Dorsal Fins, From Killer Whales to Narwhals

While some cetaceans have huge dorsals relative to their body size, others have smaller ones, and some species lack them altogether. Thus, the diversity of these fins is a continuum that ranges from imposing to non-existent.

In the killer whale, the size of the dorsal is pushed to the extreme. This species sports a larger dorsal fin than any other cetacean and its size varies according to the sex of the individual. Males feature much taller dorsals than females. This difference has yet to be fully explained, but could be attributable to females’ sexual preferences or other social or ecological pressures.

In the narwhal, the dorsal fin is completely absent, a characteristic that betrays the species’ arctic habitat. Arctic water bodies are often covered with pack ice or ice floes under which it is easier to navigate without a dorsal fin.

On the other hand, species with a small dorsal fin or none at all have less stability and, consequently, reduced ability to reach high speeds. This characteristic is offset by greater agility, however. A characteristic better suited to the lifestyle of cetaceans that inhabit more difficult-to-navigate environments such as rivers, estuaries or coastal waters. In river dolphins such as the Amazon river dolphin, manoeuvrability is key for (mostly solitary) foraging through narrow river inlets, underwater obstacles, or even forests in flooded riparian areas.

In cetaceans, the dorsal fin may also represent an additional tool in the regulation of body temperature. Using a countercurrent heat exchange system, a species with a dorsal fin can regulate the temperature of blood flowing through the arteries of this fat-free fin. This allows the animal to eliminate (when the blood is warmer) or conserve (when it is cooler) heat according to the needs of the moment.

The Pacific grey whale is another species that, much like the pilot whale, is found between temperate and cold waters. It does not have a dorsal fin, however. In this case, the thermoregulatory capacities of this fin are probably not as developed as those of the pilot whale due to the fact that its feeding technique (sucking up marine sediment) is much less active. Furthermore, it appears that contact between cold water and the inside of its enormous oral cavity offers it another solution to regulate its body temperature when foraging.