Dynamics of Locomotor Transitions from Arboreal to Terrestrial Substrates in Verreaux's Sifaka (Propithecus verreauxi)

Most primates are able to move with equal facility on the ground and in trees, but most use the same quadrupedal gaits in both environments. A few specialized primates, however, use a suspensory or leaping mode of locomotion when in the trees but a bipedal gait while on the ground. This is a rare behavioral pattern among mammals, and the extent to which the bipedal gaits of these primates converge and are constrained by the anatomical and neurological adaptations associated with arboreal locomotion is poorly understood. Sifakas (Propithecus), primates living only in Madagascar, are highly committed vertical clingers and leapers that also spend a substantial amount of time on the ground. When moving terrestrially sifakas use a unique bipedal galloping gait seen in no other mammals. Little research has examined the mechanics of these gaits, and most of that research has been restricted to controlled captive conditions. The energetic costs associated with leaping and bipedal galloping are unknown. This study begins to fill that gap using triaxial accelerometry to characterize and compare the dynamics of sifakas' leaping and bipedal galloping behavior. As this is a relatively novel approach, the first goal of this article is to explore the feasibility of collecting such data on freeroaming animals and attempt to automate the identification of leaping and bipedal behavior within the output. The second goal is to compare the overall accelerations of the body and to use that as an approximation of aspects of energetic costs during leaping and bipedalism. To achieve this, a lightweight accelerometer was mounted on freely moving sifakas. The resulting acceleration profiles were processed, and sequences of leaps (bouts) were automatically extracted from the waveforms with 85% accuracy. Both vector dynamic body acceleration and overall dynamic body acceleration (ODBA) were used to characterize locomotor patterns and energy expenditure during leaping and bipedalism. The unique kinematics of the gait of sifakas, and the mechanics of bouts involving a string of successive leaps or gallops, appear to minimize redirections of the center of mass as well as the number of acceleration peaks and ODBAs. These results suggest that bipedal galloping is not only a reflection of the unique anatomical configuration of a leaping primate, but it may also provide a musculoskeletal and an energetic advantage to sifakas. In that sense, bipedal galloping represents an advantageous way for sifakas to move when transitioning from arboreal leaping to terrestrial locomotion.