Sex differences in grey seal diet reflect seasonal variation in foraging behaviour and reproductive expenditure: evidence from quantitative fatty acid signature analysis

1. Intraspecific variation in diet can be an important component of a species niche breadth. We tested the hypothesis that sex differences in seasonal foraging behaviour and energy storage of sexually size dimorphic grey seals Halichoerus grypus (Fabrisius 1971) are reflected in differences in the diet and niche breadth. Diet composition was estimated for 496 adult (226 males, 270 females) and 91 juvenile (46 males/45 females; all 6 months old) grey seals sampled between 1993 and 2000 using quantitative fatty acid signature analysis. Niche breadth and overlap were estimated using the Shannon-Weaver diversity index (H') and the Morisita-Horn index (C-H), respectively. 2. Sand lance Ammodytes dubius (Reinhardt 1837) and redfish Sebastes sp. (Cuvier 1829) accounted for a high proportion of the diet in both sexes and age groups. However, the diets of adult males were significantly more diverse across all seasons (H': males 0.36 +/- 0.007 vs. females 0.28 +/- 0.007) and less energy dense in spring (male 5.3 +/- 0.07 kJ g(-1) vs. females 5.6 +/- 0.09 kJ g(-1)) than those of adult females. 3. Season and sex explained most of the observed variation in adult diets, but there were significant sex-season interactions. These differences were most evident during the post-breeding (spring) foraging period when energy acquisition is important to female recovery of nutrient stores needed to support pregnancy. Females selected fewer and higher quality prey species in spring than males. 4. There were no sex differences in the diets of juvenile grey seals. Although many of the species overlapped with those eaten by adults, juvenile niche breadth (H': 0.41 +/- 0.014, n = 91) was significantly broader than that of adults (H': 0.30 +/- 0.011, n = 115). Juvenile diets were also of lower energy density (5.3 +/- 0.04 kJ g(-1)) than those of adults (5.6 +/- 0.09 kJ g(-1)), suggesting less selectivity in these young and relatively naive predators. 5. Sex-specific seasonal changes in diet correspond to seasonal changes in diving behaviour and rate of body energy accumulation of adult males and females. Sex-specific reproductive requirements appear to be a primary factor generating the intraspecific variation in the seasonal foraging ecology of this large marine carnivore. However, sex differences in the breadth and energy content of diets also suggest the influence of body-size dimorphism as a factor shaping the diet of this species.