RESEARCH ARTICLE Diet of the social groups of long-finned pilot whales (Globicephala melas) in the Strait of Gibraltar R. de Stephanis Æ S. Garcı ´a-Tı ´scar Æ P. Verborgh Æ R. Esteban-Pavo Æ S. Pe ´rez Æ L. Minvielle-Sebastia Æ C. Guinet Received: 17 September 2007 / Accepted: 2 March 2008 / Published online: 27 March 2008 Ó Springer-Verlag 2008 Abstract The Strait of Gibraltar is inhabited throughout the year by a group of pilot whales (Globicephala melas), but their spatial distribution varies between Summer and Autumn. In this paper, we have used carbon ( 13 C/ 12 C) and nitrogen ( 15 N/ 14 N) stable isotope signatures to investigate the differences in diet amongst seasons, sex and stable social units. Skin samples were collected from 56 indi- vidually photo-identified pilot whales during Autumn 2005 and Summer 2006. These individuals were genetically sexed and their isotopic signature determined. The level of inter-individual association both within and between stable social units were compared to Euclidean distances between individual isotopes signatures. No differences in either d 15 N or d 13 C were found according to the sex of individ- uals, but significant seasonal differences were found in d 15 N, although not in the d 13 C values. This suggests that pilot whales are resident year round in the Strait, a finding supported by independent photo-identification. The varia- tion in d 15 N could reflect a shift in pilot whale diet through the year, with pilot whales feeding at a higher trophic level in Autumn compared to Summer. This could also represent a change in the diet of pilot whale prey species. The d 13 C values were significantly different amongst the four stable social units sampled and individual d 13 C values were significantly related to the level of inter-individual asso- ciation, while no relationship was found for d 15 N. These results suggest that within the same general area (i.e. the Strait of Gibraltar), there is some level of specialisation in habitat or prey choice between pilot whales social units. Introduction The Strait of Gibraltar is the narrow and shallow connec- tion between the Mediterranean Sea and the Atlantic Ocean (Fig. 1). The water circulation in the Strait is characterised by (a) a surface inflow of Atlantic waters which is driven by the excess of evaporation over precipitation in this basin and (b) a deep outflow of dense Mediterranean water (Lacombe and Richez 1982). The Strait is also character- ised by mixing processes through a pulsed upwelling induced by the tides and constrained by the bathymetry (Echevarrı ´a et al. 2002). The interface between the Atlantic surface waters and the deep Mediterranean waters generally takes place at a depth between 50 and 200 m, depending on the geo- graphical location and intensity of the tidal flows. The boundary between Atlantic waters and Mediterranean waters is deeper from the Spanish coast to the Moroccan coast (north to south) (Reul et al. 2002) and from the Atlantic to the Mediterranean (east to west), from Communicated by R. Lewison. R. de Stephanis (&)  P. Verborgh  R. Esteban-Pavo  S. Pe ´rez CIRCE, Conservation, Information and Research on Cetaceans, C/ Cabeza de Manzaneda 3, Algeciras-Pelayo, 11390 Cadiz, Spain e-mail: renaud@stephanis.org P. Verborgh  C. Guinet Centre d’E ´ tudes Biologiques de Chize ´, CNRS UPR 1934, 79 360 Villiers en Bois, France S. Garcı ´a-Tı ´scar Department of Ecology, Universidad Auto ´noma de Madrid, Madrid, Spain L. Minvielle-Sebastia Institut de Biochimie et Ge ´ne ´tique Cellulaires, Universite ´ Victor Segalen Bordeaux 2, CNRS, 1, rue Camille Saint Sae ¨ns, 33077 Bordeaux Cedex, France 123 Mar Biol (2008) 154:603–612 DOI 10.1007/s00227-008-0953-8