AO-4, from the Marine Mammal Science announcement. (arrow added)

On October 28, 2006, fisherman that were capturing individuals of a group of 118 bottlenose dolphin (Tursiops truncatus) near Taiji, Japan for exploitation in aquaria noticed something peculiar about one of the captured individuals. While the vast majority of dolphins have only two front flippers one particular female had a set of small pelvic flippers. Many whales (particularly baleen whales) have the vestiges of hips and leg bones inside their bodies but a whale with external pelvic fins is an even rarer find. A new paper released online in the journal Marine Mammal Science provides an update on this individual, dubbed AO-4 and now residing with another captured pod member (AO-1) at the Taiji Whale Museum.

Unfortunately that’s about all the new article says. The short communication ends with a call to organize a team to undertake a detailed study of AO-4;

In order to conduct wider interdisciplinary research, we wish to organize a project team to study the comparative morphology, genetics, physiology, and life history of these two dolphins involving interested scientists from various research fields. We believe that keeping AO-1 and AO-4 together will be useful for comparative studies and to maintain the long-term mental health of this unusual dolphin.

I definitely want to find out more about AO-4 and why she has pelvic fins (their internal anatomy, the changes during development that could have led to their expression, etc.) but I have to admit that I am a bit saddened by the story. After my last trip to Sea World in 2006 I’ve become especially concerned with how cetaceans are kept in captivity and I can only conclude, particularly in the case of marine amusement parks, that cetaceans are often kept in an ethically unacceptable manner. (See the particularly egregious case described in Lolita: Slave to Entertainment as an example.)

Many people say that they are amused by the beauty, power, and intelligence of cetaceans, but these fantastic animals are kept in cramped, plexi-glass lined pools that do not in any way even approach their natural habitat. I know I would go out of my mind if I was stuck in a concrete room every day of my life, and I do not understand why we see fit to confine some of the most intelligent animals on the planet in such unacceptable conditions for our amusement. Cetaceans in captivity often suffer from physical and mental illness, becoming bored and even aggressive towards trainers, their tank mates, and even themselves. Being that “Shamu” is Sea World’s cash cow, though, I don’t see them changing their ways anytime soon.


A bottlenose dolphin (Tursiops truncatus) with fin-shaped hind appendages


  1. #1 Chromosome Crawl
    June 20, 2008

    Is it possible that the fins could be remnants of a parasitic twin – similar to cases seen in humans?

  2. #2 Laelaps
    June 20, 2008

    C.C.; I don’t think so. The genetic underpinnings for the development of pelvic bones and fins are still present to some degree in cetaceans, they just are not expressed in the adult form. During development embryonic dolphins develop hind limb buds by about 24 days and almost entirely have them resorbed by about 48 days (just like their teeth), so unless the future research turns up something entirely different I would say this is an atavism due to a change in development.

  3. #3 Zach Miller
    June 20, 2008

    The fact that the dolphin has pelvic FINS implies that the animal’s body knew how to create fins instead of limbs. So instead of tetrapod-style tiny legs, the dolphin has tiny hindfins.

    Doesn’t THAT imply that, sometime in the history of cetaceans, whales had hindfins?

  4. #4 Laelaps
    June 20, 2008

    Zach; I think the terms fins and limbs might lead to some confusion. If you woke up tomorrow and your hand looked like an ichthyosaur fin but still had all the same bones inside, would it be a fin or a limb? Like I said, I’d like to know whether there are bones inside the fins of this dolphin and what they look like. Take Basilosaurus isis, for example. Greatly reduced hind limbs were found but they probably looked like fins on the outside, and even if they were streamlined they still may have increased drag.

    So yes, for at least some point in their history whales had “hindfins” but the internal anatomy, from what I’ve been able to tell, looked like limbs (again, think about the internal structure of ichthyosaur and cetacean flippers).

  5. #5 Zach Miller
    June 20, 2008

    I know, my terms weren’t that great. I was trying to get a point across poorly. I suppose, ultimately, my question is that: did whales have hindlimbs modified into fins/flippers in the past? And my answer, based on this dolphin, would be “yes.”

    NOW my question is, how widespread do you think hindlimb flippers are among extent (or extinct) cetaceans? If fully-formed hindlimb flippers are turning up in a modern dolphin, I imagine the genes responsible for expressing hindlimb flippers haven’t been dormant for too terribly long.

  6. #6 Steve
    June 20, 2008

    Has anyone written to the authors to see if there are observations that the fins are active during swimming and if so, are they an aid or an impediment (etymological pun intended)?

  7. #7 Laelaps
    June 20, 2008

    Zach; Like I said I am curious to see what the osteological characters of the flippers are. Internally, that is, do they resemble limbs? (Which is what I would expect.) Like I also said in the post, the vestiges of hips, femurs, and lower leg bones are known in baleen whales; they are much much rarer in odontocetes.

    The hind flippers of Basiolosaurus show the the hind flippers were almost entirely gone even in the Eocene. Unless we start finding more hind limb bones separated from the spinal column/sticking out of the body in younger cetaceans like Aetiocetus I don’t think pelvic flippers were around very long. From what I understand the genes responsible for the development of the flippers are also involved in organizing other parts of the body so even though the expression of flippers is developmentally constrained those genes still have functions elsewhere in the body and so every now and then there’s a mutation that allows the limb buds in dolphins (that are usually resorbed around 48 days of development) to be more fully expressed. Although the hind limbs/flippers of cetaceans would have preserved poorly as they became reduced it seems likely that the vestiges of the pelvic flippers might have been entirely internalized by close of the Oligocene if not earlier during the epoch.

    Steve; There was no contact information or author listings on the announcement. Maybe it is because it was released online early, but details were thin overall. I assume that one could contact the Taiji Whale Museum and find out more. From what the article says the researchers know almost nothing, as yet, about the fins and the questions you ask are ones they want to work out.

  8. #8 windy
    June 21, 2008


    Not necessarily. Development of front-and hindlimbs would have largely utilized the same genes, and the instructions for “make leg here” have been modified into “make flipper here”, so that’s what gets expressed when the body tries to make a hindlimb now.

    Compare this with the humans that have “atavistic” tails – the tails aren’t covered with monkey fur, although our most recent tailed ancestor would have had a furry tail.

  9. #9 Aaron
    April 11, 2011

    I’m still looking for x-rays of the “limbs”.

    A limb is just any protruding appendage – so a fin can be considered a limb – but these limbs don’t have to be considered “legs.”

    Indeed, these hind fins are more likely to be caused by a hox mutation where the DNA code for the front limb is expressed in the rear. These mutations can occur no matter what the proposed ancestor of a species is. Lab experiments have been done where chickens were born with extra limbs in places where no limbs are supposed to grow. It’s a matter of inserting the right genetic material in the right place.

    The fact that these appendages look exactly like front fins is indicative to me that it is just a genetic malfunction, not an atavistic reversion.

    The same is true of larger mysticetes with instances of protruding hind limbs. In the case of a sperm whale, the x-ray showed the bone structure in the hind fins and they were much more like the front flipper bones than the hind limb bones of Basilosaurus.

    Most scientists believe that Basilosaurus limbs were protruding limbs that could clasp – probably used as sexual claspers – and not in the shape of a fin (as suggested above).

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