The dual function of prokinesis in the feeding and locomotor systems of parrots

Abstract

Prokinesis, a mode of avian cranial kinesis involving motion between the neurocranium and upper beak, has long been investigated in biomechanical analyses of avian feeding and drinking. However, the modern avian beak is also used for non-feeding functions. Here, we investigate the dual function of prokinesis in the feeding and locomotor systems of the rosy-faced lovebird (Agapornis roseicollis). Lovebirds and other parrots utilize their beak both during feeding and as a third limb during vertical climbing. Thus, we experimentally measured both force-generating potential and movement of the rosy-faced lovebird mandible and maxilla (via prokinetic flexion of the craniofacial hinge) during tripedal climbing and mandibular/maxillary adduction. We found that whereas the maxilla is primarily responsible for generating force during locomotion, the mandible is primarily responsible for generating force during forceful jaw adduction, hinting at a remarkable capacity to alter prokinetic function with differing neuromuscular control. The ability of the prokinetic apparatus to perform functions with competing optimality criteria via modulation of motor control illustrates the functional plasticity of the avian cranial kinesis and sheds new light on the adaptive significance of cranial mobility.

The role of many-to-one mapping of vertebral form to function in Psittaciform tripedal locomotion

“…A key difference between the skull and neck of parrots is that dietary preference accounts for more morphological variation in the neck of parrots (up to 15% vs 2.4%, Supplementary Table 4). We had expected to observe the opposite pattern as the skull directly manipulates and processes food, not the neck (7,22,49,50). Diet is still a minor (∼15%) component of neck morphological variation however, and the differences observed here may be due to discrepancies in dietary classification schemes between the two studies (22).…”

“…Thus, the neck of birds functions across a broad range of activities as a 'surrogate forelimb' (3)(4)(5). Psittaciformes (parrots) exemplify the functional diversity of the avian cervical column as the necks of this group participate in feeding, preening, tool use and most spectacularly, locomotion (6)(7)(8). By utilising the neck and head as a third 'propulsive limb' parrots are able to ascend vertical and horizontal substrates by co-opting the craniocervical system to function within a cyclical tripedal gait pattern (6,8,9).…”

Cranial functional specialisation for strength precedes morphological evolution in Oviraptorosauria

“…It may be the case that the development of a rhamphotheca in oviraptorosaurians was linked with an adaptive pressure towards having a feeding apparatus that was more continuously replaced in terms of wear (and potentially self-sharpening) to function more reliably and consistently. In this way, they would function in an analogous way to the beaks of modern parrots (though lacking their degree of cranial kinesis 90 ), which can be entirely replaced in a few months 91 .…”