Hypothesis: Intermittently Pneumatic Avian Long Bones Function as Adaptive Mechanical Waveguides for Substrate and Body-Borne Vibration Sensing

Avian skeletal pneumaticity has historically been evaluated through the lenses of mass reduction, structural mechanics, and respiratory efficiency. Here, we present the Intermittent Pneumatic Waveguide Hypothesis, proposing a complementary, previously unrecognized sensory function for the avian postcranial skeleton. We hypothesize that thin-walled, regionally pneumatic long bones (such as the humerus and femur) act as tuned, partially damped cylindrical waveguides. The structural alternation between air-filled, air-backed segments and denser, marrow-retaining or trabecular segments creates impedance discontinuities and differential damping along the bone shaft. This spatial heterogeneity selectively filters, attenuates, mode-converts, and segregates propagating mechanical waves, shaping vibration signals before they reach mechanoreceptors in the periosteum, joints, or axial skeleton. By transforming the skeleton into an active distributed filter bank, this architecture enhances the extraction of frequency, direction, and arrival timing cues from substrate and body-borne vibrations. This framework integrates principles of thin-cylindrical-shell acoustics into skeletal biomechanics, offering testable predictions for sensory biology and evolutionary morphology.