Lateral sequence walking in infant Papio cynocephalus: implications for the evolution of diagonal sequence walking in primates.

One of the most distinctive aspects of primate quadrupedal walking is the use of diagonal sequence footfalls in combination with diagonal-couplets interlimb timing. Numerous hypotheses have been offered to explain why primates might have evolved this type of gait, yet this important question remains unresolved. Because infant primates use a wider variety of quadrupedal gaits than do adults, they provide a natural experiment with which to test hypotheses about the evolution of unique aspects of primate quadrupedalism. In this study, we present kinematic data on two infant baboons (Papio cynocephalus) in order to test the recent hypothesis that diagonal sequence, diagonal couplets walking might have evolved in primates because their limb positioning provides stability in a small branch environment (Cartmill et al. [2002] Zool J Linn Soc 136:401-420). To assess hindlimb position at the moment of forelimb touchdown, we measured hindlimb angular excursion and ankle position for 84 walking strides, across three different types of gaits (diagonal sequence, diagonal couplets (DSDC); lateral sequence lateral couplets (LSLC); and lateral sequence diagonal couplets (LSDC)). Results indicate that if a forelimb were to contact an unstable substrate, LSLC walking provides as much, and perhaps more, stability when compared to DSDC walking. Therefore, it appears that this moment in a stride was unlikely to be a particularly important selective factor in the evolution of DSDC walking. Further insight into this issue will likely be gained by observations of primate quadrupedalism in natural environments, where the use of lateral sequence gaits might be more common than currently known.