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.
After controlling for
confounding
In causal inference, a confounder is a variable that influences both the dependent variable and independent variable, causing a spurious association. Confounding is a causal concept, and as such, cannot be described in terms of correlatio ...
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, 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 () is the study of the evolutionary history of life using observable characteristics of organisms (or genes), which is known as phylogenetic inference. It infers the relationship among organisms based on empirical dat ...
lineages that appear to follow this rule include
primates
Primates is an order of mammals, which is further divided into the strepsirrhines, which include lemurs, galagos, and lorisids; and the haplorhines, which include tarsiers and simians ( monkeys and apes). Primates arose 74–63 ...
,
pinnipeds, and
artiodactyls.
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