1Division of Anatomy, University of Alberta, 2Department of Anthropology, Boston University, 3Department of Evolutionary Anthropology, Duke University
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Diagonal-sequence, diagonal couplets (DSDC) walking gaits provide two simultaneous advantages for primates and other mammals with grasping hindfeet: forelimb testing of a new support with a secured hindlimb grasp near the center of mass, while minimizing periods of bipedal support on two ipsilateral limbs. However, kinkajous rely predominantly on DSDC gaits despite lacking grasping hindfeet. Lemelin and Cartmill (2010) reconciled this paradox by proposing that walking gaits with higher diagonality maximize the linear distance between two contralateral feet (or diagonal bipods) during the much longer periods of diagonal bipedality of the gait cycle, thus potentially increasing stability for locomotion on relatively thin supports. We compared lengths of diagonal and ipsilateral bipods and other kinematic variables of two kinkajous walking quadrupedally on two poles of different diameter and a runway. Digital videorecordings of over 400 gait cycles were imported to a motion-analysis software for calculation of speed, bipod lengths, and limb joint angles. As speed did not differ across substrates, length of diagonal bipods was significantly longer on poles compared to the runway (P < 0.05). However, length also increased for ipsilateral bipods on poles. This is shown to be a necessary corollary of the phase shift between fore and hind limb cycles that increases diagonality. On thin arboreal supports where all footfalls are essentially collinear, DSDC gaits may be advantageous in increasing the length of the support base–if duty factors are low and bipods correspondingly predominate in the cycle of support patterns.
This study was funded by the Natural Sciences and Engineering Research Council of Canada.