Evolution and maintenance of sexual size dimorphism: aligning phylogenetic and experimental evidence

Abstract

Integrating the insights derived from both phylogenetic and experimental approaches offers a more complete understanding of evolutionary patterns and processes, yet it is rarely a feature of investigations of the evolutionary significance of trait variation. We combine these approaches to reinterpret the patterns and processes in the evolution of female biased sexual size dimorphism in Nephilidae, a spider lineage characterized by the most extreme sexual size dimorphism among terrestrial animals. We use a molecular phylogeny to reconstruct the size evolution for each sex and reveal a case of "sexually dimorphic gigantism": both sexes steadily outgrow their ancestral sizes, but the female and male slopes differ, and hence sexual size dimorphism steadily increases. A review of the experimental evidence reveals a predominant net selection for large size in both sexes, consistent with the phylogenetic pattern for females but not for males. Thus, while sexual size dimorphism in spiders most likely originates and is maintained by fecundity selection on females, it is unclear what selection pressures prevent males from becoming as large as females. This integrated approach highlights the dangers of inferring evolutionary significance from experimental studies that isolate the effects of single selection pressures.

Disentangling the Size and Shape Components of Sexual Dimorphism

“…As sit-and-wait hunters limited to their web space, orb-weaving spiders are exposed to predators, and may be under substantial ecological pressure to increase their survival rates through abdomen shape changes. Our study implies that these morphological changes evolve more frequently in those lineages that are highly female size biased, perhaps because of already disassociated male and female phenotypic evolution (Kuntner and Coddington 2009;Kuntner and Elgar 2014). Although speculative, we suggest that responses to predation pressure may trigger a sex-specific response affecting morphology in those lineages with giant females.…”

“…These sexually specific differences imply that abdominal shape evolution is largely driven by evolutionary pressures on females, among which literature consistently singles out fecundity selection Head 1995;Higgins 1992;Hormiga et al 2000). One common evolutionary outcome of fecundity selection is female gigantism as is the case in another orb web clade, the nephilid spiders (Head 1995;Hormiga et al 2000;Kuntner and Coddington 2009;Kuntner and Elgar 2014). An important difference between these clades is that in nephilids, size evolution between the sexes runs independently (Kuntner and Coddington 2009;Kuntner and Elgar 2014), whereas in argiopines it is highly positively correlated (Cheng and Kuntner 2014).…”

“…One common evolutionary outcome of fecundity selection is female gigantism as is the case in another orb web clade, the nephilid spiders (Head 1995;Hormiga et al 2000;Kuntner and Coddington 2009;Kuntner and Elgar 2014). An important difference between these clades is that in nephilids, size evolution between the sexes runs independently (Kuntner and Coddington 2009;Kuntner and Elgar 2014), whereas in argiopines it is highly positively correlated (Cheng and Kuntner 2014). The correlated evolution of argiopine male and female size implies that female size changes also depend on evolutionary pressures on males (Elgar 1991;Moya-Laraño et al 2009;Vollrath and Parker 1992).…”

Differential investment and size-related mating strategies facilitate extreme size variation in contesting male spiders

“…Higgins, Coddington, Goodnight, & Kuntner, 2011;Kuntner, Arnedo, Trontelj, Lokovsek, & Agnarsson, 2013;Kuntner & Coddington, 2009;Kuntner & Elgar, 2014), but beyond that, males in the genus Nephila may vary drastically in body size, with size differences reaching an order of magnitude in some species (Elgar, De Crespigny, & Ramamurthy, 2003). Therefore, Nephila species have been used in a range of empirical studies investigating potential costs and benefits of small male size, most of which have reported benefits of large size (e.g.…”

Nutrients intake determines the post-maturity molting in the golden orb-web spider Nephila pilipes (Araneae: Nephilidae)

“…In arthropods, female body size is positively correlated with female fecundity (Head, 1995;Higgins, 1992). In spiders, sexual size dimorphism is believed to be generated and maintained by fecundity selection on female body size (Head, 1995;Kuntner and Elgar, 2014). In theory, an increase in female body size via PMM may enhance reproductive performance, especially when food is abundant.…”