HOMBERGER, D.G.: Functional Morphology of a Predator-Prey relationship: Australian Cockatoos and Woody Fruits

A fresh analysis of the unique structure of the psittaciform quadrato-mandibular joint shows that this joint prevents a protraction of the mandible (in contrast to earlier interpretations), but allows lateral movements. Paradoxically, most parrots and cockatoos, which possess a psittaciform bill, do not move their mandibles laterally, except very slightly during bill-honing. They extract kernels from fruits and seeds with the transverse cutting edge of their mandible by exerting pressure on weak points of the fruit walls. However, certain red-tailed Black-Cockatoos (Calyptorhynchus lathami, Callocephalon fimbriatum, and some C. banksii sspp.), which possess a calyptorhynchid bill, regularly move their mandibles sideways to place one of their mandibular corners directly opposite their upper bill tip. They extract seeds from woody fruits with a pincer-like action, ripping through large woody Eucalyptus or Casuarina fruits, whose walls exhibit a “ply-wood” construction that is part of a complex hydraulic mechanism for releasing mature seeds. Parrots and other cockatoos (including some other C. banksii sspp.) do not crack open the walls of such fruits. The uniform quadrato-mandibular joint in combination with the diversified bills in Psittaciformes, as well as the biomechanical properties and functional morphology of the food sources, observations in the field under natural conditions, and paleoecological data suggest that the Psittaciformes originated from an ancestral form that had evolved the capacity for moving its mandible sideways to rip open woody materials, such as tree trunks and branches containing wood-boring or gallicole insect larvae. (Supported by NSF)