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.
[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.