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Gait transitions independent of speed in the Lesser Egyptian Jerboa
Moore TY*, Cooper KL, Biewener AA
Terrestrial locomotion is well documented and characterized by distinct gait transitions with increasing speed, which can optimize locomotor economy. Past research on bipeds indicates that each foot in a symmetric gait decelerates then accelerates the body to achieve the spring-loaded inverted pendulum running gait. Quadrupeds are capable of favoring deceleration by the forelimbs and acceleration by the hindlimbs within one running stride.
The Lesser Egyptian Jerboa (Jaculus jaculus) is a desert-adapted bipedal rodent with unique gait characteristics when compared to bipedal and quadrupedal animals. Jerboas perform three distinct bipedal gaits: a symmetrical, in-phase "hop;" an asymmetrical, slightly out-of-phase "skip;" and a fully out-of-phase "run." To characterize the gaits we encouraged 5 male jerboas to run along a straight track equipped with a 2-axis (vertical and fore-aft) force platform, while recording leg kinematics with a high-speed camera.
The ground reaction forces on each foot in a skip differed within the same stride: the first foot to touch decelerates then accelerates, whereas the second foot mainly accelerates. Two individuals significantly preferred to lead with one foot (p<0.05), but across individuals no significant preference for leading foot was observed (two-sided binomial test, p=0.2). Over 140 trials jerboas ran at speeds between 0.4 and 3.0 m/s (mean 1.5 ± 0.6 m/s st.dev.). All gaits were displayed across the speed range, but animals predominantly used the skip (57% of trials). The jerboas only hopped during rapid acceleration and deceleration, whereas skipping was used during acceleration, deceleration, as well as constant speeds. Animals only ran at constant speeds. Jerboa gait transitions are correlated with acceleration rather than speed, suggesting that gait selection is based on maneuverability rather than locomotor economy.
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