Ducks with 6 limbs are not caused by genetic changes!


Cool: here’s a duck with four hindlimbs.

I have to gripe about the description, though:

A rare mutation has left eight-day-old Stumpy with two extra legs behind the two he moves around on. … The mutation is rare but cases have been recorded across the world.

No, no, no. This is almost certainly not the result of a mutation, and it’s one of my pet peeves when the media makes this wrong assumption, that every change in a newborn is the product of a genetic change. This is the result of a developmental error, not a genetic one, most likely caused by a fusion of two embryos in a single egg.

(via Apostropher)


  1. says

    So if it’s a developmental error, as you say, it’s not heritable. Too bad, it’s something along these lines (in terms of phenotype, anyway) that could lead to six-limbed mythical beasts like dragons, pegasi, and hippogriffs.

    So, what kind of heritable (genetic) changes would be required to generate hexapod vertebrates?

  2. yuppituna says

    along the lines of Brummell’s question – it possible that a tendancy for such a developmental error to be heritable?

  3. says

    yuppituna: No, it’s not possible for developmental disorders to be inherited.

    Something like polydactyly, however, is a genetic mutation, thus it is inheritable. (Polydactyly is when animals have more than the usual number of digits on a hand)

  4. Sonja says

    Question: What do duckies do?

    Hypothesis: God made duckies with many parts that work together so they can do lots of things.

    Conclusion: God made this ducky with even more parts so he can do lots and lots of things!

  5. says

    So if its the fusion of two embryos into one (that’s a chimera right?), does this duck have two souls? What will happen when it goes to duck heaven?

  6. Michael says

    A duplication in a HOX gene cluster might do the trick. Such a thing has been seen before (at least in evolutionary history) in the duplication of vertebrae and isn’t too terribly unlikely (all things considered). It’s Dawkin’s “stretched DC-8 analogy).

  7. says

    A duplication in a HOX gene cluster might do the trick.

    OK, that makes sense. However, it can’t possibly be true. The reason it can’t be true is that there are a zillion species of tetrapods living and in the fossil record and not one of them has evolved … let’s see … multipedia. You would think it would happen if it could happen. (Of course, I’m a man of faith. In Natural Selection)

    The only examples I can think of are dragons and winged monkeys, and they don’t exist.

    So what is the fundamental reason this cant happen? Because they would find no one to mate with? I don’t think we can accept that answer though it is a possibility.

  8. says

    Yeah, I don’t think the Hox duplication would be sufficient. This is a case where we’d need reiteration of a pattern of Hox expression along the A/P axis, and that’s not simply a trivial consequence of making an extra copy. It’s also the case that the Hox genes are locked into multiple roles: the same genes expressed along the A/P axis are involved in patterning the limb itself along the proximo-distal axis. A change that induced an extra limb would also induce malformations in all of the limbs.

  9. T. Bruce McNeely says

    Fusion of two embryos is the explanation for conjoined twins in humans (and other animals) and also accounts for 2 headed snakes and turtles, as well as certain other congenital anomalies. This duck is most likely the same. It’s not a mutation and it’s not heritable. Chang and Eng Bunker, the original “Siamese Twins”, had a large number of children between them and none were conjoined twins. Now if we could only get the news media to stop confusing viruses and bacteria…and monkeys might fly out of my butt.

  10. SteveF says


    This is a kind of related question and hopefully you can offer a few quick thoughts; a creationist I have come across recently has been arguing that classic examples of selection are actually simply down to phenotypic plasticity (PP) and that the two have often been confused.

    He cited a couple of examples, one being Losos relatively recent work on lizards – I read the paper and they do suggest that some morphological changes are the result of PP and not evolution in action. They go to argue that PP can’t account for all variation so this rebuts the creationist.

    He also makes the claim about Darwin’s finches, stating that the changes in beak morphology (mediated by BMP4) are solely PP and not selection at all. I’m not overly familiar with Darwin’s finches, but have quickly scanned Grant’s paper in the Science (I think the 2002 one) and I’m not sure if they do explicitly differentiate between the two. I might just be missing something, but they don’t seem to refer to plasticity at all. Could selection and PP have been conflated in this particular case and are there other good examples of where the two are clearly seperated so we can see natural selection at work? Or am I just not understanding?


  11. John says

    “This is the result of a developmental error, not a genetic one, most likely caused by a fusion of two embryos in a single egg.”

    Developmental with two embryos and two yolks in a single eggshell. Production of double-yolked eggs is probably heritable and might increase incidence of chimeras, but it would still be a developmental error. Four-legged birds are not terribly rare.

    Google results:,2106,3839522a4560,00.html

  12. Hank Fox says

    I’ve been seeing a great number of stories lately in mainstream media that amount to a modernized version of the old Freak Show.

    Human or animal either one, it always bothers me. I look at this spotlighted duckling and I hear actor John Hurt’s voice as John Merrick, the Elephant Man: “I am not an animal! I am a human being! I…am…a man!”

    Or in this case, a duckie.

  13. Russ says

    How is it that we know that this is not genetic but rather developmental? To actually know that would we not need to be in possession of a gene map? My formal genetics training ended thirty years ago and consisted of only a few college classes.
    Thank you.

  14. Sonja says

    Thanks for the links John. This paragraph made my day:

    “Looking ungainly on its extra legs but twice as cute, the bird was an exception to the rule that chickens with defects are not normally born alive.”

    That’s writing!

  15. OhioBrian says

    “This is sewious!”

    (crickets chirping)

    No “Wonder Pets” viewers out there?

    (more crickets chirping)

    Never mind.

  16. says

    And the 3-eyed fish in “The Simpsons” — mutants? Related to the 3-fingered animals in Disney cartoons?

    “The Simpsons” had an episode remaking “The Night of the Hunter” — a 1955 Noir classic, based on the novel by Davis Grubb. It was adapted for the screen by James Agee and Charles Laughton (the only time he directed).

    Mitchum portrays Harry Powell–one of his hands tattooed with “LOVE” on its knuckles, the other tattooed “HATE”–who shares a prison cell with Ben Harper (Graves), and hears him talk about the money in his sleep. Harper is sentenced to hang for his part in a robbery, but hid the money from the robbery, and trusted his children John (Chapin) and Pearl (Bruce)–about ten and five years old, respectively–with the money’s location.

    Upon his release from prison, Powell masquerades as a preacher. He woos and marries Harper’s widow, Willa (Winters) in order to obtain the robbery money, and eventually kills her. A famed scene shows the dead Willa, seated in a Model T at the bottom of a river.

    Anyway, in The Simpsons, because of the reduced number of fingers, the villain has “LUV” and “HAT” tattooed on his knuckles, with a bar above the “A” to show that it’s prounced as a long A.

    Everybody talks about Cartoon Physics, running off a cliff and hanging there until noticing there’s nothing underfoot.

    I say, it’s time to talk about Cartoon Biology.

    For example, the symbiosis of Storks and human babies. The predator-prey network Dog-Cat-Mouse.

    And Donald Duck not wearing any pants when otherwise fully dressed. What’s up with that?

    Youth wants to know.

  17. MarkP says

    I predict that this poor little duckie is about to be in every creationist slide show as an example of how mutations are never beneficial.

  18. says

    Sorry for the long comment.

    I’m working* on a paper right now looking at tetraploids (salmon) and how their genomes are returning to a diploid state. Indeed, simple duplication of HOX wouldn’t work to give a vertebrate more legs – examination of HOX loci has provided some of the strongest evidence for two rounds of genome duplication around 600 million years ago (mya) in the jawed-vertebrate lineage, with another round of genome duplication around the time the Teleosts diverged (and super-radiated) from other ray-finned fish (~350 mya)(the divergence was so distinctive in its radiation that now it’s difficult to find a person, even among ichthyologists, who can rapidly name any of the non-teleost Actinopterygiians – can you?). Long story short, if duplication was all it took, then PZ’s zebrafish would have an odd number of limbs (paired fins), since they’ve got 7 HOX clusters (we have 4, Drosophila has 1)**.

    The question of how tetraploids form (and get a viable population going) is also very interesting. Fortunately for my manuscript, it happened to salmon before the time we’re interested in, and I don’t have to wrestle with that issue very much (though we’ll see what the reviewers say if/when we get around to submitting it). Anyway, the problem with drastic mutations (“hopeful monsters”) in general, including polyploids, is that they need someone to mate with. Lots of plants can self-fertilize, which probably contributes to the relatively high frequency of tetraploid (or octaploid, or…) plants in some families. Obligate-sexual animals (like most vertebrates) have a problem here. But for the purposes of this discussion, I’m not going to be concerned about ecological persistence until we’ve figured out the formative event.

    So, I’m wondering if anyone can think of some other mechanism that could result in a heritable change in a vertebrate (any vertebrate) that adds 50% more limbs. We’ll worry about finding our proto-dragon a mate after it hatches. As a random thought, is there any potential in the genes involved in the axial fins of fish? PZ is always pointing out how easy lateral symmetry is…

    *I say “working” when I’m basically stuck running in little grammatical circles. I need to send the damn thing to my co-authors, pronto, but I’m still afraid they’ll laugh at some of the things I’ve written.

    **I trust I have that essentially right, PZ. If I’m not, I have some rather significant editing to do…

  19. craig says

    “I predict that this poor little duckie is about to be in every creationist slide show as an example of how mutations are never beneficial.”

    I dunno, its on a duck farm. Unless the duck farm raises ducks for petting zoos and not for food, I predict that this little guy is going to have better survival odds than the rest of the ducklings on the farm.

  20. SEF says

    God made this ducky with even more parts so he can do lots and lots of things!

    Not necessarily …

    Because they would find no one to mate with?

    For a start, it depends on how well endowed it is. It needs at least one functional (both externally available and internally connected) cloaca. Then the developmental abnormality would have to have some sort of a heritable tendency (like six-fingeredness) to stand any more chance of being repeated in a descendant than in some random unrelated duckling. Otherwise the mating is irrelevant.

  21. says

    “heritable tendency (like six-fingeredness)”

    I can count such examples on the fingers of one hand. Or two cartoon hands.

    Were there Babylonian priests with hexadactyly, and thus the base-60 sexigesimal notation, still used in time measurements and angles?

  22. oddman says

    The imaginary god produced this abnormality to show the glory of an imabinary god. Much like births defects in children show the glory of an imaginary god.

  23. Dunc says

    Were there Babylonian priests with hexadactyly, and thus the base-60 sexigesimal notation, still used in time measurements and angles?

    Oh, I hate it when that comes up… I know that there is a perfectly good and rational reason for there to be 60 minutes in an hour (or a degree), but I can never remember exactly what it is… Something to do with constructing your instruments without any sophisticated measuring tools, I think.

    The reason it can’t be true is that there are a zillion species of tetrapods living and in the fossil record and not one of them has evolved … let’s see … multipedia.

    Doesn’t necessarily mean it can’t happen, but it’s a pretty strong indication that it’s not advantageous.

  24. jtdub says

    Re: SteveF comment

    Hey Steve.

    A book that seems to address your questions quite nicely is The Plausibility of Life by Kirschner et al. They argue that greater phenotypic plasticity is itself selected for (because it facilitates variation), and is still representative of evolutionary advance.

    Have any biologists read this book (alas, I am but a neuroscience journeyman)? And if so, what do you think of it?

  25. SteveF says


    Thanks for the reference, I definately need to read around the subject a little more. I have scanned this paper and importance of plasticity in evolution seems reasonable. I wonder if PZ has read this and what he thinks? I certainly enjoyed it and recommend the read:

    Pigliucci, M. et al. (2006) Phenotypic plasticity and evolution by genetic assimilation. Journal of Experimental Biology, 209, 2362-2367.

    In partial answer to my own question, I notice that this paper differentiates between PP and evolution:

    Latta, L.C. et al. (2007) Rapid evolution in response to introduced predators II: the contribution of adaptive plasticity. BMC Evol Biol, 7, 21-

    I still wonder about Darwin’s finches though; where does the line between plasticity and selection lie and has it been adequately demonstrated? I wouldn’t be surprised if it has, but I just don’t know about it!

  26. Ken Mareld says

    I’m not sure if this little guy will ever be able to fly, but better than any duck out there he will surely be a hell of a dancer. (two left feet and all).


  27. yeila says

    When you incubate eggs in an incubator you must turn the eggs regularly. If you don’t, you will get chicks with legs on backwards and stuff like that. I imagine the backwards legs are also developemental errors rather than genetic. You do see livestock born with defects like this quite often (if you see a lot of livestock that is). I’ve seen calves with an extra (short) leg twice.

  28. Caledonian says

    There are 360 degrees in a circle because the number 360 has the factors of 2, 2, 2, 3, 3, and 5. As a result, it can be evenly divided by many numbers – an important point if you haven’t developed the concept of fractions yet.

  29. says

    vertebrates evolving more limbs… surely snakes spend some time evolving more vertebrae? how does THAT work? what about those long centipedes? seems to me that the common ancestor in THERE had fewer in number. anybody know anythign about this?

    p.s. you can say that hymenoptera evolved ONE extra limb by messing with the segments between the thorax and abdomen making a flexible egglaying/stinging/squirting (formicines) limb out of the abdomen. but that was cheating.

  30. kurage says

    I’m betting there’s an excellent chance that this little ducky has double the usual number of genitals and anuses. Moral of the story: God made him this way so he could have lots of kinky duck sex.

  31. miko says


    I love the Kirschner and Gerhart book. I don’t think it says stunningly new things, but it says them very well and they explicitly articulate a framework for thinking about the role of development in evolution. Much deeper and richer than Carroll’s book, for my money. Right after I read it, I went to a seminar about the evolution of limb development, and how the fins-to-limbs transition may have involved the ability of muscle precursors to undergo an epithelial to mesenchymal transition. That is, a sheet of cells becomes an army of individually migrating cells. This allows a classic exploratory phenomena (in the Kirschner / Gerhart sense) that can accomodate a wide range of skeletal changes, and seems like a necessary condition for the dazzling array of vertebrate limbs we see around us.

    SteveF, the line is clear. I think you’re refering to Cliff Tabin’s work on finches, where the expression domains of BMPs have changed in size or position. This cannot be due to phenotypic plasticity and is almost definitely due to genotypic changes that lead to alterations in transcriptional regulation. Remeber, all the finches of a given type on a given island have the same beak phenotype. For PP to underly this observation, you would have to argue that there is something in the environment on one island–a chemical in the water, something about the temperature–that causes thicker upper beaks by regulating BMP expression, which just so happens to be good for eating the food on that island. It lacks parsimony and evidence. It would be nice to see the changes in regulatory DNA sequences that are causal for this, and I think we will once there are a lot more vertebrate genomes for comparative analysis.

  32. SteveF says


    Thanks for your thoughts. Yes, thats the work I’m referring to, specifically:

    Abzhanov, A. et al. (2004) Bmp4 and morphological variation of beaks in Darwin’s finches. Science, 305, 1462-1465.

    In the following paper, Losos et al, investigate the observation that species of Anolis lizards that use broad substrates have long legs, whereas those that use narrow surface have only short legs. Previously this had been assumed to be the result of selection, but the authors demonstrate that phenotypic plasticity plays a major role (though this can then presumably be acted on by selection) and so evolution and plasticity had actually been confused:

    Losos, J.B. et al. (2000) Evolutionary implications of phenotypic plasticity in the hindlimb of the lizard Anolis sagrei. International Journal of Organic Evolution, 54, 301-305.

    So, I wondered on this basis, how exactly the Grant’s experiments explicitly differentiate between plasticity and selection. I haven’t read their work in enough detail to be able to evaluate this question in any great depth (so I thought I’d ask PZ).

    In the lizard example, plasticity is effectively mimicing selection; the environment is such that a particular phenotype is advantageous and plasticity acts in such a way as to favour this. Could not something similar be happening here, with beak size being modulated by Bmp4 expression to suit a particular niche?

    As this scenario wouldn’t necessarily invoke genetic changes would it not arguably more parsimonious? This is particularly the case as some of the examples of morphological change that the Grants refer to happened very rapidly and throught the entire population:

    Grant, P.R. and Grant, B.R. (2002) Unpredictable evolution in a 30-year study of Darwin’s finches. Science, 296, 633-635.

  33. miko says

    Hi SteveF,

    I think the problem is there appears to be a correlation between beak shape and food resource exploitation. You’d have to argue that the environmental factors that act on the embryo are somehow biasing development toward a morphology that happens to be good for that environment in an indirect way… hard to imagine. Also, I don’t think there is intraspecies variation in beak morphology, which you would expect with PP. Likewise, closely related species in the same environment have very different beak morphologies (they exploit different resources), again arguing against PP.

    In Losos et al, they are looking at a feature (leg length) that develops in direct contact with the environmental determinant (substrate geometry). BMP patterning of the beak, and the development of recognizable beak morphology, occurs in the egg, and again it’s hard to imagine how the availability of a certain nut or insect could affect what’s going in there at all, let alone bias it toward a tool for getting that nut/insect.

  34. SteveF says


    Greatly appreciate your thoughts; this isn’t an area that I’m overly knowledgable about (to put it mildly).

    Good point about intraspecies variation; I guess you would expect to see some sort of continuum (or at least range of some kind) within a particular phenotypically influenced morphological category. I’m not sure if that is seen in the Losos paper, I’ll have to check later.

    With regards to the closely related species point, I’m not entirely sure I follow; the differentiation in environmental resource (irrespective of what it is), is what plasticity is responding to. Could this not be the case with the very different beak morphologies you mention.

    Next, I’m going to ask what might be a rather dumb question. Lizard leg size is responding directly to the environmental cue; is beak size not doing something similar? What is the developmental difference? Is it that changes beak morphology (and its control by Bmp4) are occuring in the egg as you say, but lizard leg length can change during the lifetime of the lizard? Does Bmp4 only act during development and is not an influence when an individual finch is hopping around the Galapagos?

    If the above is the case, then I see your point. However (and I’m about to ask an even dumber question), could you not apply to same logic to the genetic changes required for selection to occur? Is the availability of food resources any more likely to affect genetic change (particularly one that is felt repidly through a population) than plasticity regulated by protein expression during development? I guess it isn’t overly parsimonious for a population to simultaneously change its Bmp4 expression, unless it somehow “knew” to do so (which couldn’t be the case). That still leaves me amazed at the speed at which the genetic changes could be felt by a group of finches; how does a population feel these effects s rapidly (maybe I need to read the Grant paper again).

    Thanks again (I’ve been thinking aloud a bit here, so hopefully it all makes sense)

  35. Cris says

    Jonathan Vos Post: That Simpsons episode was actually based on the 1991 version of Cape Fear. In that film, Robert Deniro’s character had “Love” and “hate” tattooed on his knuckles — surely inspired by Night of The Hunter. So the connection is there, but it’s once removed.

  36. bennyp says

    1 : a significant and basic alteration

    I would think the extra 2 legs rate as ‘significant’.

  37. miko says

    Hi SteveF,

    Yes, the BMP functions during development to determine beak morphology, and not in the adult. The surfaces the lizard walk on act not embryonically, but during juvenile growth (while the young lizard is walking around) to influence final leg length. It is analogous (though probably mechanistically dissimilar) to the way the muscles we use more grow, while the ones we don’t use atrophy. It is fascinating to speculate on the plasticity mechanism (which itself is probably an evolved adaptation): one could imagine that the way the muscles are used on different substrates locally affects bone growth; or maybe mechanosensory information received in the brain triggers an endocrine (hormonal) response that controls limb growth.

    The genetic change that alters BMP expression was presumably selected for by resource availability, but it was not caused by it. Mutations are random, or at least pseudo-random. It likely occured in a single individual, not simultaneously throughout the population. It is non-intuitive how rapidly beneficial alleles can spread through a population until you look at population genetics models. Finches probably have a generation time of a year or two, so even in a “short” evolutionary time that is tens of thousands of generations. (Imagine what can happen in the nematode (round worm), which has a generation time of a couple days…). Cave fish completely lost their eyes, which requires several independent mutations, within 10s of thousands of years of living in a lightless environment.

    There are a lot of books on evolution for non-biologists that cover this stuff in as much detail as you like. I really like the graphics-rich “Introducing…” series for topics I am unfamiliar with. They are short, to the point, and have pictures!

  38. Steve Cuthbertson says

    I reckon that the FSM wanted to try for the duck waterspeed record – all four flippers at once should get it hydroplaning across the pond like nothing else! Sure beats snail racing (even my two african giants don’t move that quickly, but boy, can they eat!)

  39. SteveF says

    Hi Miko,

    Thanks very much for the help, that clarifies a lot. One final thing; when I was talking about rapid evolution, I was actually referring to a couple of years. Check out this paper:

    Grant, P.R. and Grant, B.R. Unpredictable evolution in a 30-year study of Darwin’s finches. Science, 296, 633-635.

    Evolution can be predicted in the short term from a knowledge of selection and inheritance. However, in the long term evolution is unpredictable because environments, which determine the directions and magnitudes of selection coefficients, fluctuate unpredictably. These two features of evolution, the predictable and unpredictable, are demonstrated in a study of two populations of Darwin’s finches on the Galapagos island of Daphne Major. From 1972 to 2001, Geospiza fortis (medium ground finch) and Geospiza scandens (cactus finch) changed several times in body size and two beak traits. Natural selection occurred frequently in both species and varied from unidirectional to oscillating, episodic to gradual. Hybridization occurred repeatedly though rarely, resulting in elevated phenotypic variances in G. scandens and a change in beak shape. The phenotypic states of both species at the end of the 30-year study could not have been predicted at the beginning. Continuous, long-term studies are needed to detect and interpret rare but important events and nonuniform evolutionary change.

    This was why I was thinking about plasticity; it is easier to imagine plasticity creating significant morphological change on these timescales than nat selection (or I find it easier anyway).

    I had in mind the kind of plasticity exhibited by the lizards, during their lifetimes as opposed to embryonically as with finches; resource changes, individual lizards can adapt plastically as they live and all members of the population do so (because it is appropriate to do so). With it occurring in the embryo in finches, there would have to be an approximately coincidental and simultaneous change in Bmp4 in the egg in all individuals and this is hardly likely. A link is required between the individuals within a population, i.e. genetic change and evolution. Moreover, the resource itself would have to directly affect the egg as you say and this is also not likely without some concious activity on behalf of the mother. Right, I’ve thought that through now and got it straight in my head!

    The speed of evolution in the Grants’ study still amazes me though! I’ll have to read it again and see what they say about that.

    Thanks again for your help.

  40. says


    I stand corrected. Good work! I may have been reviewing Science Fiction films online for 12 years, and may have helped to keep an underground music newspaper going in the early 1980s with movie theatre ads, but I do make mistakes. I’m wrong, you’re right. [took my wife years to teach me to say that].

    Transposons leaping from film to film, and then to TV animation?