Biomechanically distinct filter-feeding behaviors distinguish sei whales as a functional intermediate and ecologically flexible species

Abstract

With their ability to facultatively switch between filter-feeding modes, sei whales represent a functional and ecological intermediate in the transition between intermittent and continuous filter feeding. Morphologically resembling their lunge-feeding, rorqual relatives, sei whales have convergently evolved the ability to skim prey near the surface of the water, like the more distantly related balaenids. Because of their intermediate nature, understanding how sei whales switch between feeding behaviors may shed light on the rapid evolution and flexibility of filter-feeding strategies. We deployed multi-sensor bio-logging tags on two sei whales and measured the kinematics of feeding behaviors in this poorly understood and endangered species. To forage at the surface, sei whales used a unique combination of surface lunges and skim-feeding behaviors. The surface lunges were slow and stereotyped, and were unlike lunges performed by other rorqual species. The skim-feeding events featured a different filtration mechanism from the lunges and were kinematically different from the continuous filter feeding used by balaenids. While foraging below the surface, sei whales used faster and more variable lunges. The morphological characteristics that allow sei whales to effectively perform different feeding behaviors suggest that sei whales rapidly evolved their functionally intermediate and ecologically flexible form to compete with larger and more efficient rorqual species.

Evidence of resource partitioning between fin and sei whales during the twentieth-century whaling period

“…For example, off the coast of the Falkland Islands in the western South Atlantic, fin and sei whales were both historically observed by local inhabitants predominantly between January and June (Frans and Augé 2016). The Falkland Islands are known feeding grounds for sei whales (Segre et al 2021); however, limited information is available for fin whales in this region and sightings are rare (GBIF 2021). In the North Pacific, fin and sei whales are observed during summer off the coast of British Columbia; however, sei whales are observed further offshore, likely associated with the abundance and distribution of copepods, whilst fin whales are associated with the availability of euphausiids (Flinn et al 2002).…”

Morphological variation of the relictual alveolar structures in the mandibles of baleen whales

“…This pattern was true across all four taxonomic families and in the fetal and subadult specimens as well. Only one whale in our dataset fell notably outside these values: USNM 571340 (Balaenoptera borealis) has an alveolar groove that is only 41% of the mandible's CLL.This anomalous datapoint may be the result of an unobserved pathology, ontogenetic variation, or linked to the unusual feeding biomechanics of sei whales (Segre et al, 2021).…”

Scaling of maneuvering performance in baleen whales: larger whales outperform expectations

“…Importantly, tail ruddering represents a method of rolling that does not rely on forward speed to generate lift over the flippers, meaning that we would not expect these rolls to follow the flipper-generated lift model for performance. The two sei whales were also performing a unique surface feeding behavior where they maintained very slow translational speeds while lunging and rolling at high rates for several hours ( Segre et al, 2021 ). For both gray whales and sei whales, the slow swimming speeds biased our rolling metric, which penalizes faster translational velocity (Eqn 5).…”