On the gular sac tissue of the brown pelican: Structural characterization and mechanical properties

The brown pelican (Pelecanus occidentalis) wields one of the largest bills of any bird and is distinguished by the deployable throat pouch of extensible tissue used to capture prey. Here we report on mechanical properties and microstructure of the pouch skin. It exhibits significant anisotropy, with the transverse direction having maximum nominal tensile strains of 200% to 300%, triple the value in the longitudinal direction. This is a higher extensibility than most conventional skin and is the result of the requirement of the sac to net fish; it should expand laterally, with controlled longitudinal stretch. Transmission electron microscopy provides microstructural evidence of the directionality of the collagen fibers and reveals the individual collagen fibrils with a bimodal diameter distribution having peaks at 100 and 170 nm. These dimensions are similar to collagen in mammal skin. In the lateral direction, the fibers form a curvy pattern with a radius of approximately 2 µm wherein the fibrils reorient, straighten, slide, and stretch elastically under tensile load. A second mechanism operates in the transverse direction; the membrane forms a corrugated pattern that, upon straightening of collagen fibrils, confers additional extensibility. This elicits the anisotropic response observed in tensile testing. This work focuses on the mechanical characterization based on the effect of relative bird age, sample location on the pouch, and strain rate. Anterior-posterior location and strain rate are not major influencers on exhibited strengths and extensibilities. However, bird age and dorsal-ventral location are found to affect the mechanical response of the pouch significantly. A physically-based constitutive model is developed for the middle layer of the gular sac, based on observations, which predicts maximum stresses, strains, and the shape of the stress-strain curve consistent with the experimental results.