The Clever Badger To add insult to injury, the platypus is leading.

Woodpeckers and Evolution – Part 2

(This took longer than I expected to put together, so the first post has rolled off of the front page and may be found here.)

In the first post on woodpeckers and evolution, I touched on some background material on how a woodpecker's tongue is put together and how the whole mechanism operates.  Ordinarily, I wouldn't give much consideration to a woodpecker's tongue, but as it happens, it's one of those things that certain creationist writers like to point at and say "Evolution can't explain that, so creationism must be true!"

Ignoring for a moment that even if evolution couldn't explain the woodpecker's tongue, it wouldn't automatically imply that creationism (or any other alternative) was true,¹ the fact of the matter is that the woodpecker's tongue is quite readily explained in an evolutionary context, and that's what I'll be discussing here.

The original article at Mental Floss that inspired me to write this included a link to a write-up at TalkOrigins that get into the guts and feathers (if you'll pardon the expression) of woodpecker tongues.  I'll be discussing that article a lot here, so I'd suggest having that article open in another tab or window for reference.  Rusty Ryan does a first-rate job of discussing the material, but the article seems to assume a bit more background than a lot of people might have, so I'm going to attempt to fill in the blanks.²

Despite the tendency of creationist writers to portray the woodpecker's tongue as some mysterious, freakish organ for which there's no good evolutionary explanation, it's really pretty easy to characterize in very basic evolutionary terms.

Let's start with what I consider to be a key concept in evolutionary discussions.

The raw material of evolution is heritable variation amongst individuals within a population.  This variation is everywhere you look, too - your siblings don't all look just like you, the squirrels in your back yard don't all have the exact same color fur, some of the kittens in the litter grow to be slightly larger than the others, and some woodpeckers have slightly longer tongues than others of the same species.

Another key concept is that variations that tend to confer an advantage in a given environment will, on average, increase within a population.³

For example, to stick with woodpeckers, a slightly longer tongue allows a bird to get a few more insects per day than his neighbors since his reach is deeper.⁴ This could easily be an advantage in times of drought where food is relatively scarce, but even in times of plenty, the extra nutrition could make him healthier and more likely to find a mate.  If he passes the genes for a longer tongue to his offspring, that trait increases within the population, perhaps to the point that none of the shorter tongued birds can find mates.

There's no reason to suspect that the process stops - longer tongues, more feeding opportunities, more reproductive success.  Where it gets interesting in the case of the woodpecker isn't so much that the tongue got long, but how characteristics that were already present were tinkered with to arrive at the somewhat exotic anatomy that we see today (and we will see that the tinkering is actually quite minimal).

And on that note, let's dive into Rusty Ryan's article.

After a bit of preamble, Ryan notes that while human tongues are mostly muscular, bird tongues are supported by the cartilage and bone of the hyoid apparatus.  I discussed this some in the previous post, but Ryan does a much better job of detailing just how this works.⁵

One of the most important points to keep in mind comes next, and it's worth quoting in its entirety:

The Y- shaped hyoid apparatus of birds, however, extends all the way to the tip of their tongues. The fork in the "Y" sits just in front of the throat, and it is in this area that most of the muscles of the hyoid attach. Two long structures, the "horns" of the hyoid, grow backwards from this area and provide insertion sites for protractor muscles which originate on the lower jaw.The hyoid horns of some species of woodpeckers are quite startling in appearance, as they can grow all the way up to the top of the head and, in some species, grow around the eye socket, or even extend into the nasal cavity. (Sources: Goodge, WR. 1972. Anatomical Evidence for Phylogenetic Relationships Among Woodpecker, The Auk, 89: 65-85; Short, LL. Woodpeckers of the World. 1982, Delaware Museum of Natural History)

Take special note of the third and fourth sentences.  The hyoid horns grow backwards from the throat area.  Not just in woodpeckers, but in birds in general.  The interesting thing about woodpeckers is how long the horns can get and how they manage to accommodate that extra length.

What happens isn't very mysterious or unexpected when you think it through.  Juvenile woodpeckers have hyoid horns that aren't particularly long, and the bone is exactly where it should be in a bird - anchored in the front of the throat with the horns pointing backwards.  Note that the fork of the hyoid can't move backwards - the internal structure of the throat is in the way.

As the woodpecker matures, the hyoid horns grow in the only direction they can - towards the back of the skull.  (The horns are surrounded by a sheath of tissue in which they can slide.)

Despite the length, the horns are still attached to the mandible of the bird via the branchiomandibularis muscles (remember those muscles from Part 1?), and when those muscles contract, they act on the hyoid, pulling the horns tight against the skull.  As the slack is taken up, the tongue is extended.  (You can see this in Ryan's Figure 4, most clearly in the two skulls on the right.)

Hopefully the mechanics of the woodpecker's tongue are now clear, and we can turn our attention to the detail that makes some folks think that the whole Theory of Evolution has cracked a wobbly when it comes to woodpeckers:  the seemingly drastic routing of the horns up around the back of the skull and down the front, in some cases even into the nasal cavity, as the woodpecker matures.

Ryan summarizes it thus:

It is only with age that the hyoid horns of the flicker grow up to the top of the head, forward, and into the nasal cavity, where the sheath fuses to the nasal membrane. This makes adaptive sense, since the young flicker is fed by its parents, and a long tongue would only get in the way.

The genetic changes necessary for such a modification are quite minor. No new structures are required, merely an extended period of growth to lengthen an existing structure. It is likely that in ancestral woodpecker species which began to seek grubs deeper in trees, those woodpeckers with mutations for increased hyoid horn growth had a fitness advantage, as they could extend their tongue farther to reach prey.

In other words, you can get to a hyoid bone with extremely long horns simply by delaying the cue to stop growing.  This is such a basic and obvious concept that it's difficult to express it more simply.  The technical term for this process is heterochrony, and it's the same process that has allowed dog breeders to select for variations that lead to such diverse breeds as Chihuahuas and Great Danes.  There is no need to introduce any novel structure to the bird's anatomy.

Now, at this point, one might wonder why the hyoid horns of some woodpeckers manage to route around one eye socket and into their nasal cavity, rather than, say, one horn routing around each  eye socket and into the nasal cavity on either side in a symmetrical manner.⁶

Ryan doesn't address this specific question directly, but I strongly suspect that the routing of the horns is a spandrel.  By this I mean that there isn't really any functional reason that the horns would have taken that path, but as they grew, they had to go somewhere and the routing that we see now is just coincidence rather than the result of any particular selection pressure .  They could just as easily have taken a different route.  In point of  fact, Ryan's only reference to this matter, (and it's a somewhat oblique acknowlegement) cites John James Audubon's comments on the variation in routing present in different species of woodpeckers:

"There is a very curious gradation in the degree of elongation of the horns of the hyoid bone in the different American Woodpeckers, some of which consequently have the power of thrusting out their tongue to a much greater extent than others. Thus: In Picus varius [Yellow-bellied Sapsucker], the tips of the horns of the hyoid bone reach only to the upper edge of the cerebellum, or the middle of the occipital region. In Picus pubescens, they do not proceed farther forward than opposite to the centre of the eye. In Picus principalis, they reach to a little before the anterior edge of the orbit, or the distance of 1/2 inch from the right nostril. In Picus pileatus, they extend to half-way between the anterior edge of the orbit and the nostril. In Picus erythrocephalus, they reach to 3 twelfths of an inch from the base of the bill. In Picus tridactylus, they reach the base of the ridge of the upper mandible. In Picus auratus [Nothern Flicker] , they attain the base of the right nasal membrane. In Picus canadensis, they curve round the right orbit to opposite the middle of the eye beneath. Lastly, in Picus villosus, they receive the maximum of their development, and, as represented in the accompanying figures, curve round the right orbit, so as to reach the level of the posterior angle of the eye." [Note - many of the latin names used by Audabon do not reflect current usage and classification].

Audubon, John James. 1840-1844. The Birds of America - As an addendum to the entry for "golden-winged woodpecker" (Northern Flicker), Audubon writes:

The last thing to point out is that we aren't discussing fossils here.  We're talking about living birds, whose anatomy can be observed in real-time, from hatching to adulthood.  Consequently, we can watch how the tongue and its supporting structures develop and don't have to speculate.

In this post, we've looked at Rusty Ryan's article and picked out a few of the most significant points.  Namely, we've revisited the anatomy of the woodpecker's tongue, considered how that anatomy is simply explained by an extended period of growth for the hyoid apparatus, and that the specific routing is somewhat variable in woodpeckers and doesn't appear to serve any particular purpose in and of itself.  In short, we've seen that the woodpecker's tongue is in no way a challenge for the Theory of Evolution to explain, and is actually a very straightforward anatomical feature that is very easily accounted for without any head scratching whatsoever.

The next post will take a look at several creationist claims about the woodpecker's tongue, and will point out where those claims fall short.

CB
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¹Remember when I talked about false dilemmas?

²TalkOrigins should be on the bookmark list of anyone who has any interest in the Evolution/Creationism issue. There's a wealth of great information at the site itself, and the sites linked from there are as comprehensive as you're likely to find in any one place.  That said, a lot of the articles assume some degree of familiarity with evolutionary concepts.  This can present a problem in that folks that come to the discussion from an anti-evolution background tend to have a lot of incorrect ideas about evolution that have to be corrected first, and that's not something that can be done in 10 or 15 minutes.

³This concept is frequently oversimplified to "survival of the fittest", which is a workable approximation so long as you keep in mind that "fittest" isn't as intuitive a term as you might like to think, and that what makes one "fittest" for one environment might very quickly get one eaten in another.

⁴This can lead to an evolutionary arms race of sorts. Such arms races are interesting for a lot of reasons, not the least of which is that they demonstrate the intimate interconnectedness of life. In this example, birds that feed on insects that live in wood put a selective pressure on the insects, namely the pressure to avoid that type of bird. One thing that might result is that insects that tend to live deeper in the wood get eaten less frequently than insects that live near the surface. It doesn't have to be much deeper - just slightly deeper than the bird's tongue can reach. An unusually long-tongued bird makes that pressure that much stronger. This sort of situation is called a Red Queen race.

⁵I'm not going to quote too liberally from Ryan's article because I want folks to read the whole thing for themselves. I will mostly be calling attention to what I think are the key points in his presentation.

⁶We're so used to the visible external symmetry of animals that we find asymmetries to be uncomfortable. As it happens, a lot of the asymmetries we might think of, like the fact that our heart is to the left of center or that our digestive tract twists all over the place go away if you unpack everything. Spend a little time thinking about it and you'll figure out that our body plan is basically a donut with a few infoldings and outgrowths here and there.