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.

褐鹈鹕（Pelecanus occidentalis）挥舞着所有鸟类中最大的钞票之一，其特点是可展开的可展开组织的喉袋用来捕获猎物。在这里，我们报道了囊袋皮肤的机械性能和微观结构。它表现出明显的各向异性，其横向具有200％至300％的最大名义拉伸应变，是纵向值的三倍。这是比大多数常规皮肤更高的延展性，并且是由于需要囊囊来捕捞净鱼的结果。它应横向扩展，并控制纵向拉伸。透射电子显微镜提供了胶原纤维定向性的微观结构证据，并揭示了具有双峰直径分布且在100和170nm处具有峰值的单个胶原原纤维。这些尺寸类似于哺乳动物皮肤中的胶原蛋白。在横向方向上，纤维形成弯曲的图案，其半径约为2 µm，其中，原纤维在拉伸载荷下重新定向，拉直，滑动和弹性拉伸。第二机构在横向方向上操作。膜形成波纹状的图案，一旦胶原蛋白原纤维拉直，就赋予了额外的延展性。这引起在拉伸试验中观察到的各向异性响应。这项工作的重点是根据相对鸟龄，小袋上样品位置和应变率的影响进行机械表征。前后位置和应变率对所显示的强度和可扩展性不是主要影响因素。但是，发现鸟龄和背腹位置会显着影响小袋的机械反应。