The relationship between limb muscle mass distribution and the mechanics and energetics of quadrupedalism in infant baboons (Papio cynocephalus)

Primates have more distally distributed limb mass than many other mammalian quadrupeds because of their adaptations for grasping hands and feet. Although researchers have noted primates’ unique limb shape, the effects of this limb mass distribution pattern on quadrupedal mechanics have yet to be examined. Some researchers have predicted that distally distributed limb mass produces relatively low stride frequencies. Others have predicted that distally distributed limb mass leads to increased energetic costs of locomotion because distal mass requires more work to move the limbs during locomotion. This study uses an ontogenetic sample of infant baboons (Papio cynocephalus) to test these predictions. Infants have mass distributed more distally than adults because infant primates must grasp their mothers’ hair during their early development. Infants’ limb mass migrates proximally as their need to grasp their mothers’ hair decreases with age. To examine the effects of limb mass distribution on the mechanics of quadrupedalism, inertial properties, quadrupedal kinematics, and mechanical work were measured during ontogeny in a sample of infant baboons. When the infant baboons are young and have limb mass most distally concentrated, they use lower stride frequencies and longer strides compared to older ages, when limb mass is more proximally concentrated. These kinematic changes have important effects on their mechanical energy outputs. The lower stride frequencies used by infant baboons at young ages mitigates the amount of work it takes to move their limbs relative to their body. In comparison to other mammals, infant baboons have lower stride frequencies and longer strides. They also do less work to move their limbs, but due to longer strides and therefore larger vertical oscillations of their centers of mass, the infant baboons do larger amounts of work moving their whole body center of mass. As a result, the infant baboons do similar amounts of total work compared to other mammals, suggesting a trade-off mechanism that allows total work to be independent of limb shape. Overall, the results from this study suggest that the adaptations for grasping hands and feet in primates have had profound effects on their kinematics, while likely having little effect on their total energy expenditure.