STEVENS, N.S.; HEESY, C.P.*: Effects of substrate orientation on inclination of the orbital plane in Loris tardigradus.

The arboreal environment provides a complex three-dimensional network for balance and movement. One of the most obvious challenges is that branches differ in angular orientation. Primates have developed various techniques for negotiating inclines and declines. On oblique supports, an animal must not only avoid toppling or sliding off of a branch, it may also adjust its visual field to reflect the path of movement. Indeed, this is superficially supported by findings that when walking on top of horizontal supports, arboreal quadrupeds have a more inferiorly directed orbital plane, while the orbital plane of suspensory primates moving beneath branches is more superiorly directed. If a primate adjusts its visual field to reflect where it is going, it should exhibit a more inclined orbital plane on inclines than on declines. Maintaining the position of vestibular structures may provide additional constraints to head position while balancing on oblique supports. Lorisids are adept arboreal quadrupeds that routinely negotiate inclines and declines. They are unique in having less frontated orbits than other primates, making them an interesting taxon for examining head posture during locomotion. Orbital plane inclinations were recorded for two adult slender lorises (Loris tardigradus) at touchdown, midstance and lift-off during locomotion on supports oriented horizontally, and at 30 degrees. In general, orbits were more superiorly directed on inclines, and more inferiorly directed on declines. However, these differences were not as great as would be predicted by substrate orientation alone, suggesting that other locomotor and balance issues may also influence the orientation of the head.