Why do some belugas have turned-up pectoral fins?

Fin curvature in some St. Lawrence belugas was first reported in 1943 by biologist Vadim D. Vladykov. While studying carcasses, he noticed that several individuals showed curved pectoral fins. This deformity is believed to be more prevalent with increasing age, in addition to being more pronounced and more frequent in males.

The aerial photograph gives the impression that the fins are flexible and undulate as they slip through the water. In reality, they are too rigid for that. Inside the fins of a beluga are bones that are very similar to those found in the human hand. But the phalanges of their five “fingers” are no longer articulated and therefore do not allow for movement. Stéphane Lair, veterinarian responsible for beluga necropsies, regularly observes this phenomenon and confirms that fins are fixed in this position and that “they cannot really be unfolded.” Moreover, Vadim D. Vladykov explains that beluga hunters used the curved fins of old bulls as a handle to pull the carcasses out of the water more easily.

Aside from formerly facilitating the work of beluga hunters, what good can come from these curved fins? Is there an advantage or disadvantage to this trait? The Vladykov study does not provide any conclusive answers. And the phenomenon does not seem to have been studied in more recent times. According to Stéphane Lair and Bill Van Bonn, VP of Animal Health at the Shedd Aquarium, curved fins do not negatively affect the animals’ ability to swim. Data on this deformity are therefore not collected, as it is considered non-pathological. For Stéphane Lair, it is likely a “geriatric change” (related to aging), which might be attributable to the constant pressure of water on the fins. Perhaps we can compare the curled fins of older belugas to wrinkles or baldness in humans, two phenomena associated with aging?

Fin deformities are observed in several other species of cetaceans. For example, narwhals, close cousins ​​of belugas, also have pectoral fins and a caudal fin that are curved upward in adulthood. On the other hand, some older killer whales have a caudal fin whose edges are curved downward.