“Why keep your mouth shut?” Inshore South African Bryde’s whales abort lunges within feeding dives

Rorqual whales exhibit foraging plasticity to maximize their energy intake and reduce the cost of feeding manoeuvres. Bryde’s whales are known for their inter-population differences in the way they lunge, typified by acceleration, engulfment and filtration. This study presents the first description of potentially intentional interruption of lunge feeding, with whales not opening their mouth at the point of maximum speed during the acceleration, which usually corresponds to engulfing the prey. Three different hypotheses are proposed to explain the reason for aborting a lunge: (1) when aborted lunges occur isolated in time and are not followed by true lunges–defined as conventional lunges with engulfment and filtration–the reason may be that whales simply fail at catching the prey; (2) when whales abort lunges in sequence within the same feeding dive, a prey herding scenario is suggested, where whales may actively work around the prey to either reduce the prey ball size or herd the prey into a single bigger ball to maximize food intake during the next lunge; (3) to minimize the cost of the lunge, whales may also not open their mouth after sensing insufficient prey density to justify the drag costs of the lunge operation. The documentation of this foraging behaviour forms the basis to investigate further how rorquals may adjust their feeding ecology around the optimal foraging theory.