Energy expenditure of bonobos across their locomotor repertoire

Unlike quadrupedal mammals, primates use diverse locomotor modes adapted to arboreal environments. Such diversity of movements challenges allometric models, which underestimate energetic costs by focusing on steady-state walking. To address this, we combined ethological and biomechanical approaches to estimate the net metabolic cost of transport in bonobos (Pan paniscus). Using three-dimensional tracking and a point-mass model, we derived this estimate from net mechanical power and muscle efficiency across locomotor modes. Our results show that quadrupedalism in bonobos (10-12 J kg⁻¹ m⁻¹) exceeds mammalian predictions (approx. 3 J kg⁻¹ m⁻¹), whereas vertical ascent (26-32 J kg⁻¹ m⁻¹) is the most costly mode. However, arboreal locomotion incurs no major energetic penalty, given the high potential for energy recovery via kinetic/potential energy exchange during vertical descent and suspensory locomotion. By integrating these data with field-derived locomotor repertoires, we estimate that daily travel distances (0.4-6 km) account for 3-53% of bonobos' daily energy expenditure. Reversing quadrupedal/bipedal proportions in this model shifts costs to 4-62%. Notably, even with 35% bipedalism, 2 km daily travel remains energetically feasible for bonobos. This suggests that early hominins could have incorporated bipedalism into diverse repertoires without facing energetic constraints, and that increased travel distance may have driven selective pressure for greater bipedal efficiency.