Narwhals can be identified by their fins, which are smaller than those of most other cetaceans and have a distinctive tip that curls up (Nweeia, et al., 2014). These whales do not possess a dorsal fin. A dorsal fin's main use is to keep marine animals from spinning while they swim. The reason they do not have this fin is that they have such strong flukes that they can use to stabilize themselves. The fluke is what the narwhal uses to push itself forward. Narwhals use their flippers to steer and navigate themselves to where they want to go. In place of a dorsal fin, narwhals have a dorsal ridge. "The dorsal ridge is about 5 centimeters in height and extends along the posterior half of its back. The pectoral fins are 30-40 centimeter in length and the tail flukes are normally 100-120 centimeters long. Narwhals have convex posterior margins around their tail flukes rather than concave or straight. Female narwhals have tail flukes with frontal edges that are swept back, whereas, a male narwhal has tail flukes that appear slightly concave in shape" (about animals, 2018). In a typical narwhal, 30 - 35% of their weight is blubber. This layer of fat, ranging in thickness from 50-100 mm, is essential for survival in Arctic waters. 

 

It is believed that this lack of a pronounced dorsal fin has evolved as an adaptation to navigate beneath ice-covered waters (Dietz, et al., 2007). A shallower dorsal ridge will allow narwhals to maneuver more easily when ventral side up closer to the inlet bottom. If the narwhal swam flat against the bottom with its dorsal side up, it would have the advantage of having its mouth closer to benthic prey than if it were upside-down. However, the lower jaw of a narwhal is fragile because it is hollow and thin-boned probably because it is used for sound reception. An open-jaw hitting a hard bottom at a speed of 12 m/s could cause substantial damage. Hence, protection of the jaw could be another reason for upside-down swimming (Dietz, et al., 2007). This study observed the supine behavior among both males and females, suggesting that supine behavior is related to the use of sonar and protection of the fragile lower jaw which is common in both sexes. The most likely explanation related to the presence of a dorsal ridge is the improved use of sonar as well as the protection of the fragile lower jaw.

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Narwhals have flexible rib cages that compress as water pressure increases while they descend. This supports their organs in deep water where pressures are extreme (Heide-Jorgensen & Garde 2011). Narwhal muscles, like those of other deep-diving whales and seals, are packed with myoglobin, a protein that boosts their oxygen-carrying capacity so they can dive for 25 minutes or more on a single breath. Narwhal muscle is made of primarily slow-twitch muscle fibers. These muscle fibers use oxygen more efficiently and are used for endurance instead of speed (Williams, et al. 2011).