Phenotypic variation in dorsal fin morphology of coastal bottlenose dolphins (Tursiops truncatus) off Mexico

Abstract

Geographic variation in external morphology is thought to reflect an interplay between genotype and the environment. Morphological variation has been well-described for a number of cetacean species, including the bottlenose dolphin (Tursiops truncatus). In this study we analyzed dorsal fin morphometric variation in coastal bottlenose dolphins to search for geographic patterns at different spatial scales. A total of 533 dorsal fin images from 19 available photo-identification catalogs across the three Mexican oceanic regions (Pacific Ocean n = 6, Gulf of California n = 6 and, Gulf of Mexico n = 7) were used in the analysis. Eleven fin shape measurements were analyzed to evaluate fin polymorphism through multivariate tests. Principal Component Analysis on log-transformed standardized ratios explained 94% of the variance. Canonical Discriminant Function Analysis on factor scores showed separation among most study areas (p < 0.05) with exception of the Gulf of Mexico where a strong morphometric cline was found. Possible explanations for the observed differences are related to environmental, biological and evolutionary processes. Shape distinction between dorsal fins from the Pacific and those from the Gulf of California were consistent with previously reported differences in skull morphometrics and genetics. Although the functional advantages of dorsal fin shape remains to be assessed, it is not unlikely that over a wide range of environments, fin shape may represent a trade-off among thermoregulatory capacity, hydrodynamic performance and the swimming/hunting behavior of the species.

Bottlenose dolphin ecotypes of the western South Atlantic: the puzzle of habitats, coloration patterns and dorsal fin shapes

“…data), as well as previous findings of similarities in external morphology (i.e. dorsal fin falcateness) between dolphins inhabiting deeper waters in the eastern Pacific (Morteo et al 2017, Félix et al 2018, we aimed to investigate the potential phenotypic similarity in the dorsal fin shape be tween offshore dolphins of other oceanographic regions with the ecotypes of the wSAO. Therefore, we used 1 dorsal fin image obtained from the offshore waters of the Galapagos Islands (and included in the ordination procedure -see Sections 2.2 and 2.3) and another from the western North Atlantic (recognized as an offshore ecotype by morphological and genetic analyses, W. McFee & P. E. Rosel pers.…”

“…ImageJ software (Schneider et al 2012) was used to define 6 landmarks on the dorsal fins, modified from Morteo et al (2017), and to measure the distance between them (Fig. 1).…”

“…Each landmark was established based on angles that depicted the shape of the dorsal fin. Distances between landmarks were used to define 4 indices for each image through the following standardized ratios (see Weller 1998, Morteo et al 2017): C30B/AB, C20B/AB, C10B/AB, and C5B/AB (Fig. 1).…”

Form, function, and divergence of a generic fin shape in small cetaceans

“…The spread out falcate shape of the dorsal fin can be observed across the different Odontoceti species but the contribution of fin to the stability and maneuverability control seems to be different in a 200 kg dolphin and a 7 ton bottlenose whale. On the intraspecific level, the distinct phenotypic variation of the dorsal fin was found in coastal and offshore ecotypes of dolphins [ 73 – 76 ].…”

High similarities in dorsal fin ratios of common bottlenose dolphins (Tursiops truncatus) in the Caribbean Sea

“…; Würsig & Jefferson, 1990). In common bottlenose dolphins (Tursiops truncatus), differences in dorsal fin shape have been also associated with geographic variation (Morteo et al, 2017), habitat specialization between ecotypes (Félix et al, 2018, Simões-Lopes et al, 2019, and ontogenetic factors (Félix et al, 2018).…”