Woodpeckers: barbed tentacles and the avoidance of brain injury

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...

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.

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].

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'].

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.