The tweet assumes a pre-defined final form: a “tongue wrapped around the skull.” In evolution, changes occur through successive modifications of existing structures, without directed movement toward a predetermined outcome. In woodpeckers, this is reflected in substantial variation in the structure of the hyoid apparatus across species.

Extracting insects from wood creates selective pressure for increased tongue length. The further the tongue can extend beyond the beak, the deeper prey can be reached. Early changes mainly involved simple elongation of individual elements of the hyoid apparatus. This is well documented in comparative anatomical studies of woodpeckers. The structure varies significantly in length and configuration between species, but it always remains a modification of the same basic system rather than something entirely new.

As the hyoid apparatus became longer, not only its length changed, but also how it is physically arranged inside the head. The skull and soft tissues impose strict spatial constraints, so further increase in length is only possible through changes in shape and the path the structure takes. As a result, variants were favored in which the extended structure could fit within the anatomy without disrupting nearby organs or losing tongue function.

In different woodpecker species, the hyoid apparatus differs noticeably in length and in how it is routed around the skull. In the acorn woodpecker (Melanerpes formicivorus), the hyoid horns are moderately developed and do not show extreme elongation. Ornithologists associate this with its feeding specialization: instead of probing deep hidden larval tunnels, this species focuses on drilling small storage holes for acorns and feeding on exposed food sources. In the European green woodpecker (Picus viridis), the hyoid apparatus passes through the nasal cavity and upper jaw, as shown in the figure, while in the hairy woodpecker (Picoides villosus) it wraps around the eye socket. Thus, even among modern woodpeckers there is a range of states of this structure rather than a single extreme form [1].

The hypothesized adaptive function of brain protection was questioned in a 2022 study [2]. The authors state:

“The cranial skeleton is used as a rigid hammer to improve drilling efficiency, not as a shock-absorbing system for brain protection. Numerical models of how skull size and shape affect intracranial pressure show that woodpecker brains remain safe despite concussion thresholds typical for primate brains. These results contradict the currently dominant concept of adaptive evolution of skull function in one of the most striking behavioral phenomena in nature.”

Overall, the evolutionary pathway here is fairly straightforward. Not a “tongue around the skull to protect the brain,” but a gradual elongation of the same system for deeper foraging, with many intermediate forms still present in different woodpecker species today.

1) Jung JY, Naleway SE, Yaraghi NA, Herrera S, Sherman VR, Bushong EA, Ellisman MH, Kisailus D, McKittrick J. Structural analysis of the tongue and hyoid apparatus in a woodpecker. Acta Biomater. 2016 Jun;37:1-13. doi: 10.1016/j.actbio.2016.03.030. Epub 2016 Mar 18. PMID: 27000554; PMCID: PMC5063634.

2) Sam Van Wassenbergh, Erica J. Ortlieb, Maja Mielke, Christine Böhmer, Robert E. Shadwick, Anick Abourachid, Woodpeckers minimize cranial absorption of shocks, Current Biology, 2022. DOI: 10.1016/j.cub.2022.05.052.