The beak of the toco toucan is integral to their vital social, feeding, foraging, and nest building behaviors. Since the toucan beak does not fully regenerate, it is evolutionarily favorable for it to have robust mechanical properties while still being light enough to allow flight. The beak makes up one-third of the toucan's body length, while contributing to less than three percent of the toucan's weight. The beak structure largely influences its mechanical properties and the lifestyle of the toucan. The beak's properties are increasingly becoming popular in the realm of biomimicry as several industries such as architecture, transportation, and protective equipment can utilize trends of the biological beak structure in manmade designs.
Structure
Outer beak The toucan beak has a synergistic sandwich structure with a thin rigid outer shell encapsulating the bulk of the volume which is a cellular solid. The exterior layer, known as the rhamphotheca, is composed of several layers of overlapping sheets of beta keratin . Each keratin sublayer is between 2 and 10 micrometers thick with the entire rhamphotheca stack being 0.5 to 0.75 mm thick, varying by position along the length of the beak. Hexagonal keratin cells with a diameter around 50 micrometers make up the majority of the laminate shell. Each cell boundary and interior are interwoven with a matrix of intermediate filaments with randomly distributed orientations, allowing for the isotropic character of the material. The purpose of the matrix is to act as a viscoelastic medium for dispersing severe impacts to the beak over a greater area, to reduce the effects of local imperfections, protecting it from cracking damage.
Hydroxyapatite mineralization is present but to a lesser extent than in the inner foam. Keratin in the rhamphotheca of the toucan beak has a much lower abundance of sulfur, and therefore the amino acid, cystine, compared to other keratin structures like hair. This indicates less disulfide crosslinking.
Inner beak The inner beak is a collagen foam, with a high glycine content and subsequent hardness as in most bones. The fibrous network of collagen trabeculae varies from 70 to 200 micrometers in thickness and are often hollow with an edge connectivity of 3 or above. These, along with even thinner (2 to 25 micrometer) membranes characterize a closed cell foam network which provides the optimal strength at low density for flight consideration. The membranes and trabeculae have higher calcium mineralization content than the dense outer beak. Micro- and nano- indentation testing supports the notion that the inner collagen trabeculae are stronger pound for pound than the outer shell.
Mechanical and material properties Research has been conducted in order to determine mechanical and material properties of the Toucan Beak. The apparent density of the overall beak is estimated to be between 0.1 and 0.25 grams per cubic centimeter.
Outer beak properties The exterior shell of the beak, the rhamphotheca, has a tensile strength of around 50 MPa and a Young's Modulus of 6.7 GPa. From testing the keratin-layer of the beak, it was found the mean value of the Yield Strength is 30 MPa. Further, the yield strength of the beak is sensitive to the strain rate and is associated with the viscoelasticity of the inter-scale glue for the keratin scales. When the yield stress of the beak nears or exceeds the ultimate tensile strength, the fracture of the keratin scales is preferred for the beak over the viscoelastic deformation of the inter-scale glue. In regard to the Young's Modulus and Yield Strength of the keratin in the beak, these values do not change along the longitudinal and transverse direction. The keratin shell can therefore be considered transversely isotropic. The fracture modes of the keratin shell demonstrate a dependence on the strain rate at which the fracture occurs. Testing at lower strain rates revealed a slipping of the scales that is a result of the organic glue being released. For higher strain rates, the keratin scales fractured. Hardness tests were conducted to determine microhardness and nanohardness values for the keratin shell, which are 0.22 ± 0.01 GPa and 0.48 ± 0.06 GPa, respectively.
Inner beak properties For the internal foam part of the toucan beak, which is composed of trabeculae, the Young's Modulus was found to be 12.7 GPa, which is higher than that of the outer shell. This difference is due to the calcium content being higher in the foam than that in the keratin shell. The relative density of the foam was found to be 0.09. Tests were conducted to determine the hardness values of the trabecula. The Microhardness value was found to be 0.28 ± 0.03 GPa, and the nano-hardness value was found to be 0.55 ± 0.12 GPa. For the foam of the beak, the crushing stress increased with its density. The mean value found for crushing stress is 0.17 MPa.
Functions The toucan beak serves several functional roles for the toucan such as acting as a thermal regulator for heat exchange, a tool in feeding, and a method of defense. The toucan has an extreme beak size, with the beak accounting for approximately forty percent of the bird's total body surface area. When compared to other birds, it is noted that the toucan is exposed to some of the warmest monthly temperatures and has the longest beak length. Geographic and temperature variation in beak size has been presumed to relate to resource exploitation and reproduction. Many early researchers assumed to associate beak size with sex-specific traits such as mate selection or vocalization. However, more recently the beak has more closely been linked to its role in physiological homeostasis.
… excerpt ends here. Continue reading the full article.
