The evolution of sexual dimorphism in Chondrichthyes: drivers, uncertainties, and future directions

Abstract

Sexual dimorphisms are abundant in natural systems; however, their ecological and evolutionary significance have largely been neglected with respect to Chondrichthyes. A number of dimorphisms have been reported in this ancient clade, yet there remains considerable uncertainty regarding the disparity and variation in dimorphisms present in extant taxa, and the evolutionary processes that have resulted in their manifestation. In this review, I summarise our current understanding of sexual dimorphisms in chondrichthyans and consider the extent to which existing studies favour the two predominant theories regarding their evolution. Throughout, I consider the major limitations and open questions in the field, arguing ultimately that additional studies are required (both with regard to the phenomenon of sexual dimorphism itself, and several related fields including evolutionary genetics) if we wish to fully understand the evolutionary and ecological significance of sexual dimorphism in Chondrichthyes.

Ontogenetic morphometry of the brown smoothhound sharkMustelus henleiwith implications for ecology and evolution

“…The sexual dimorphism in caudal morphometry exhibited by adult M. henlei individuals implies either that differences in caudal scaling are related to sexual reproduction, or that there is some significant ecological difference between males and females of this taxon (Gayford, 2023). While there is no obvious association between caudal morphology and sexual reproduction, we present two potential ecological drivers of this sexual dimorphism: Smaller individuals are likely to be subjected to greater predation pressure than larger individuals (Guttridge et al, 2012; Heithaus, 2007), and sexual size dimorphism has been reported in some M. henlei populations (Yudin & Cailliet, 1990).…”

“…Due to the energetic requirements of migration (Lawson et al, 2019;Del Raye et al, 2013), and the increased predation pressure that often results (Heithaus, 2007;Heupel et al, 2007), it is unlikely that M. henlei juveniles exhibit migratory behaviour. Rather, juveniles may be subject to greater predation pressure than adults, or may travel greater distances during foraging-which may be focussed The sexual dimorphism in caudal morphometry exhibited by adult M. henlei individuals implies either that differences in caudal scaling are related to sexual reproduction, or that there is some significant ecological difference between males and females of this taxon (Gayford, 2023). While there is no obvious association between caudal morphology and sexual reproduction, we present two potential ecological drivers of this sexual dimorphism: Smaller individuals are likely to be subjected to greater predation pressure than larger individuals (Guttridge et al, 2012;Heithaus, 2007), and sexual size dimorphism has been reported in some M. henlei populations (Yudin & Cailliet, 1990).…”

The biology and ecology of the Pacific sharpnose shark Rhizoprionodon longurio

“…No existing studies expand on the potential for sexual dimorphism in R. longurio despite the abundance of sex-based morphological differences in other sharks and their potentially significant ecological and behavioural implications (Ritter & Amin, 2019;Whitehead et al, 2022). A lack of dental sexual dimorphism may suggest that sexual conflict is relatively weak compared with other elasmobranchs, however the absence of dental sexual dimorphism alone is insufficient to confirm this (Gayford, 2023). The morphological characteristics of R. longurio are relatively well understood compared with other aspects of the species' ecology, and therefore there are no major knowledge gaps in this area that are specific to R. longurio.…”

The origins and drivers of sexual size dimorphism in sharks

“…Moreover, as the sister taxon to Osteichthyes, macroevolutionary trends within Chondrichthyes are of great relevance to our understanding of character transitions and trait evolution in jawed vertebrates (Hara et al., 2018; Stein et al., 2018; Venkatesh et al., 2014). Sexual dimorphism is abundant in sharks (Gayford, 2023), and indeed, SSD has been reported in many species (Colonello et al., 2020; Sims, 2005). Existing studies have speculated that the magnitude and direction of SSD in sharks may relate to differences in reproductive mode and/or the intensity of sexual selection (Colonello et al., 2020; Sims, 2005).…”