Beyond Size Dimorphism: The Past, Present, and Future of Rensch's Rule

AbstractA pattern of allometry in which the degree of male-biased sexual size dimorphism (SSD) increases with species body size is known as "Rensch's rule." Over the past decades, a growing amount of Rensch's rule studies has advanced our understanding of SSD, our knowledge of its prevalence in nature, and our comprehension of 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 Rensch's 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 Rensch's pattern beyond its applications to SSD. The strong bias toward body size in the study of Rensch's 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.