Head posture and visual orientation in Loris tardigradus during locomotion on oblique supports

Abstract

Primates moving through the trees must cope with a three-dimensional network of branches that differ in angular orientation. On oblique supports, an animal must not only avoid toppling or sliding off of a branch, it may also need to adjust its visual field orientation along the path of movement. Previous studies have found that primate quadrupeds walking on top of horizontal supports direct the orbital plane more inferiorly, whereas suspensory primates moving beneath branches direct the orbital plane more superiorly. If primates adjust the visual path to re flect substrate position, they should incline the orbital plane more on inclines than on declines. Alternatively, eye mobility within the orbits may permit collection of suf ficient visual information without reorienting angular posture of the head. Lorisids are adept arboreal quadrupeds that routinely negotiate inclines and declines. We collected 150 strides of kinematic data on head postures for two adult slender lorises ( Loris tardigradus ) during locomotion on horizontal and oblique supports. In general, lorises adjusted head posture as predicted, directing orbits more superiorly on inclines and more inferiorly on declines. However, we observed higher angles on declines than predicted by substrate angle alone, suggesting that other locomotor and vestibular issues also in fluence head orientation.