Minke whale feeding rate limitations suggest constraints on the minimum body size for engulfment filtration feeding

Bulk filter feeding has enabled gigantism throughout evolutionary history. The largest animals, extant rorqual whales, utilize intermittent engulfment filtration feeding (lunge feeding), which increases in efficiency with body size, enabling their gigantism. The smallest extant rorquals (7-10 m minke whales), however, still exhibit short-term foraging efficiencies several times greater than smaller non-filter-feeding cetaceans, raising the question of why smaller animals do not utilize this foraging modality. We collected 437 h of bio-logging data from 23 Antarctic minke whales (Balaenoptera bonaerensis) to test the relationship of feeding rates (lambda(f)) to body size. Here, we show that while ultra-high nighttime lambda(f) (mean +/- s.d.: 165 +/- 40 lunges h(-1); max: 236 lunges h(-1); mean depth: 28 +/- 46 m) were indistinguishable from predictions from observations of larger species, daytime lambda(f) (mean depth: 72 +/- 72 m) were only 25-40% of predicted rates. Both lambda(f) were near the maxima allowed by calculated biomechanical, physiological and environmental constraints, but these temporal constraints meant that maximum lambda(f) was below the expected lambda(f) for animals smaller than similar to 5 m-the length of weaned minke whales. Our findings suggest that minimum size for specific filter-feeding body plans may relate broadly to temporal restrictions on filtration rate and have implications for the evolution of filter feeding. Theoretical models of foraging efficiency suggest that lunge-filter-feeding marine vertebrates could be as small as 10 kg. However, here the authors show with bio-logged data from filter-feeding minke whales that in practice there are minimum body-size constraints on filter feeders, below which this becomes an unviable feeding strategy.

Automated summary

Bulk filter feeding has allowed for gigantism in evolutionary history. The largest rorqual whales use lunge feeding, which becomes more efficient with body size. However, smaller rorquals exhibit lower daytime feeding rates compared to larger species, suggesting a minimum size for efficient filter feeding.