The origins and drivers of sexual size dimorphism in sharks

Abstract

While sexual size dimorphism (SSD) is abundant in nature, there is huge variation in both the intensity and direction of SSD. SSD results from a combination of sexual selection for large male size, fecundity selection for large female size and ecological selection for either. In most vertebrates, it is variation in the intensity of male–male competition that primarily underlies variation in SSD. In this study, we test four hypotheses regarding the adaptive value of SSD in sharks—considering the potential for each of fecundity, sexual, ecological selection and reproductive mode as the primary driver of variation in SSD between species. We also estimate past macroevolutionary shifts in SSD direction/intensity through shark phylogeny. We were unable to find evidence of significant SSD in early sharks and hypothesise that SSD is a derived state in this clade, that has evolved independently of SSD observed in other vertebrates. Moreover, there is no significant relationship between SSD and fecundity, testes mass or oceanic depth in sharks. However, there is evidence to support previous speculation that reproductive mode is an important determinant of interspecific variation in SSD in sharks. This is significant as in most vertebrates sexual selection is thought to be the primary driver of SSD trends, with evidence for the role of fecundity selection in other clades being inconsistent at best. While the phylogenetic distribution of SSD among sharks is superficially similar to that observed in other vertebrate clades, the relative importance of selective pressures underlying its evolution appears to differ.

Sharks Violate Rensch's Rule for Sexual Size Dimorphism

“…Data regarding SSD and body size in sharks were extracted from Gayford and Sternes (2024) . Specifically, 2 measures of SSD were included in this study: SD% (percentage of maximum total length corresponding to the difference in median length at sexual maturity between the two sexes) and MFR (the ratio of median length at sexual maturity between males and females).…”

“…Elasmobranchii (sharks and rays) is one major vertebrate clade in which the validity of Rensch's rule has not been tested. SSD is widespread in elasmobranchs and appears to be predominantly female-biased ( Conrath and Musick 2012 ; Gayford 2023 ; Gayford and Sternes 2024 ), although there is evidence that male-biased SSD has evolved multiple times independently within the clade ( Gayford and Sternes 2024 ). Gayford and Sternes (2024) recently performed a comparative phylogenetic analysis of SSD in sharks, finding support for the hypothesis that differences in reproductive mode underlie the phylogenetic distribution of SSD in the clade ( Sims 2005 ; Colonello et al.…”

“…2007 ; Colonello et al. 2020 ; Gayford and Sternes 2024 ). This result is indicative of spatiotemporal variation in reproductive effort modulating selection on female body size ( Gayford and Sternes 2024 ).…”

“…2020 ; Gayford and Sternes 2024 ). This result is indicative of spatiotemporal variation in reproductive effort modulating selection on female body size ( Gayford and Sternes 2024 ). To date, no study has investigated (directly or indirectly) the potential for associations between SSD and body size in Elasmobranchii.…”

“…In this study, we use Gayford and Sternes (2024) ’s SSD database to test for consistent allometric trends in SSD in sharks at multiple ontogenetic stages, and consequently test the validity of Rensch's rule in selachians. This is the first formal investigation of Rensch's rule or SSD allometry in Chondrichthyes and amongst the first to apply a multi-species, comparative approach to studying SSD allometry in fish.…”