Life Hack: Grow a Dorsal Fin
It’s the sure sign of a good time: six jagged dorsal fins circling the lifeboat of huddled shipwreck survivors. Somewhere in the distance a tuba slowly alternates between F and F sharp. The dorsal fin has become one of the most recognizable cinematic elements of all time, but … what does it do? Besides informing humans of their imminent peril, the dorsal fin is one of the most useful and ubiquitous aquatic appendages ever invented.
Everybody’s Doing It
Dorsal fins are one of the primary examples of convergent evolution. When completely unrelated organisms develop similar outward characteristics, it is likely due to similarities in ecological niche and environment. As an example: the most recent common ancestor of the North American porcupine and the African Crested porcupine was not a porcupine. Sharks, ichthyosaurs, and porpoises all descended from very distinct ancestors, yet they converged on very similar body plans (all of which include dorsal fins). Contrary to structures that arise from convergent evolution are vestigial structures. These are structures that were once useful to an ancestor, but no longer of use to the descendent. The tiny leg bones in snakes and the human tailbone are just a couple of examples. Structures that arise from convergence, independent of ancestry, are generally considered particularly useful to the survival of any animal in a similar environment.
Stability
So why would a fish, a reptile, and a mammal require the same fin to be successful hunters? One well understood answer is that dorsal fins provide stability while swimming. Swimming (and flying) bodies are subject to three different forces: yaw, pitch, and roll.
When a body turns quickly, it is naturally subject to rolling forces that try to turn it on its side. If you’ve ever seen someone in a movie roll their car up onto two wheels after turning at high speed it’s similar to that. Almost all fish have dorsal fins, which allow them to counteract these rolling forces. By increasing the surface area that opposes the rolling motion, drag is increased above the body and rolling is prevented. This allows large aquatic hunters to make quick turns without losing sight of their prey.
Thermal Regulation
A second, less understood, advantage of the dorsal fin to large aquatic life is thermal regulation. Computers have finned surfaces called ‘heat sinks’ that allow for more effective cooling of circuit elements. By increasing the surface area exposed to air, fins increase the amount of convective heat transfer out of the element. The ears of a jackrabbit and the dorsal sail of the Dimetrodon both provide this same advantage to animals seeking to cool off.
The use of fins is especially justifiable when placed on a surface exposed to air rather than water. The rate of convective heat transfer is the product of the surface area, temperature difference, and fluid convection coefficient. Water is generally about 25 times faster than air at receiving convective heat from a surface. For this reason, whenever a surface separates a liquid from a gas, like the walls of a car radiator, fins are used on the gas side to aid in convection.
So if fins are only justifiable when exposed to air, what advantage could they provide to aquatic animals? Living in the ocean as a mammal is a constant uphill battle against heat transfer from the body (37 °C) to the ocean (17 °C on average). Aquatic mammals have developed some ways to maintain a thermal steady state.
The ideal body shape for conserving heat is a perfect sphere. Any dimple or bump that deviates the shape from a sphere increases the surface area-to-volume ratio, and increases the outbound heat flux. A perfectly spherical dolphin wouldn’t be able to do many of the things dolphins enjoy, so there is some compromise. Porpoises and whales have rounded body shapes to reduce their surface area-to-volume ratios. Most of them have lost even vestigial structures of hind legs to further decrease that ratio.
Keeping this in mind, a dorsal fin looks like a deliberate attempt to increase the surface area-to-volume ratio. Why go to such great lengths to decrease surface area, and then turn around and grow a dorsal fin?
The answer is that we’ve only talked about conserving heat so far. Maintaining a somewhat steady body temperature includes the occasional release of excess heat, such as during exercise or swimming in warm water. It is useful to think of the body of a porpoise or whale as divided into a heat-conserving volume and a heat-releasing volume. The central trunk, where most of the vital functions occur, is shape-optimized to conserve heat. The appendages, including the dorsal fin, are shape-optimized to release heat. The body can modify its spatial allocation of blood flow to indicate to what degree these volumes will perform their thermoregulative functions.
The dorsal fin, thanks to its vascular anatomy, is a dynamic thermal surface. When heat conservation is needed, blood returning to the heart from the dorsal fin passes by a large artery full of warm outbound blood. This counter-current heat exchanger — called the rete mirabile (Latin for “wonderful net”) — prevents cold blood from entering the heart and decreasing the internal body temperature. When heat expulsion is needed, the artery dilates and crushes the neighboring veins, cutting off their blood flow. Blood returning from the dorsal fin follows an alternate path where it is not heated before returning to the heart, lowering the internal body temperature.
Sharks have a similar need to dissipate excess body heat despite being technically cold-blooded. If dinosaurs have taught us anything as a scientific community, it’s that warm-bloodedness falls on a spectrum. Great whites and makos are able to generate a large amount of internal heat to be more energetic predators. These sharks use the same rete mirabile to maintain a high internal body temperature.
