The study was conducted at La Mare aux Elephants (46°22′S, 51°40′E) located on Possession Island, Crozet Archipelago. Antarctic fur seals (Arctocephalus gazella) and subantarctic fur seals (A. tropicalis) breed sympatrically at this site, with a pup production of 164 Antarctic fur seals and 80 subantarctic fur seals during the study period [27] . Randomly selected breeding males and lactating females of unknown age were captured during December 2001 and January 2002, respectively. Male seals were sedated by intramuscular injection of a tiletamine-zolazepam mixture (Zoletil), while females were captured using a hoop net and placed on a restraint board. From each individual, a single whisker was collected by cutting with a pair of scissors as close to the skin as possible.
Prior to isotopic analysis, whiskers were hand-washed in 100% ethanol and then cleaned in distilled water for 5 min in an ultrasonic bath. Whiskers were measured, dried and cut into 3 mm-long consecutive sections starting from the proximal (facial) end. Sections were weighed on a microbalance within a range (0.03–2.06 mg) that produced meaningful isotopic measurements. Samples were then packed in tin containers, and carbon and nitrogen isotope ratios were determined by a continuous flow mass spectrometer (Thermo Scientific, Delta V Advantage) coupled to an elemental analyser (Thermo Scientific, Flash EA 1112). Results are presented in the conventional δ notation relative to PeeDee belemnite marine fossil limestone and atmospheric N2 for δ13C and δ15N, respectively. Replicate measurements of internal laboratory standards (acetanilide) indicate measurement errors of <0.15‰ for both δ13C and δ15N. Isotopic data on male A. gazella whiskers, previously reported in [26] (link)), have been incorporated in the present study to investigate intra-and inter-specific variations in fur seal foraging strategies.
Five additional lactating female A. gazella were sampled at Bird Island (South Georgia) in 1992 as part of other studies [28] . They were aged by counting growth layers of tooth dentin. Seal whiskers were cut into 5 mm long consecutive sections from the root. Isotopic analyses were performed in the laboratory of Donald Schell at Fairbanks (Alaska) in 1994.
Keratinous tissues (including whiskers) are approximately 3‰ 13C enriched in pinnipeds compared to their diet [29] . Taking into account both the keratinous effect and the latitudinal gradient in blood δ13C values of top predators in the Southern Ocean [14] , [30] , the isotopic positions of the polar front (PF) and the subtropical front (STF) for fur seal whiskers were estimated at approximately −19 and −16‰, respectively. The subtropical zone (STZ) is defined as the area north of the STF, the subantarctic zone (SAZ, where the Crozet Islands are located) as the area between the STF and the PF, and the Antarctic zone (AZ) as the area south of PF.
Prior to isotopic analysis, whiskers were hand-washed in 100% ethanol and then cleaned in distilled water for 5 min in an ultrasonic bath. Whiskers were measured, dried and cut into 3 mm-long consecutive sections starting from the proximal (facial) end. Sections were weighed on a microbalance within a range (0.03–2.06 mg) that produced meaningful isotopic measurements. Samples were then packed in tin containers, and carbon and nitrogen isotope ratios were determined by a continuous flow mass spectrometer (Thermo Scientific, Delta V Advantage) coupled to an elemental analyser (Thermo Scientific, Flash EA 1112). Results are presented in the conventional δ notation relative to PeeDee belemnite marine fossil limestone and atmospheric N2 for δ13C and δ15N, respectively. Replicate measurements of internal laboratory standards (acetanilide) indicate measurement errors of <0.15‰ for both δ13C and δ15N. Isotopic data on male A. gazella whiskers, previously reported in [26] (link)), have been incorporated in the present study to investigate intra-and inter-specific variations in fur seal foraging strategies.
Five additional lactating female A. gazella were sampled at Bird Island (South Georgia) in 1992 as part of other studies [28] . They were aged by counting growth layers of tooth dentin. Seal whiskers were cut into 5 mm long consecutive sections from the root. Isotopic analyses were performed in the laboratory of Donald Schell at Fairbanks (Alaska) in 1994.
Keratinous tissues (including whiskers) are approximately 3‰ 13C enriched in pinnipeds compared to their diet [29] . Taking into account both the keratinous effect and the latitudinal gradient in blood δ13C values of top predators in the Southern Ocean [14] , [30] , the isotopic positions of the polar front (PF) and the subtropical front (STF) for fur seal whiskers were estimated at approximately −19 and −16‰, respectively. The subtropical zone (STZ) is defined as the area north of the STF, the subantarctic zone (SAZ, where the Crozet Islands are located) as the area between the STF and the PF, and the Antarctic zone (AZ) as the area south of PF.
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