Disentangling the Size and Shape Components of Sexual Dimorphism
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Many organisms are sexually dimorphic, reflecting sex-specific selection pressures. But although sexual dimorphism may consist of different variables from size to shape and physiology, most research emphasizes a single aspect of sexual dimorphism, notably size, without specifying its components and their relationship. Among terrestrial animals, spiders exhibit most extreme sex-specific differences in size and abdominal shape, and therefore represent ideal models to address this question. Here, we dissect sexual dimorphism in spiders at two phylogenetic hierarchical levels. At the species level, we employ comparative phylogenetic tests to explore the association between sexual shape dimorphism (SShD) and sexual size dimorphism (SSD) in the orbweb clade Argiopinae. At the genus level, we then explore such patterns on a phylogeny of orb weavers (Araneoidea). Female argiopines had more diverse abdominal morphotypes than the males and the abdominal shape evolution was only poorly correlated between the sexes. Phylogenetic and comparative data suggested that evolution of SShD in argiopines was related to geographic history, but that sexually shape monomorphic cases arose through selection for male size, perhaps acting against fecundity selection. While in argiopines there was no clear association between SShD and SSD, we detected a significant correlation in all orb weavers at the genus level. The shape and the size components of sexual dimorphism may thus respond independently to selection pressures, but at certain phylogenetic levels SSD may be a prerequisite for SShD. Research on other animal groups is needed to establish whether the here detected patterns on spiders are general.
KeywordsSexual shape dimorphism Sexual size dimorphism Abdominal shape Sexual selection Intraspecific niche divergence Spiders
We thank D. Li, J. Schneider, X. Xu, I. Agnarsson, I. M. Tso, P. Jäger, W. Chotwong, J. P. Huang, C. P. Lin, Y. C. Su, V. Settepani, S. Huber, M. Gregorič, G. Uhl, E. A. Yağmur, M. A. Herberstein, and J. N. Huang for their help with material for size measurement and molecular work. We also thank C. P. Liao for helping on statistics and EZ Lab members for various support: N. Vidergar, T. Lokovšek and S. Kralj-Fišer. Our paper was independently reviewed by Axios Review, and we thank Tim Vines, Matthew Symonds and three anonymous reviewers for their feedback. This work was supported by the Slovenian Research Agency (P1-10236 to MK and a Young Researcher fellowship to RCC).
Conflict of interest
The authors declare no conflict of interest.
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