In the extensive geographical distribution of the common dolphin, several morphotypes of uncertain taxonomic status, identified by the relative length of their rostra, have been established. We investigated variation in skull morphometrics and isotopic signatures of carbon and nitrogen (δ13C and δ15N) in individuals from the subtropical waters of the North Atlantic Ocean, in order to assess population structure and taxonomic status. The distribution of the relative rostral lengths (RL) of individuals followed a cline with no subgrouping. Both δ13C and δ15N showed high variability, which suggests that individuals use habitat heterogeneously. δ15N correlated with RL, indicating that longer beaked individuals either feed at a higher trophic level and/or inhabit waters located further offshore than shorter beaked animals. Although δ13C and δ15N were correlated, RL and δ13C failed to show any correlation, possibly because the incremental effect of trophic level on δ13C has been offset by the potential allopatric distribution of the morphotypes. We conclude that both the long-beaked and short-beaked forms of common dolphin do occur off Mauritania but, in contrast to other areas, the existence of more than one species in the region is questioned because both stable isotopes and skull morphometric appear to reflect differential use of habitat rather than taxonomy. Even though proposed previously by some authors, this is the first time that skull differentiation in common dolphins has been demonstrated to be likely due to niche segregation and not to speciation. This reveals that caution is needed when considering that long-beaked and short-beaked common dolphins from outside the eastern North Pacific fall into the taxonomic model described for this region.