How do feeding biomechanics, extreme predator-prey size ratios and the rare enemy effect determine energetics and ecology at the largest scale?

Abstract

The most recent and largest radiation of marine filter feeders are edentulous baleen whales (Mysticeti) that use keratinized racks of fringed and matted baleen to filter zooplankton (e.g. krill) or small schooling fish (e.g. anchovies, sardines). Rorqual whales (Balaeopteridae) exhibit the greatest size range among mysticetes and employ a unique lunge-feeding mechanism whereby engulfment and filtration are temporally decoupled. As a result, lunge feeding confers the ability to rapidly engulf large prey aggregations, such as krill or schooling fish, followed by a prolonged filter phase. In contrast, engulfment and filtration occur at the same time in all other gigantic filter feeders (e.g. basking sharks, whale sharks) at slow speeds. Although lunges in rorquals occur at higher speeds, the extreme predator-prey ratios at play suggest that whales may not be able to overcome the escape abilities of scattering prey. These types of prey have been engaged in evolutionary arms races with smaller predators for tens of millions of years prior to the rise of today's ocean giants. Extant rorqual whales evolved gigantism only in the last few million years; thus, they represent rare enemies of small prey such that flight responses may be delayed until escape is less likely. Data from whale-borne movement-sensing tags, looming stimulus experiments and stomach contents suggest a potential trade-off in capture efficiency for different prey types (e.g. fish versus krill) with increasing whale body size. Such constraints likely shaped the ecology and energetics of foraging at the largest scales.