Shape variation in the talus and medial cuneiform of chimpanzees and bonobos

Abstract

Establishing form-function relationships between anatomy and locomotor behaviours in extant taxa provides critical context for interpretations of extinct species. This study used 3D geometric morphometric methods to explore talus and medial cuneiform shape variation among taxa in Pan and to determine whether and to what extent any shape variation may be related to differences in climbing behaviour. Current locomotor behaviour data suggest that bonobos, western, and eastern chimpanzees do not differ from one another in total frequencies of arboreality as much as once thought. However, these data do suggest that bonobos and eastern chimpanzees more often use smaller diameter substrates (<10 cm) when climbing, while western chimpanzees tend to climb larger diameter (>15 cm) tree trunks and boughs. The morphology of the talus and medial cuneiform in Pan was predicted to reflect differences in locomotor behaviour, showing a greater emphasis on hallucial grasping in bonobos and eastern chimpanzees, and on an inverted foot set in western chimpanzees. This difference in emphasis was expected to be more pronounced between bonobos and western chimpanzees, both of which appear to climb more frequently than do eastern chimpanzees. The results of this study suggest that the shapes of the bonobo talus and medial cuneiform covary as a functional unit that emphasizes hallucial grasping in the medial cuneiform but not inversion at the talocrural joint. The exact opposite pattern was observed in western chimpanzees, with features that emphasize inversion at the ankle joint but not hallucial grasping. Eastern and central chimpanzee (and possibly Nigeria-Cameroon chimpanzee) talus and medial cuneiform shapes fall in between these two extremes. Overall, these results are reasonably consistent with the different styles of arboreality observed in bonobos and western chimpanzees. Interestingly, the pattern of covariance observed in this study among chimpanzees and bonobos does not exist in modern humans. If the pattern of covariance observed in Pan also characterized the Pan-Homo ancestor, then it must have become dissociated at some point during early hominin evolution in order to produce the combination seen in modern humans (i.e., an everted foot set combined with an adducted hallux).