Global latitudinal and bathymetric gradients in body size among cartilaginous fishes (Gnathostomata: Chondrichthyes)

Joel Harrison Gayford, Julia Türtscher, Patrick L Jambura, Duncan J Irschick, Andrew Chin, Jodie L Rummer

Understanding the macroecological rules governing body size variation across environmental gradients has long been a central focus of biology for centuries. Bergmann’s rule – the tendency for animals to reach larger body sizes in colder environments – has been studied in endotherms but with mixed support. However, phylogenetically informed tests of this rule in ectotherms remain scarce, and there is very limited evidence at higher taxonomic scales. Here, we provide robust evidence for Bergmann’s rule in Chondrichthyes (sharks, rays, and chimaera), a predominantly ectothermic clade, using a phylogenetically informed dataset of over 900 species extracted from existing databases and primary literature. We show that chondrichthyans tend to mature at larger sizes and also reach larger maximum body sizes at higher latitudes and in deeper habitats – a pattern that is consistent with Bergmann’s rule. We suggest that increased survivorship in these environments may promote evolutionary shifts toward greater resource allocation to growth and larger body sizes. This work fills a critical gap in testing Bergmann’s rule among ectotherms in a phylogenetic context, demonstrating the potential for environmental gradients to shape the evolution of body size in marine vertebrates.