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First published online February 6, 2004
Journal of Experimental Biology 207, 1027-1042 (2004)
Published by The Company of Biologists 2004
doi: 10.1242/jeb.00863

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Stabilization and mobility of the head and trunk in wild monkeys during terrestrial and flat-surface walks and gallops

Donald C. Dunbar^1,*, Gyani L. Badam^2,, Benedikt Hallgrímsson³ and Stéphane Vieilledent⁴

¹ Department of Anatomy and Caribbean Primate Research Center, University of Puerto Rico Medical School, PO Box 365067, San Juan, PR 00936-5067, USA
² Department of Archaeology, Deccan College Post-graduate and Research Institute, Pune 411006 (Maharashtra), India
³ Department of Cell Biology & Anatomy, University of Calgary, 3330 University Drive NW, Calgary, AB T2N 4N1, Canada
⁴ Département STAPS, Centre Universitaire de Recherche en Activités Physiques et Sportives (CURAPS), Université de La Réunion, Site du Tampon, 117 rue du Général Ailleret, 97430 Le Tampon, France

^* Author for correspondence (e-mail: ddunbar{at}rcm.upr.edu)

Accepted 31 December 2003

This study investigated the patterns of rotational mobility (>20°) and stability (20°) of the head and trunk in wild Indian monkeys during natural locomotion on the ground and on the flat-topped surfaces of walls. Adult hanuman langurs (Semnopithecus entellus) and bonnet macaques (Macaca radiata) of either gender were cine filmed in lateral view. Whole-body horizontal linear displacement, head and trunk pitch displacement relative to space (earth horizontal), and vertical head displacement were measured from the cine films. Head-to-trunk pitch angle was calculated from the head-to-space and trunk-to-space measurements. Locomotor velocities, cycle durations, angular segmental velocities, mean segmental positions and mean peak frequencies of vertical and angular head displacements were then calculated from the displacement data. Yaw rotations were observed qualitatively. During quadrupedal walks by both species, the head was free to rotate in the pitch and yaw planes on a stabilized trunk. By contrast, during quadrupedal gallops by both species, the trunk pitched on a stabilized head. During both gaits in both species, head and trunk pitch rotations were symmetrical about comparable mean positions in both gaits, with mean head position aligning the horizontal semicircular canals near earth horizontal. Head pitch direction countered head vertical displacement direction to varying degrees during walks and only intermittently during gallops, providing evidence that correctional head pitch rotations are not essential for gaze stabilization. Head-to-space pitch velocities were below 350 deg. s^–1, the threshold above which, at least among humans, the vestibulo-ocular reflex (VOR) becomes saturated. Mean peak frequencies of vertical translations and pitch rotations of the head ranged from 1 Hz to 2 Hz, a lower frequency range than that in which inertia is predicted to be the major stabilizer of the head in these species. Some variables, which were common to both walks and gallops in both species, are likely to reflect constraints in sensorimotor control. Other variables, which differed between the two gaits in both species, are likely to reflect kinematic differences, whereas variables that differed between the two species are attributed primarily to morphological and behavioural differences. It is concluded that either the head or the trunk can provide the nervous system with a reference frame for spatial orientation and that the segment providing that reference can change, depending upon the kinematic characteristics of the chosen gait.

Key words: natural locomotion, kinematics, segmental stabilization, sensorimotor control, spatial orientation, reference frames, graviceptors, vestibular apparatus, vestibulo-ocular reflex, inertia, free-ranging monkeys, hanuman langur, bonnet macaque, Semnopithecus entellus, Macaca radiata

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