Yesterday I got out for my first cross-country ski in an embarrassing number of years. The occasion was a birding field trip on the Grand Mesa organized by my local Audubon Society chapter- the Grand Valley Audubon Society. The weather was overcast with light snow falling- it made for great skiing conditions, but the birds were lying low. We heard some chickadees in the distance and got a glimpse of what might have been a junco darting into the base of a spruce. I only got a good look at one bird, but it was a great one: an American three-toed woodpecker.
This woodpecker gets its name because it has- you guessed it- three toes. Most birds have four toes- three toes pointing forward and the first toe, or hallux, pointing backward. This arrangement of toes is called anisodactyl. Most woodpeckers also have four toes but in a slightly different arrangement. Their first and fourth toes typically point backward with the second and third toe pointing forward. This arrangement is called zygodactyl. But the American three-toed woodpecker has lost the first toe over the course of evolution and has only the fourth toe pointing backward with the second and third pointing forward. Okay, but why does the three-toed woodpecker only have three-toes?
As a former ornithology TA, I felt like I should know the answer. After all I had enjoyed torturing undergrads with just this kind of trivia. I had other, much more pressing, work that I needed to be doing today, but my obsessive little mind kept circling back to Google search and dig around through old ornithology studies to find an answer. I feel I should share the answer I pieced together with the world. I am sure, somewhere out there in the vastness of the Internet, someone else’s freaky little mind is obsessing over the same question.
According to my informal investigation, American three-toed woodpeckers are not the only woodpeckers with only three toes. There is a Eurasian three-toed woodpecker that was once thought to be the same species as the American three-toed, but is distinct (Zink et al., 2002.) There is also one other species of woodpecker in North America that has three toes, the black-backed woodpecker. Apparently the first toe has been “lost” over the course of evolution in multiple lineages of woodpecker, but why? For a bird that makes a living clinging to the side of a tree with its feet, you would think losing a toe would be a disadvantage. Not so according to an analysis by Walter J. Bock. Bock (1999) used the “Method of Free-Body Analysis” to determine all the physical forces acting on the clinging bird. According to his analysis, the first toe does not supply much support in the clinging woodpecker. Even in woodpecker species that have not lost a toe, one of the rear toes usually points sideways when the bird is climbing, not backwards. The zygodactyl arrangement of toes is ancestral and is advantageous for perching with the foot wrapped around a twig rather than clinging to the side of a trunk. For instance, parrots, old-world cuckoos and some owls all have zygodactyl feet and they do not typically cling to the sides of trees. When a bird is clinging to the side of a tree, the toes doing most of the work are those that are opposing the force of gravity- the forward facing ones (or toes two and three.)
So if over the millennia a woodpecker were to be born without a first toe, it would not be at a disadvantage. Perhaps it might even be advantageous in some way? Weight reduction perhaps? This three-toed mutation could be passed on to its offspring and eventually become the norm for the species. Losing a digit has happened many times and by various mechanisms in the evolution of vertebrates (Cooper, 2014.) Perhaps someday humans will be exploring a future version of the Internet minus a pinkie finger. Of course, in the distant future (if we haven’t driven ourselves extinct, or become enslaved by artificial intelligence) we probably won’t need fingers at all- we will control everything with our freaky little minds.
Bock, W.J. (1999) Functional and evolutionary morphology of woodpeckers. Ostrich 70 (1): 23-31.
Cooper, K.L. et al. (2014) Patterning and post-patterning modes of evolutionary digit loss in mammals. Nature 511: 41-45.
Zink, R.M. et al. (2002) Holarctic phylogeography and species limits of three- toed woodpeckers. The Condor, 104(1): 167-170.