Tetrapod Zoology

The Great spotted woodpecker shown here yesterday was, I think, an unusual individual, and thanks to everyone who had a go at explaining what it was that made her so odd. Unfortunately no-one got it right. Several of you noted that she appeared to be tridactyl on at least one foot, whereas she should be four-toed, with two toes pointing forwards and two pointing backwards (the zygodactyl arrangement). Incidentally, the common assumption that the zygodactyl foot is a climbing specialisation is probably not right (Bock & Miller 1959), but that’ll have to be a subject for another time. Anyway…

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I’m afraid that the apparent tridactyly of this individual is a quirk of death pose and camera angle, and in fact her feet were perfectly normal for her species (as usual, I took the opportunity to examine the ventral surfaces of the claws. As seems to be typical for birds, the lateral and medial margins of the claw sheaths descended further ventrally than the flattened ventral surfaces. This is quite different from the bony ungual, which is simply convex across its ventral surface). What was odd about this individual is the notched bill-tip she possessed on her left side: both upper and lower mandibular tomia possess concave margins, and together form an opening. The tomial margins look worn as if this was the result of some repetitive behaviour, and it didn’t seem to have arisen from breakage, post-mortem damage or anything like that. Unfortunately nothing is known about this individual’s life, and the bill notch wasn’t noticed when she was collected. I’ve never seen it before in a woodpecker and can’t find a record of anything similar.

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It’s also true (as Neil pointed out) that her bill was relatively long compared to that of other Great spotted woodpeckers. However, the species seems to be quite variable in bill length: I have another Great spotted woodpecker in my collection but it’s at Portsmouth and not with me now, but I’ll try and locate its rhamphothecae so I can compare them. Entirely coincidentally, a long-billed freak Great spotted woodpecker was photographed in North Yorkshire earlier this year: here’s a photo of it. Bizarre. Despite its incredible bill (c. 120% longer than it should be), the bird looked healthy and was apparently doing fine. You might recall from the article on bill dimorphism that some woodpeckers are sexually dimorphic in bill length.

Tongues, skulls, brains

It would be wrong to talk about a woodpecker’s head and not mention some of the other neat stuff we know about woodpecker bills, tongues and skulls. All woodpeckers – even those incapable of drilling and excavating wood (namely the wrynecks) – have an incredibly long, protrusible tongue anchored to hypertrophied hyoid bones that have grown backwards around the skull base and then up and over the skull roof. Different lineages have evolved different ways of storing the tongue’s base when the tongue is retracted. In some groups (e.g., megapicins and dendropicins) the tongue base is wound around the right eye, while in others (like malarpicins) it is stored in a cavity in the upper mandible. Except in wrynecks, the tongue’s tip is barbed, with bristles that vary among taxa in their number, size and stiffness. The tongue can be moved laterally when extended, and enlarged salivary glands coat it with a very sticky fluid [tongue of - I think - Red-bellied woodpecker Melanerpes carolinus below. From that episode of QI].

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This incredibly versatile organ is used almost like an adhesive, barbed limb or tentacle. It’s mostly used to grab insects, but can also be used to obtain nectar and pollen. Indeed some woodpeckers exploit flowers a lot and might be more important in pollination than previously thought (Rocca et al. 2006). Wrynecks, the most basal woodpeckers (Benz et al. 2006), lack the specialisations for excavation present in other woodpeckers but exhibit the very long tongue. This indicates that the long tongue evolved first and was initially employed in extracting insects from cavities; the excavation habit came later [image below shows two deceased Eurasian wrynecks Jynx torquilla. Although wrynecks belong to the group of birds that we call 'woodpeckers', they don't really peck wood and might be better thought of as 'proto-woodpeckers'].

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The woodpecker skull is relatively thick-boned, robust and with an immobile fronto-nasal hinge zone (in some woodpecker clades the frontals actually overhang and reinforce the base of the rostrum). Again, wrynecks are far less specialised and have a far more open palatal structure and a less reinforced skull (Burton 1984, Bock 1999). A surprisingly simple question that seems not to have been answered is whether woodpeckers hit wood with a closed or open bill. The action happens so quickly (at 600-700 cm/sec) that it’s not possible to tell with the naked eye, and I don’t know if high-speed photography has yet been used to answer this question. Winkler et al. (1995) thought that the bill tips were open on impact, and that the bill didn’t function as a single solid chisel.

How are woodpeckers able to withstand rapid, repeated percussion without sustaining brain injury? This was looked at by Gibson (2006), and the rather disappointing conclusions were that the small size of the brain and short duration of the impacts helped the brain withstand high deceleration, as did the shape of the brain (it’s longest axis is arranged dorsoventrally rather than anteroposteriorly). I say that these conclusions were ‘disappointing’ as I imagined that woodpeckers had evolved some sort of unique, shock-absorbing, brain-cushioning specialisations. It has in fact been suggested that the muscles at the tongue base might serve this function, but this can’t be true as these muscles wrap around the back and top of the skull and don’t have any contact with the brain itself.

As always there’s tons more to say – the woodpecker lineages vary a great deal in the sort of excavation behaviour they’re capable of – but I’ve run out of time and need to press on with other stuff.

Refs – -

Benz, B. W., Robbins, M. B. & Peterson, A. T. 2006. Evolutionary history of woodpeckers and allies (Aves: Picidae): placing key taxa on the phylogenetic tree. Molecular Phylogenetics and Evolution 40, 389-399.

Bock, W. J. 1999. Functional and evolutionary morphology of woodpeckers. The Ostrich 70, 23-31.

- . & Miller, W. D. 1959. The scansorial foot of the woodpeckers, with comments on the evolution of perching and climbing feet in birds. American Museum Novitates 1931, 9-45.

Burton, P. J. K. 1984. Anatomy and evolution of the feeding apparatus in the avian orders Coraciiformes and Piciformes. Bulletin of the British Museum of Natural History (Zoology) 47, 331-443.

Gibson, L. J. 2006. Woodpecker pecking: how woodpeckers avoid brain injury. Journal of Zoology 270, 462-465.

Rocca, M. A., Sazima, M. & Sazima, I. 2006. Woody woodpecker enjoys soft drinks: the blond-crested woodpecker seeks nectar and pollinates canopy plants in south-eastern Brazil. Biota Neotropica 6 (2), 1-4.

Winkler, H., Christie, D. A. & Nurney, D. 1995. Woodpeckers. Pica Press, Mountfield.

Comments

  1. #1 Diego
    August 12, 2008

    I concur, that looks like a red-bellied in your woodpecker tongue picture.

    I wonder– could the great spotted specimen have developed the notch through repeated tool use? I don’t know what she would have been manipulating and it seems pretty far out there, but it would be a deeply cool possibility.

  2. #2 Oruga
    August 12, 2008

    The photo of the long-billed woodpecker is amazing. I start to think that sometimes it’s really possible to achieve a new organ lenght or shape just after a few generations.

  3. #3 Neil
    August 12, 2008

    Hooray I was (sort of) right! I wonder if the bill notch could have been due to ware from repeatedly using the same peanut or other artifical bird feeder?

  4. #4 cubicle charlie
    August 12, 2008

    With respect to the brain damage comment: How do we know that they don’t sustain brain damage over a lifetime of tapping away at trees? It seems within the realm of possibility that brain damage could occur over the course of their lifetime without impeding them during their prime reproductive years. Other animals have behaviors that allow them to survive, but over the long haul it proves detrimental, if not deadly–a certain species of seal comes to mind. It gnaws at the edges of an opening in sea ice to keep it open, but over time wears its teeth down to the gumline and thus starves through its inability to catch fish. Any thoughts on this?

  5. #5 Alan
    August 12, 2008

    Is it known when the advanced woodpeckers appeared? I gather that both modern Aye-Aye and an opposum from New Guinea use rodent like incisors and enlarged fingers to extract grubs from wood, and both of these occur on islands without woodpeckers, which suggests competition. I gather the Apatemyids showed similar adaptations, which implies that the appearance of modern woodpeckers was one factor in their extinction

  6. #6 DDeden
    August 13, 2008

    Alan, interesting observation. I have wondered also about apes using twigs/sticks/”spears” to access termites/ants/grubs in soil and rotting wood, and the habit of gorillas and chimps of pounding hollow tree trunks. Some tribes in New Guinea systematically cultivate wood grubs, turning over the logs to harvest them.

    The three-toe vs 4-toe arrangement is interesting, I hope Darren goes further into it sometime.

  7. #7 Alan Kellogg
    August 13, 2008

    OT: Darren, considering your interest in the subject, here’s a bit of news on an upcoming formal announcement. Go to “Georgia Gorilla” to read about it. I tried getting a link to the pictures Loren published, but couldn’t connect.

  8. #8 Alan Kellogg
    August 13, 2008

    Update: Connected.

    The pictures could be better, if real the specimen is badly decayed, and it could have been better handled. If it is real it’s going to raise a ruckus.

  9. #9 Max Paddington
    August 13, 2008

    Perhaps what I’ve read is bunk, but I always thought Woodpeckers avoided brain injury by having an elastic sort of cushioning between beak and skull.
    Though come to think of it, it seems counter-intuitive. Why have a weak soft spot where the ‘blade’ of your tool connects to the ‘handle’ so to speak?
    Anyone care to enlighten me?

  10. #10 Max Paddington
    August 13, 2008

    Nevermind, my mistake, I missed the part where you mentioned that. But why then do you seem to imply that this isn’t sufficient to protect the brain?

  11. #11 Michael Suttkus, II
    August 14, 2008

    “Transitional woodpeckers are impossible!” is a common creationist canard. Wrynecks have long been a favorite of mine for debunking it.

  12. #12 David Marjanovi?
    August 16, 2008

    AFAIK the extinction of the apatemyids made the evolution of woodpeckers possible… I don’t know the exact timing, though.

  13. #13 KidBengal
    February 17, 2011

    umm that doesn’t make sense…. you need to do more research. The reason they do not get a headache is because the neck muscles stiffen before they strike which keeps the skull from moving much… and for the brain. There is no cerebral fluid that humans have. wood peckers do not. instead they have shock absorbers inside their skull. Its not that wood peckers have small brains. If anyone has already commented these facts i apologize for i cannot read comments on this school computer for some reason. But if i made valid points your welcome and if i just made u mad or insulted u that is… My bad

  14. #14 Kidbengal
    February 17, 2011

    umm sorry but that doesn’t make sense. For one. Its not that wood peckers have small brains. Its that instead of cerebral fluid like humans have in their skulls. Woodpeckers have shock absorber plates. Also. The neck muscles of woodpeckers before they strike the tree stiffens to keep the skull from being shocked too much. I do not know what anyone else commented so if someone already covered this then i apologize for repeating. This computer at the school cannot read comments its blocked but i am able to make comments. If anyone went and commented some God made everything comment go elsewhere. This is pro evolution and obviously most of us believe there is NO god so please do not Sass us about something we do not really care about. Ty goodbye

  15. #15 David Marjanović
    February 18, 2011

    Kidbengal… why on the planet do you comment on a thread without having read all previous comments?

    Why?

    Read them when you get home. And shame on your school for censoring.

  16. #16 Darren Naish
    February 18, 2011

    Kidbengal: thanks for your comments. But you need to understand the difference between a speculative cool idea and a conclusion reached via proper study. As noted in the article, some people have indeed suggested that shock-absorption of some sort goes on in the woodpecker skull.

  17. #17 Jeff Brinker
    June 27, 2011

    Dr. Naish,

    I know this is an old article, but I’m a fairly serious birder in the Eastern U.S., and incidents of beak deformation have been a noticeably growing trend. Several (non-scientific) birding publications have been publishing updates in recent years, and various Audubon and other birding groups are actively collecting data.

    I also found a link the British Trust for Ornithology that had details of a program tracking woodpecker and other avian deformities like your great spotted- , called the Big Garden BeakWatch…

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