Beyond size dimorphism: the past, present, and future of Rensch's Rule

Renschs Rule is known as a pattern of allometry in which the degree of male-biased sexual size dimorphism (SSD) increases with species body size. Over the last decades, a growing amount of Renschs Rule studies has advanced our understanding of SSD, its prevalence in nature, and the mechanisms underlying its evolution. However, Bernhard Rensch, when describing the pattern for the first time, considered the allometry of SSD only as a special case of a more general pattern in which dimorphism in any relative sexual difference increased with body size. In this perspective I revisit the history of Renschs Rule, starting with its popularization in recent decades, then diving into the original works by Rensch to rediscover his original observations, and finally discussing the implications of studying Renschs pattern beyond its applications to SSD. The strong bias towards body size in the study of Renschs Rule has proven valuable regarding our understanding of the evolution of SSD. Using empirical examples I propose, however, that expanding the study of the pattern to other traits might prove insightful for the general study of sexual dimorphism and phenotypic diversity.