HILLENIUS, Willem/J; MADERSON, Paul/F*; Coll. of Charleston; Brooklyn Coll. of CUNY: DOES PITH STRETCH A FEATHER? Does pith stretch a feather?

The proverbial lightness of feathers, usually attributed to an aerodynamic role, results from delicate pulp caps within the tubular calamus and spongy pith within spathe components. In both rachis and barbs, the medullary pith, enclosed by denser cortical epithelia, certainly provides a strong, yet flexible and light, beam structure. However, we propose an additional, developmental role for the pith tissues. Immature keratinocytes initially appear as a sheet of tightly packed, polygonal cells situated between the epithelial sheath and the turgid dermal core. Within this sheet patterned &beta – keratinogenic rachideal and barb ridges then appear. At first rectangular, these ridges gradually transform into the approximately rounded rachis and flattened, branched barbs. On their own, the mechanical environments provided by sheath and core tissues neither hinder nor help the elongation and flattening of cortical epithelial cells or barbule elongation. We propose that the unique swelling of differentiating pith cells, analogous to expanding foam insulation, provides a critical mechanical force permitting epithelialization of cortical tissues. By the time the sheath splits and dermal regression occurs, breakdown of intercellular contact between adjacent barbs and barbules, followed by terminal dehydration of the constituent cells, allows spathe deployment. This explanation of pith development in modern feathers supports an evolutionary model of feather origins via patterned splitting of a keratinized extension of an ancestral scale. It implies that pith, originally serving to facilitate splitting, was an excessive construction (sensu Gans, Evolution 1979) whose strength and lightness were later selected for in the context of flight.