Understanding the adaptive value of diagonal‐sequence gaits in primates: A comment on Shapiro and Raichlen, 2005

In a recent article in this journal, Shapiro and Raichlen (2005) took issue with our hypothesis (Cartmill et al., 2002, 2007; Lemelin et al., 2003) that primates adopt their distinctive diagonal-sequence, diagonalcouplet (DSDC) gaits to ensure that they are securely balanced over a protracted hindfoot when their forefoot comes down on an untested support. Shapiro and Raichlen (2005) suggest that lateral-sequence, lateral-couplet (LSLC) gaits provide equal or greater stability at the point of forelimb touchdown, and that stability at forelimb touchdown, therefore, cannot be invoked as a selective advantage of DSDC walking gaits in primates. To support their contention that LSLC gaits are more stable at forelimb touchdown, Shapiro and Raichlen (2005) videotaped two infant baboons walking on a laboratory floor, and measured their footfall timing and limb excursion angles. Their data show that these animals, like other infant cercopithecoids observed by Rollinson and Martin (1981) and Nakano (1996), used a variety of different walking gaits. One infant used mainly (83%) DSDC walks of the usual primate sort, but also a few LSLC walks like those of a camel (7%), and some lateral-sequence, diagonal-couplets (LSDC) walks like those of a horse (8%). The other infant showed a decided preference for camel-type walking (62%) and practically never walked like a typical monkey. Shapiro and Raichlen compared the phase relationships between the fore and hind pairs of limbs (‘‘gait number’’ or ‘‘diagonality’’) with the angle of hindlimb protraction in their data, and discovered the following facts, on which they based their rejoinder to our hypothesis: Fact 1. Hindlimb protraction at the moment of forefoot touchdown was not maximized by the intermediate phase values seen in DSDC or LSLC walking. Fact 2. Rather, hindlimb protraction peaked when the fore and hindlimb cycles were either exactly inphase (the pace) or exactly 1808 out-of-phase (the trot). Fact 3. At forefoot touchdown, the hindlimb was just as protracted in many of the infants’ LSLC walks as in typical DSDC walks. Fact 4. In a DSDC (primate-type) walking gait, the animal is standing precariously on two ipsilateral feet placed close together at the moment when the opposite forefoot comes down. Fact 5. When the forefoot comes down in LSLC walks, the animal is standing on two diagonally opposite feet, which probably makes it more stable at that moment than in DSDC walks.