Chimpanzee locomotor energetics and the origin
of human bipedalism
Michael D. Sockol*, David A. Raichlen†, and Herman Pontzer‡§
*Department of Anthropology, University of California, Davis, CA 95616;†Department of Anthropology, University of Arizona, Tucson, AZ 85721;
and‡Department of Anthropology, Washington University, St. Louis, MO 63130
Edited by David Pilbeam, Harvard University, Cambridge, MA, and approved June 12, 2007 (received for review April 9, 2007)
Bipedal walking is evident in the earliest hominins [Zollikofer CPE,
Ponce de Leon MS, Lieberman DE, Guy F, Pilbeam D, et al. (2005)
Nature 434:755–759], but why our unique two-legged gait evolved
remains unknown. Here, we analyze walking energetics and bio-
mechanics for adult chimpanzees and humans to investigate the
long-standing hypothesis that bipedalism reduced the energy cost
of walking compared with our ape-like ancestors [Rodman PS,
McHenry HM (1980) Am J Phys Anthropol 52:103–106]. Consistent
with previous work on juvenile chimpanzees [Taylor CR, Rowntree
VJ (1973) Science 179:186–187], we find that bipedal and quadru-
pedal walking costs are not significantly different in our sample of
adult chimpanzees. However, a more detailed analysis reveals
significant differences in bipedal and quadrupedal cost in most
individuals, which are masked when subjects are examined as a
group. Furthermore, human walking is ?75% less costly than both
between bipedal and quadrupedal walking, as well as between
chimpanzees and humans, is well explained by biomechanical
differences in anatomy and gait, with the decreased cost of human
walking attributable to our more extended hip and a longer
hindlimb. Analyses of these features in early fossil hominins,
coupled with analyses of bipedal walking in chimpanzees, indicate
that bipedalism in early, ape-like hominins could indeed have been
less costly than quadrupedal knucklewalking.
biomechanics ? human evolution ? locomotion ? limb length ? inverse
divergence of the human lineage from the other apes. One
enduring hypothesis for this transition is that bipedalism evolved
to reduce locomotor costs in early hominins, relative to the
ape-like last common ancestor (LCA) of chimpanzees and
humans (3). Under this scenario, reducing the energy cost of
walking provided early hominins with an evolutionary advantage
over other apes by reducing the cost of foraging. Such an
advantage may have been especially important given the cooler,
drier climate that prevailed at the end of the Miocene (4, 5) and
that would have increased the distance between food patches.
Testing this hypothesis requires comparative data not only on
the cost of locomotion (COL) in humans and chimpanzees but
also on the biomechanical determinants of these costs. The only
previous study of chimpanzee locomotor cost used juvenile
chimpanzees and indicated that bipedalism and quadrupedalism
were equally costly in chimpanzees and that both were more
costly than human locomotion (6). Although this study has been
central to the debate over energetics and the evolution of
bipedalism (3, 7), the reliability of these data has been ques-
tioned because adult and juvenile locomotor mechanics and
costs can differ substantially (7) and because of recent evidence
that bipedalism is more costly than quadrupedalism in other
did not include a biomechanical analysis of the determinants of
chimpanzee locomotor costs, limiting the potential application
of the study to the hominin fossil record.
of the earliest hominins (2) and thus marks a critical
Here, we compare human and adult chimpanzee locomotor
energetics and biomechanics to determine links among anatomy,
gait, and cost. Our study focuses on two primary questions. First,
do adult chimpanzees follow the pattern of costs found previ-
ously for juveniles (6)? Second, do differences in anatomy and
gait between bipedal and quadrupedal walking, as well as
between chimpanzees and humans, explain observed differences
in cost? Using this biomechanical approach to link differences in
anatomy and gait to cost, we then examine what changes, if any,
would lower the cost of bipedalism for an early hominin, such
that bipedalism would be more economical than the ape-like
quadrupedalism of the last common ancestor.
We focused on walking speeds because walking is the gait
commonly used during terrestrial travel in wild chimpanzees (9).
We tested two sets of predictions; first, based on recent studies
of primate mechanics and energetics (8, 10), we predicted that
bipedal and quadrupedal (i.e., ‘‘knucklewalking’’) costs will
differ in adult chimpanzees and that both bipedal and quadru-
pedal walking in chimpanzees will be energetically more costly
relative to other quadrupeds and humans. Second, following
previous work (11, 12), we predicted that these differences in
cost would be explained by corresponding differences in (i) the
force required to support bodyweight during each step and (ii)
the volume of muscle activated to generate one unit of ground
To test these predictions, we collected metabolic, kinematic,
and kinetic data during walking from five chimpanzees, aged
6–33 years, and four adult humans (see Table 1 and Methods).
The magnitude of ground force was estimated as the inverse of
the duration of foot–ground contact time, tc, per step (11, 13),
and the volume of muscle activated per unit of ground force,
Vmusc, was estimated by using inverse dynamics (14) (see Meth-
ods). Following Roberts et al. (12), we predicted that the COL
(ml of O2kg?1s?1), varies as the ratio of Vmuscand tc, Vmusc/tc.
Thus, any difference in Vmusc/tc, either between species or
between gaits, should lead to a proportional difference in COL.
Results and Discussion
The mass-specific cost of transport (ml of O2 kg?1m?1) for
chimpanzees was greater than expected for their body size (15)
(Fig. 1). In contrast, human walking was less expensive than
Author contributions: M.D.S., D.A.R., and H.P. contributed equally to this work; M.D.S.,
D.A.R., and H.P. designed research; M.D.S., D.A.R., and H.P. performed research; M.D.S.,
D.A.R., and H.P. analyzed data; and M.D.S., D.A.R., and H.P. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission.
Abbreviations: COL, cost of locomotion (measured as the mass-specific rate of oxygen
force during locomotion.
§To whom correspondence should be addressed at: Department of Anthropology,
Washington University, 119 McMillan Hall, St. Louis, MO 63130. E-mail: hpontzer@
This article contains supporting information online at www.pnas.org/cgi/content/full/
© 2007 by The National Academy of Sciences of the USA
July 24, 2007 ?
vol. 104 ?
no. 30 ?