The above photo might look like something from Ripley's Believe It or Not — but believe it. Some populations of spinner dolphin that live in the eastern tropical Pacific have bizarre backwards dorsal fins, a phenomenon that has left scientists and marine biologists scratching their heads.
"We've known about these dolphins for 50 years," said Matt Leslie, a graduate student at the Scripps Institution of Oceanography, in an interview with Scientific American. "but not a lot has been done to actually study why it's on backwards."
Leslie is currently raising funds for an experiment that he hopes will shed some light on the mystery. His plan is to build a model of the dolphin and put it in a flow tank. As water moves over the model, Leslie will be able to observe whether the backwards fin affects the mechanics of the dolphin's swimming ability.
Though the experiment has yet to be completed, there are a few key clues from aeronautical technology that suggest how the backwards fin may aid the dolphin's maneuverability.
Take, for instance, the illustration below exhibiting the unique, anti-intuitive aerodynamics of the Grumman X-29 experimental fighter plane:
Like the spinner dolphin's fin, these fighter planes showcase backwards wings. The design allows airflow around the wings to terminate at the body of the plane rather than at the wingtips, providing better stability and maneuverability. Could this also explain the dolphin's fin?
It's possible — but it turns out that the nautical efficiency of the dolphin's backwards fin is only half the mystery. The really peculiar thing about these dolphins is that only adult males exhibit the odd trait. Females and juveniles have normal-shaped fins. This leaves open an important question: Even if the backwards fin is proven to provide benefits for the dolphin, why would the trait only be seen in adult males?
Leslie has a hunch that the fin discrepancy has something to do with mate selection and/or sexual competition. Sexual dimorphism — phenotypic differences present between males and females of the same species — is common among animals in which one sex uniquely competes to attract a mate. Peacocks are a good example. Male peacocks exhibit flamboyant tail feathers that they use in their mating displays used to attract females.
Since only adult male spinner dolphins have the backwards fins, it's likely that females prefer males that feature them.
It may also be important to remember where spinner dolphins get their name: they are known for making acrobatic, playful leaps, often twisting their bodies around many times before falling back to the water. It's therefore possible that their dramatic leaps are actually mating displays, and that their backwards fins evolved because they help males make more theatrical aerial maneuvers.
Until Leslie completes his flow tank experiment, it will be impossible to know for sure. He offers a presentation regarding the mysterious backwards-finned dolphin, and a plea for funding, in the video below: