Sex differences in diving at multiple temporal scales in a size-dimorphic capital breeder

1. Air-breathing marine predators can modify their diving behaviour to accommodate different patterns of prey abundance and distribution and may do so at multiple temporal scales (individual dives, bouts of dives and foraging trips). 2. The grey seal, Halichoerus grypus, is a size-dimorphic, capital breeding species in which sex differences in foraging ecology have previously been found at the scale of individual dives. In this study, diving behaviour of male and female adult grey seals was examined at the temporal scales of dive bouts (41 males/46 females) and foraging trips (7 males/9 females) during the 8-month prebreeding foraging period between 1992 and 1999 to investigate whether sex differences were evident at longer temporal scales. 3. Foraging trip duration did not differ between males and females. However, females (the smaller sex) spent significantly more time diving and less time hauled-out on land between trips than males. This suggests that females are more selective than males when searching for prey. These differences are consistent with the effects of body-size dimorphism on diving behaviour. 4. Virtually all dives occurred in bouts. At this scale, males and females showed significantly different seasonal patterns of most bout characteristics, including bout duration, percentage of bout spent at depth and effort (hours spent in bouts day(-1)). 5. Grey seals used four different types of bouts, differing in duration, depth, percentage bout spent at depth and the shape of individual dives within the bout. Males and females differed in the proportion of each bout type used and in the seasonal and day-night distribution of bout types. 6. Few bout characteristics varied significantly among years, despite interannual variation in prey abundance. Where interannual differences did occur, both sexes behaved in a similar way. 7. Different seasonal patterns in bouts of diving by males and females could not be accounted for by body-size dimorphism or niche divergence, but were consistent with the hypothesis that females must recover condition earlier in the year than males to support the energetic costs of reproduction.