Allometry for sexual size dimorphism: Pattern and process in the coevolution of body size in males and females

Sexual size dimorphism (SSD) is common in both plants and animals, and current evidence suggests that it reflects the adaptation of males and females to their different reproductive roles. When species are compared within a clade, SSD is frequently found to vary with body size. This allometry is detected as beta not equal 1, where beta is the slope of a model II regression of log(male size) on log(female size). Most frequently, beta exceeds 1, indicating that SSD increases with size where males are the larger sex, but decreases with size where females are larger, a trend formalized as "Rensch's rule.' Exceptions are uncommon and associated with female-biased SSD. These trends are derived from a sample of 40 independent clades of terrestrial animals, primarily vertebrates. Their extension to plants and aquatic animals awaits quantitative assessments of allometry far SSD within these groups. Many functional hypotheses have been proposed to explain the evolution of allometry for SSD, most featuring sexual selection on males or reproductive selection on females. Of these, the hypothesis that allometry evolves because of correlational selection between the sexes appears most promising as a general model but remains untested.