounced ), commonly known as seals, are a widely range (biology), distributed and diverse clade of carnivorous, fin-footed, semiaquatic, mostly marine mammal, marine mammals. They comprise the extant taxon, extant family (biology ...

, and

artiodactyls The even-toed ungulates (Artiodactyla , ) are ungulates—hoofed animals—which bear weight equally on two (an even number) of their five toes: the third and fourth. The other three toes are either present, absent, vestigial, or pointing poster ...

. 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.

References

{{Biological rules Animal size Biological rules Sexual dimorphism