Abdominal fat pads act as control surfaces in lieu of dorsal fins in the beluga ( Delphinapterus )

Almost all cetaceans have a dorsal fin, which acts as a surface to control attitude and prevent roll, yaw, and side-slip (Fish 2002, 2004). Among odontocetes, only right whale dolphins (Lissodelphis), the finless porpoise (Neophocaena), and both members of Monodontidae, the beluga or white whale (Delphinapterus) and narwhal (Monodon), lack a dorsal fin. Both Delphinapterus (whose generic name refers to its absence of a fin) and Monodon possess, like Neophocaena, an irregular, notched dorsal ridge running along the caudal portion of the back (Struthers 1895, Brodie 1989, Hay and Mansfield 1989), but the low (4–5 cm high) dorsal ridge of monodontids (also called a cuticular crest by Kleinenberg et al. 1964) appears insufficiently large to act as a suitable hydrodynamic control surface on these 4–5 m long odontocetes. Based on kinematic analysis of locomotion in captive and wild beluga whales and dissection of abdominal musculature of Delphinapterus, we present a previously undescribed morphological feature that we believe allows belugas to enhance vertical stabilization and especially to control roll during turns and during swimming while in a partially rotated or wholly inverted position. This stabilization involves a pair of distinct fat (blubber) pads running longitudinally along the ventrolateral aspect of the abdomen, parallel to the dorsal ridge. These pads form large, elevated ridges that can be seen clearly (Fig. 1–3) in various views of Delphinapterus. The ventrolateral fat pads are in line with the pectoral flippers and extend from just caudal of the axilla to the level of the pelvic bones. The abdominal fat comprising the elevated ridges does not consist of discrete, paired bodies of isolated, encapsulated adipose tissue (as in the mandibular fat body of odontocetes). Rather, it involves a pair of unusually thick, longitudinal blubber deposits; it entails more than mere bulging of uniformly thick abdominal blubber. We contend that during some swimming maneuvers (inverted swimming, longitudinal rolls, and whole body turns), the fat pads formed by these markedly thickened blubber deposits are tensed and raised by abdominal muscles to form a pair of large, rounded erect structures (elevated 10–14 cm higher than the adjacent trunk and tailstock; 107–119 cm in length and 8–12 cm in width, averaging roughly 128 cm2 in area), often including a focal bulging of the ridges near their caudal terminus which results in fin-like protuberances surrounding a crater-like, semicircular depression on the ventral surface of the animal (estimated at up to 16 cm deep; Fig. 3). Based on our qualitative and quantitative kinematic