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Chapter One: General Introduction
Delphinids likely arose in the mid- to late Miocene (10-11 mya) from kentriodontid-like
ancestors (Barnes 1985). They are relatively uncommon in the fossil record of the
latest Miocene and Pliocene deposits, suggesting that the present diversity of
delphinids is the result of an explosive species radiation occurring in the later part of
the Pliocene (Barnes 2002). Delphinids represent the most diverse family of marine
mammals with 37 currently recognised (and generally accepted) species, including
the recently described Orcaella heinsohni (Beasley et al. 2005). It is very likely,
however, that this number will continue to increase as new species are still being
described, for example, the recognition of Sotalia as two distinct species: S. fluviatilis
and S. guianensis (Cunha et al. 2005, Caballero et al. 2007).
Dolphins show wide variation in their external morphology, including the length of the
beak, colour pattern and size (ranging from 1.5 m in some Cephalorhynchus spp. and
Stenella longirostris to 9.8 m in Orcinus orca). Several species of cosmopolitan,
largely pelagic dolphins also exhibit a large degree of morphological variation
throughout their geographic distribution (e.g., spinner dolphins, Perrin 1990). Overall,
dolphins are widespread in the world’s oceans, with the killer whales being the most
widely distributed. The highest species-level diversity is found in tropical and warm
temperate latitudes with many species showing a pantropical distribution (e.g.,
Stenella spp.). In contrast, several species are distributed anti-tropically, including
notably, the six species of Lagenorhynchus (Cipriano 1997).
1.2.1. Mechanisms of speciation in dolphins
The rapid radiation of delphinids is not fully understood, but several mechanisms of
speciation have been proposed. Allopatric speciation events might have occurred
following large geographical changes and the appearance of new barriers to gene
flow. Davies (1963) suggested that the African continent could have played a
significant role in cetacean evolution, acting as a geographical barrier to dispersal of
tropical cetacean species during the Pleistocene glaciations. This hypothesis was
later revisited by Perrin et al. (1978) and Rosel et al. (1994) to explain the current
distribution of Stenella and Delphinus spp.
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