Fig. 1. Defining sciurid arboreal locomotion. Discrete airborne postures are used to define locomotor mode. (A) Chipmunks are relatively unspecialized semiarboreal leapers that exhibit an adducted limb posture in mid-air. (B) Red squirrels are arboreal parachuters that exhibit a flattened posture in mid-air characterized by abduction of the proximal limb elements and flexion of the distal limb elements. (C) Flying squirrels are arboreal gliders that exhibit an abducted posture in mid-air with extension of the distal limb elements. (D) Phylogenetic relationships of the sciurid taxa included in this study (taken from data in Hight et al., 1974; Oshida et al., 1996; Roth, 1996).