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Time-scaled molecular phylogeny displaying the taxa and interrelationships utilised in this study. Branch lengths were obtained from Stein et al. (2018). Silhouettes are representative taxa from various selachimorph radiations included in the study. Silhouette images obtained online have been dedicated to the public domain.
Source publication
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 compet...
Citations
... Longer asymptotic length (L ∞ ) for females than males and faster growth (k) of males than females has been found elsewhere (Natanson, Mello, and Campana 2002;Bishop et al. 2006;Barreto et al. 2016). Sexual dimorphism in growth is likely related to higher female reproductive energy demand, which causes slower growth of females than males (Gayford and Sternes 2024). Differences between our estimates of L ∞ and those of other studies may be related to small sample sizes of small (< 100 cm TL) and large (> 240 cm TL) sharks of both sexes, the presence of which often depends on the sampling area (Cliff, Dudley, and Davis 1990), distribution in the water column Stevens 2008), and possible escape from sampling gear due to their jaw strength (Pratt Jr. and Casey 1983). ...
The shortfin mako shark (Isurus oxyrinchus) is a widely distributed predatory species. However, critical aspects of its biology remain poorly understood in several regions, including the Ecuadorian Pacific Ocean, where it is one of the most commonly captured shark species. Vertebral samples of 238 specimens were analyzed, including 119 females (82-228 cm TL; 0-15 years), 115 males (76.4-248 cm TL; 0-16 years), and 4 unsexed individuals (194-215 cm TL; 10-14 years). The von Bertalanffy model provided the best fit for males (L ∞ 271.31 cm TL, L 0 = 75.85 cm TL, k = 0.10 year −1), and females (L ∞ 347.53 cm TL, L 0 = 76.04 cm TL, k = 0.06 year −1). The results indicated sexual dimorphism, with females reaching a larger size and exhibiting slower growth rates compared with males. The shortfin mako shark in the Ecuadorian Pacific Ocean grew slow, and based on its life history characteristics, is vulnerable to overexploitation, thereby necessitating harvest management to sustain a fishery.
... Although females were, on average, larger than males in this study, no statistical sexual size dimorphism (SSD) was found, consistent with previous studies on the genus (Romero-Caicedo et al. 2014;Briones-Mendoza et al. 2021;Calle-Moran et al. 2023). While SSD is a common condition in sharks, these results suggest that factors typically associated with SSD, such as body size, fecundity, and sexual selection, may not be as influential (Gayford and Sternes 2024). SSD may be more apparent when considering the size at sexual maturity (males = 130 cm PCL, females = 139 cm PCL; Carrillo-Colín et al. ...
The pelagic thresher shark (Alopias pelagicus) is considered overexploited; however, information about its biology in the Eastern Pacific is scarce, particularly age estimation and growth modeling. Age was estimated by counting growth bands in the cervical vertebrae. Specimens, ranging in size from 85.00 to 174.90 cm precaudal length (PCL), were collected by pelagic longliners in 2020 and 2021. Three different growth models (von Bertalanffy, Gompertz, and logistic) were applied to the length-at-age data. Bayesian methods using the Markov chain Monte Carlo algorithm were employed for parameter estimation. Based on the Watanabe–Akaike information criterion for model selection, the von Bertalanffy was selected as the best model for both sexes combined, with the estimated parameters being PCL0 = 86.20 cm (95% credible interval (CI95%), 83.03‒89.32), PCL∞ = 180.66 cm (CI95%, 172.96‒189.81), and k = 0.09 year⁻¹ (CI95%, 0.07–0.10). These results suggest that females and males have the same growth pattern. Based on known size at maturity, age at maturity for the pelagic thresher shark was estimated at 7 years for males and 9 for females, with a maximum lifespan of 22 to 38 years. This study provides crucial life history information that can contribute to managing and conserving this endangered species.
... 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 ). ...
... 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 ). ...
... 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 ). ...
Synopsis
Systematic trends in body size variation exist in a multitude of vertebrate radiations, however their underlying ecological and evolutionary causes remain poorly understood. Rensch's rule describes one such trend—in which the scaling of sexual size dimorphism (SSD) depends on which sex is larger. Where SSD is male-biased, SSD should scale hyperallometrically, as opposed to hypoallometrically where SSD is female-biased. The evidence for Rensch's rule is mixed, and comes from a small subset of total vertebrate diversity. We conducted the first empirical test of Rensch's rule in sharks, seeking to confirm or refute a long-hypothesied trend. We find that sharks violate Rensch's rule, as the magnitude of SSD increases with body size despite sharks predominantly exhibiting female-biased SSD. This adds to a growing literature of vertebrate clades that appear not to follow Rensch's rule, suggesting the absence of a single, conserved scaling trend for SSD amongst vertebrates. It is likely that selection associated with fecundity results in the “inverse Rensch's rule” observed in sharks, although additional studies will be required to fully reveal the factors underlying SSD variation in this clade.
