Locomotion in bonobos (Pan paniscus): differences and similarities between bipedal and quadrupedal terrestrial walking, and a comparison with other locomotor modes

Abstract

One of the great ongoing debates in palaeo-anthropology is when, and how, hominids acquired habitual bipedal locomotion. The newly adopted bipedal gait and the ancestral quadrupedal gait are most often considered as very distinct, with each habitual locomotor mode showing corresponding anatomical adaptations. Bonobos (Pan paniscus), along with common chimpanzees (P. troglodytes), are the closest living relatives to humans and their locomotion is valuable for comparison with other primates, and to gain an insight in the acquisition of human bipedalism. Bonobos are habitual quadrupeds, but they also engage in bipedal locomotion, both on terrestrial and in arboreal substrates. In terms of kinematics and dynamics, the contrast between bipedal and quadrupedal walking seems to be more subtle than one might expect. Apart from the trunk being approximately 37 degrees more erect during bipedal locomotion, the leg movements are rather similar. Apart from the heel, plantar pressure distributions show subtle differences between bipedal and quadrupedal locomotion. Regardless, variability is high, and various intermediate forms of locomotion (e.g. tripedal walking) exist both in captivity and in the wild. Moreover, there is overlap between the characteristics of walking and other locomotor modes, as we show with new data of walking on an inclined pole and of vertical squat jumps. We suggest that there is great overlap between the many locomotor modes in bonobos, and that the required polyvalence is reflected in their anatomy. This may hamper the development of one highly specialized gait (i.e. bipedalism), which would constrain performance of the other types of locomotion.

Fig. 1

Gait parameters during bipedal and quadrupedal walking in bonobos (after Aerts et al. 2000). Dimensionless values are presented (see Aerts et al. 2000, for details), with a stride being a full gait cycle. For clarity, only the regression lines through the actual data points are shown.

Fig. 2

Joint angles during bipedal and quadrupedal walking in bonobos (after D’Août et al. 2002). The angles are defined as the enclosed angle in the sagittal plane between the segments that form the considered joint (i.e. full flexion = 0°, full extension = 180°). The foot is modelled as one segment from the posteroventral edge of the heel bone to the distalmost toe.

Fig. 3

Example vertical ground-reaction force profiles from a bonobo walking bipedally (A), a bonobo walking quadrupedally (B), a bonobo walking over a 30° inclined pole (C) (K. Schoonaert et al., unpublished data) and a human (D, after Farley & Ferris, 1998). For comparison, all forces are scaled to 100% peak force.

Fig. 4

Time course of plantar pressure under selected zones of the bonobo foot while walking bipedally and quadrupedally (data from Vereecke et al. 2003). All profiles are normalized to 100% contact time.

Fig. 5

Example body postures during terrestrial walking. Note that there is a continuum from left to right, with A being a typical quadrupedal ‘knuckle-walking’ posture and E a typical bipedal ‘bent-hip, bent-knee’ posture. B illustrates tripedal walking.

Fig. 6

Hip angle during during terrestrial bipedal and guadrupedal walking, climbing on a 30° inclined pole, vertical climbing (after Isler, 2003) and vertical jumping (M. Scholz et al., unpublished data). Definition of angles as in Fig. 2. The jumping curve is an average of five high sequences of one individual, and the plot goes from maximal flexion (corresponding well with the start of jumping) to toe-off.

Fig. 7

Knee angle during during terrestrial bipedal and guadrupedal walking, climbing on a 30° inclined pole, vertical climbing (after Isler, 2003) and vertical jumping (M. Scholz et al., unpublished data). Definition of angles as in Fig. 2. The jumping curve is an average of five high sequences of one individual, and the plot goes from maximal flexion (corresponding well with the start of jumping) to toe-off.