Sexual selection and flight as predictors of sexual size and shape dimorphism in stick and leaf insects

Sexual size dimorphism (SSD) and sexual shape dimorphism (SShD) are widespread in animals, yet their macroevolutionary drivers remain poorly understood. We analyzed 196 stick and leaf insect species (Phasmatodea), a clade with exceptional diversity in size, shape, flight capacity, and mating systems, using phylogenetic comparative methods to evaluate the roles of fecundity selection, sexual selection, and ecological factors. SSD was universally female-biased and varied over an order of magnitude. Allometric scaling supported the inverse of Rensch's rule, and clade-level patterns matched predictions from quantitative genetic models in which stronger directional selection on females than on males explains female-biased SSD. However, interspecific variation in SSD was not explained by female fecundity. Instead, mating system and flight dimorphism best predicted variation in SSD and SShD: males engaging in short-term mate guarding (less than a few days) were larger and stockier than searching males or those exhibiting prolonged mate guarding, reducing SSD, whereas species with flight-capable males and flightless females showed increased dimorphism, largely due to smaller males. Habitat and climate had limited effects. Sex differences in growth rate and development duration contributed equally to SSD, with females growing faster and for longer, consistent with widespread protandry. These results indicate that while fecundity selection historically drove female-biased SSD, contemporary variation is primarily shaped by male-specific selection through mating system and flight-related ecological pressures.