Bipedality in chacma baboons (Papio ursinus griseipes) in Gorongosa National Park: a multi-contextual, socially-relevant and demographically-structured behavior.

Obligate bipedalism is a defining trait of the human lineage, yet the selective pressures underlying its emergence and fixation remain debated, particularly within the woodland-grassland mosaic environments associated with early hominins. Modern semi-terrestrial primates inhabiting comparable ecosystems provide a useful model for evaluating candidate functional contexts of early bipedal behavior in the human lineage. Here, we present the first systematic description of bipedal behavior in chacma baboons (Papio ursinus griseipes). Using all-occurrence sampling across two troops of baboons in Gorongosa National Park, Mozambique, we recorded 231 bipedal bouts over 12 observation days. Bipedalism was predominantly postural and most frequently occurred in vigilance contexts. Several social contexts of bipedalism were recorded, including play, grooming, socio-sexual mounting, and attempted infant snatching. Arboreal bipedalism was rare and primarily linked to foraging. Rates and contexts of bipedalism varied across age-sex classes. Infants and juveniles exhibited higher bipedalism rates than other classes and expressed the broadest bipedal context repertoire. Vigilance was the only context prompting bipedalism across all age-sex classes. We discuss these patterns to evaluate how multi-contextual, socially-relevant and age- and sex-structured bipedalism behavior in baboons may inform reconstructions of early hominin bipedalism.