Savanna chimpanzees (Pan troglodytes verus) as a referential model for the evolution of habitual bipedalism in hominids
Observations of the bipedal behavior of wild savanna chimpanzees (Pan troglodytes verus) can provide insight into the evolution of habitual bipedalism in early hominids. This study provides data on the bipedal behavior of eight adult male chimpanzees at the Fongoli field site in southeastern Senegal. Data were collected during transition months at the end of the dry season and beginning of the wet season. Focal instantaneous data on positional and locomotor behavior indicate that bipedalism in Fongoli chimpanzees is a rare, infrequent behavior, accounting for only 2.3% of all positional and locomotor behaviors. Focal all-occurrences data provides a more detailed look at ecological and behavioral variables favoring bipedal behavior here. Fongoli chimpanzees exhibited bipedal postures and locomotion in both arboreal and terrestrial contexts. Bipedal postures were most frequent during feeding and foraging in either context. All bipedal feeding and foraging postures involved forelimb assistance. Bipedal locomotion occurred most often terrestrially during agonistic bipedal threat displays, which often included using hands to throw rocks, and drag or wave branches and loose leaves. In particular, the results of this study indicate that Fongoli chimpanzees are significantly more bipedal than chimpanzees at other sites, exhibiting a rate of 1.05 bipedal bouts per observation hour. These findings suggest that both postural and locomotor bipedalism should be considered in scenarios seeking to reconstruct the evolution of bipedalism in a variety of arboreal and terrestrial contexts. In addition, the mosaic savanna-woodland habitat shared by Fongoli chimpanzees and recent reconstructions of the paleoenvironment of early hominids may have been an important variable in favoring the evolution and origin of habitual bipedal behavior.