Abstract
Feathers do not have to be especially strong but they do need to be stiff and at the same time resilient and to have a high work of fracture. Syncitial barbule fibres are the highest size-class of continuous filaments in the cortex of the rachis of the feather. However, the rachis can be treated as a generalized cone of rapidly diminishing volume. This means that hundreds of syncitial barbule fibres of the rachis may have to be terminated before reaching the tip – creating potentially thousands of inherently fatal crack-like defects. Here I report a new microstructural architecture of the feather cortex in which most syncitial barbule fibres deviate to the right and left edges of the feather rachis from far within its borders and extend into the barbs, side branches of the rachis, as continuous filaments. This novel morphology adds significantly to knowledge of β-keratin self-assembly in the feather and helps solve the potential problem of fatal crack-like defects in the rachidial cortex. Furthermore, this new complexity, consistent with biology’s robust multi-functionality, solves two biomechanical problems at a stroke. Feather barbs deeply ‘rooted’ within the rachis are also able to better withstand the aerodynamic forces to which they are subjected.
Highlights
Feathers of flying birds are subjected to extraordinary aerodynamic forces during flight[1]
SBFs were first discovered in the feather rachis by Lingham-Soliar et al.[18] and subsequently identified by Bode et al.[26]
The importance of SBFs is implicit given that they form the bulk of the physical and chemical makeup of the rachis and barbs as concentrated in their cortices
Summary
Feathers of flying birds are subjected to extraordinary aerodynamic forces during flight[1]. Histodifferentiation and TEM13 have produced important answers in feather molecular structure and developmental biology but my attempt to resolve questions at the microstructural level of fibre hierarchy needed a new form of investigation This involved lab-based microbial hydrolysis of the amorphous polymer matrix of the rachidial cortex[18,19,21] which in turn freed or delineated for the first time the highest fibre size-class of the feather rachis, the syncitial barbule fibres (SBFs hereafter). Most of the fibres of the feather rachis (apart from a thin band or two aligned around the hoop) were considered to be aligned along the longitudinal axis[4,19,24,25,26] This contrasts with proposals in a recent study[27] in which the authors suggest that the rachidial cortex is comprised of multiple laminae of differentially oriented fibres.
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