1. Diving capacity generally increases with body size both within and among taxonomic groups because of the differential scaling between body oxygen stores and metabolic rate.
2. Despite being some of the largest animals of all time, rorqual whales exhibit very short dive times relative to other large divers because of the high energetic costs incurred during lunge feeding. This mode of filter feeding requires high drag for the engulfment of large volumes of prey-laden water, and the magnitude of both drag and engulfment volume is largely determined by the size and shape of the skull.
3. The positive allometry of rorqual skulls increases mass-specific engulfment capacity in larger whales, but the energetic requirements of feeding are also predicted to increase and thus further limit diving capacity.
4. To test the hypothesis that the energetic cost of a lunge is disproportionately higher in larger rorquals, we compared diving and lunge-feeding performance among three different-sized species (blue, fin and humpback whales) foraging on krill.
5. Our hydrodynamic analyses indicate that the mass-specific energy expenditure will increase with body size if rorquals lunge at length-specific speeds (in body lengths per second) that are independent of body size, a condition that is supported by tag data.
6. Although the absolute time required filtering each volume of water increased with body size, maximum dive duration and depth were not significantly different among species. As a consequence, the maximum number of lunges executed per dive decreased with body size.
7. These data suggest that, unlike all other true divers, adult rorqual species do not exhibit a positive relationship between body size and diving capacity. Larger rorquals forfeit diving capacity for greater engulfment capacity, a trade-off that favours the efficient exploitation of patchily dense prey aggregations. Such a trade-off may underlie different foraging strategies associated with resource partitioning, life history and ecological niche.