The structure of the toucan beak teaches us principles of composite material design for light-weight strength and stiffness. Despite its large size (a third of the length of the bird) and considerable strength, the toucan beak comprises only one twentieth the bird’s mass. While the large strong beak is useful in foraging, defense and attracting mates, its low density is essential for the toucan to retain its ability to fly. The beak’s solid outer shell sandwiches within it a closed-cell, foam-like structure made of struts which, together with thin protein membranes, enclose variably shaped air spaces. The solid shell layer is built of overlapping, hexagonally-shaped thin plates of keratin protein held together by an organic glue. The internal closed-cell structural support is comprised of keratin fibers with greater mineralization, by calcium and other salts, than in either the membranes or the solid shell layers to increase hardness. The closed cell structure offers a more complex energy absorption capacity and resistance to compression than the bending deformation typical of open celled structures. The rotational deformation of cell walls, stretching of membranes, and the internal gas pressure all contribute to those features. There is a synergistic effect of the shell layer and foam-like interior elements that together gives it greater strength than the sum of the strengths of those individual parts. Material designs inspired by the structure of the toucan beak could offer the properties of low weight with high stiffness and strength, as well as good energy absorption capacity and insulation value, such as could be useful in developing crash resistance in vehicles without compromising fuel economy.
"The toucan beak, which comprises one third of the length of the bird and yet only about 1/20th of its mass, has outstanding stiffness. The structure of a Toco toucan (Ramphastos toco) beak was found to be a sandwich composite with an exterior of keratin and a fibrous network of closed cells made of calcium-rich proteins. The keratin layer is comprised of superposed hexagonal scales (50 µm diameter and 1 µm thickness) glued together. Its tensile strength is about 50 MPa and Young’s modulus is 1.4 GPa. Micro and nanoindentation hardness measurements corroborate these values. The keratin shell exhibits a strain-rate sensitivity with a transition from slippage of the scales due to release of the organic glue, at a low strain rate (5 · 10-5/s) to fracture of the scales at a higher strain rate (1.5 · 10-3/s). The closed-cell foam is comprised of fibers having a Young’s modulus twice as high as the keratin shells due to their higher calcium content. The compressive response of the foam was modeled by the Gibson–Ashby constitutive equations for open and closed-cell foam. There is a synergistic effect between foam and shell evidenced by experiments and analysis establishing the separate responses of shell, foam, and foam + shell. The stability analysis developed by Karam and Gibson, assuming an idealized circular cross section, was applied to the beak. It shows that the foam stabilizes the deformation of the beak by providing an elastic foundation which increases its Brazier and buckling load under flexure loading." (Seki et al. 2005:5281)