Toucan and hornbill beaks: a comparative study.

The structure and mechanical behavior of Toco Toucan (Ramphastos toco) and Wreathed Hornbill (Rhyticeros undulatus) beaks were compared. The beak of both species is a sandwich-structured composite, having an exterior, or rhamphotheca, consisting of multiple layers of keratin scales and a core composed of a fibrous network of bony closed-cell foam. The rhamphotheca is an arrangement of approximately 50microm diameter, overlapping, keratin tiles. The hornbill rhamphotheca exhibits a surface morphology on the ridged casque that is distinguishable from that observed on the bill proper. Intermediate filaments in the keratin matrix were observed by transmission electron microscopy. The Young's modulus measurements of toucan rhamphotheca indicate isotropy in longitudinal and transverse directions, whereas those of hornbill rhamphotheca may suggest anisotropy. The compressive response of beak foam is governed by brittle crushing behavior. The crushing strength of hornbill foam is six times higher than that of toucan foam. Micro- and nanoindentation hardness values were measured for rhamphotheca and foam trabeculae of toucan and hornbill specimens. The sandwich design of beaks was analyzed using the Karam-Gibson and Dawson-Gibson models. The presence of a cellular core increases the bending resistance (Brazier moment) by a factor of 3-6 while decreasing the compressive strength by only 50%.