The biomechanical demands of arboreal locomotion are generally thought to necessitate specialized kinetic and kinematic gait characteristics. While such data have been widely collected across arboreal quadrupeds, no study has yet explored how arboreal substrates influence the locomotor behavior of birds. Parrots – an ancient arboreal lineage that exhibit numerous anatomical specializations towards life in the trees – represent an ideal model group within which to examine this relationship. Here, we quantifiy limb loading patterns within the rosy-faced lovebird (Agapornis roseicollis) across a range of experimental conditions to define the circumstances under which arboreal gaits are triggered, and how, during arboreal walking, gait patterns change across substrates of varying diameter. In so doing, we address longstanding questions as to how the challenges associated with arboreality affect gait parameters. Arboreal locomotion was associated with the adoption of a sidling gait, which was employed exclusively on the small and medium diameter poles but not terrestrially. When sidling, the hindlimbs are decoupled into a distinct leading limb (which imparts exclusively braking forces) and trailing limb (which generates only propulsive forces). Sidling was also associated with relatively low pitching forces, even on the smallest substrate. Indeed, these forces were significantly lower than mediolateral forces experienced during striding on terrestrial and large diameter substrates. We propose that the adoption of sidling gaits is a consequence of avian foot morphology and represents a novel form of arboreal locomotion where inversion/eversion is impossible. Such movement mechanics is likely widespread among avian taxa and may also typify patterns of arboreal locomotion in humans.

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