Osteological differences between two sympatric forms of bottlenose dolphins (genus Tursiops) in Chinese waters

Abstract

Although there has been extensive research on bottlenose dolphins (genus Tursiops), much controversy over the classi®cation within this genus still exists. Even though many morphological variants occur, the prevailing view is for a single species, Tursiops truncatus (Montagu, 1821). To evaluate this view, two sympatric forms of bottlenose dolphins inhabiting Chinese waters were examined using multivariate statistical analyses of osteological data for two meristic and 18 cranial morphometric characters from 71 specimens. Cluster and principal components analyses revealed clear osteological separation of the two forms. Furthermore, the distributions of the total number of vertebrae and several proportions of cranial characters were non-overlapping between the two forms. These results provided strong evidence that the two sympatric forms of Tursiops in Chinese waters are isolated reproductively and do not support the current view of a monotypic genus. Classi®cation functions of a discriminant analysis and a key of several characters were developed to help researchers identify unassigned specimens. Provisional names (T. truncatus and T. aduncus) were suggested, but a formal taxonomic revision of this genus is still required. However, regardless of the nomenclature, immediate amendments to all present wildlife conservation legislation are strongly urged.

Dolphin sympatric ecology

“…For instance, bottlenose dolphins and Indo-Pacific bottlenose dolphins appeared to be in direct sympatry around the Chinese waters of the Penghu archipelago and were observed in mixed schools that frequently included other dolphin species as well (Yang 1976;Zhou & Qian 1985). These two species, however, differed ecologically: bottlenose dolphins preferred the coastal and shallow waters of the continental shelf, feeding upon benthic or reef-dwelling fish and cephalopods, whereas Indo-Pacific bottlenose dolphins favoured offshore waters, feeding mostly on schooling epipelagic and mesopelagic species (Wang et al 2000).…”

“…Sympatric species can also display ecological separation utilizing inshore and offshore waters, as observed by Wang et al (2000) for bottlenose dolphins living in Chinese waters, Dolar (1999) for spinner dolphins and Fraser's dolphin (Lagenodelphis hosei ) in the Sulu Sea, Bearzi (2003) for bottlenose dolphins in sympatry with short-beaked common dolphins in Californian waters, and Baird & Dill (1995) for transient and resident killer whales in British Columbia and Washington State. Resident and transient killer whales also used the same habitat but with different travel routes, sometimes related to the bottom topography (Morton 1990;Felleman et al 1991;Baird 2000).…”

“…Sympatric bottlenose dolphins and Indo-Pacific bottlenose dolphins also seemed to exist in south African waters (Wang et al 2000), although Ross (1977) described these species as being typically Table I. Accounts of associations and interactions between different species of the family Delphinidae observed in the field.…”

“…Only a few sympatric populations of small odontocetes, however, have been well investigated in the field (common bottlenose dolphins, Tursiops truncatus Á/ hereinafter bottlenose dolphin Á/ and Indo-Pacific bottlenose dolphins, Tursiops aduncus: Hale et al 2000;Wang et al 2000; short-beaked common dolphins, Delphinus delphis , and bottlenose dolphins: Politi et al 1998;Bearzi et al 2005; short-beaked common dolphins, long-beaked common dolphins, Delphinus capensis, and bottlenose dolphins: Bearzi 2003; short-beaked common dolphins and striped dolphins, Stenella coeruleoalba : Frantzis & Herzing 2002; transient and resident killer whales, Orcinus orca : Bigg et al 1987Bigg et al , 1990Baird et al 1992;Baird 1994;Ford et al 1998;Saulitis et al 2000). In this paper, sympatry is defined as the co-occurrence of two or more dolphin species in the same immediate habitat, which might be called direct sympatry, where broad sympatry simply means two or more species occurring over the same wider geographical area.…”

Shape analysis of odontocete mandibles: Functional and evolutionary implications

“…These studies have examined sexual dimorphism or compared and contrasted features of the same species that inhabit different localities, questioning whether they are different ecotypes (Wang et al, 2000;Murphy et al, 2006;Westgate, 2007). Odontocete mandibular studies examining higher taxonomic levels have used traditional morphometrics (Werth, 2006;Nummela et al, 2007).…”