Functional and evolutionary insights from postnatal skull and cervical development in woodpeckers (Aves: Picinae)

Abstract

Woodpeckers possess specialised cranial and cervical skeletal adaptations that enable them to excavate wood, yet how these structures form and integrate during development is still largely unknown. While several cranial skeletal traits have been described in this context, their postnatal development has received little attention. This study examines the postnatal skeletal development of four species, from nestlings to juveniles, using cleared and stained specimens to assess ossification and bone fusion. Woodpeckers show delayed cranial and cervical skeletal ossification typical of altricial birds, with many elements remaining cartilaginous in the first post-hatching days. Lineage-specific features—including the dorsal process of the pterygoid and the rostral process of the paroccipital process—ossify later. The rostral process originates in the exoccipital, with minor contribution from the squamosal, supporting its reinterpretation as processus rostralis paroccipitalis (new term). The lacrimal is consistently absent, indicating a true secondary loss probably linked to cranial kinesis. The mesethmoid may contribute to the frontal overhang in species where present. The epiotic forms part of the external cranial vault, also reported in other birds, suggesting it is more widespread than previously assumed. Cervical vertebrae, in turn, follow the altricial pattern, with late ossification of the atlas and unfused neural arches at early stages, showing considerable heterogeneity among altricial birds. Collectively, these findings show how postnatal skeletal development integrates functional, mechanical, and evolutionary constraints, offering new insights into the ontogeny and specialisation of the woodpecker skull and neck.