Arboreal locomotion in rats - the challenge of maintaining stability

Arboreal locomotion has mainly been looked at to date in the context of investigations into the specialization of primates and other 'arboreally adapted' animals. The feat of moving on branches as small or smaller than the body's diameter was tested in rats (Rattus norvegicus) as they moved on horizontal poles of different diameters. The data were compared with data pertaining to terrestrial locomotion. We investigated three-dimensional kinematics and dynamics using biplanar cineradiography with simultaneous substrate reaction force (SRF) measurements. As predicted, rats flexed fore- and hindlimbs and reduced vertical forces during pole locomotion. In addition, the orientation of the mediolateral substrate reaction force resultant (SRR) and impulses switched from lateral to medial. In order to maintain stability during arboreal locomotion, lateral spine movements increased. We propose that the combination of lateral sequence gaits, similar travel speed of the animals and similar contact times, higher or similar peak vertical forces as well as similar mediolateral impulses in forelimbs and hindlimbs are typical of clawed mammals moving on thin supports. Clawed mammals and primates share the reduction of vertical oscillations and side-to-side fluctuations, a crouched posture as well as the increase in lateral spine movements. We conclude that these features are behavioral adaptations caused by the biomechanical constraints of small branch locomotion, regardless of the way they make contact with the substrate.