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Brief communication Arboreal bipedalism in Bwindi chimpanzees.

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Brief Communication: Arboreal Bipedalism in
Bwindi Chimpanzees
Craig B. Stanford*
Jane Goodall Research Center, Department of Anthropology, University of Southern California, Los Angeles,
California 90089-0032
arboreal bipedalism; chimpanzees; positional behavior
Evidence of the form and function of bipedal behavior in nonhuman primates provides critical
evidence to test theories about the origins of hominid
bipedalism. Bipedalism has long been considered an evolutionarily interesting but rare behavior in wild chimpanzees. During May 2001, chimpanzees of the Ruhija community in the Bwindi Impenetrable National Park,
Uganda, engaged in an exceptional frequency of arboreal
bipedalism when feeding in large Ficus trees. Seventyeight bipedal bouts of at least 5 sec duration were recorded
for the entire community (0.49 bouts/hr), with a mean
duration of 13.7 sec (⫾1.6 sec). The animals employed
many variations on the bipedal postural theme, ranging
from erect standing on the largest substrates while grasp-
Theories to account for the earliest stages of hominid bipedalism comprise a spectrum of postural adaptations ranging from obligate terrestrial bipedal
walking (Lovejoy, 1978, 1988) to bipedalism with
substantial retained arboreality (Stern and Susman, 1983; Susman et al., 1984). Some researchers
have advocated anatomical/behavioral precursors
for terrestrial bipedalism in the vertical climbing
and brachiation patterns of great apes (Fleagle et
al., 1981), while others have argued for a terrestrial
bipedal phase following arboreal bipedal locomotion.
The likeliest selection pressure on the transition
from an arboreal ape with an unknown degree of
bipedalism to a ground-dwelling biped is a subject of
much debate (e.g., Isaac, 1978; Rodman and
McHenry, 1980; Steudel, 1994; Wheeler, 1984). Jolly
(1970), Wrangham (1980), and Rose (1984, 1991)
argued that bipedalism arose from adaptations to
terrestrial feeding on small objects such as seeds or
small fruits. They pointed out that chimpanzees and
baboons sometimes shuffle bipedally short distances
between feeding sites rather than switching between bipedal feeding and quadrupedal locomotion.
More recently, Hunt (1994, 1996) provided an arboreal perspective on small-object feeding and bipedalism. In the most detailed field study conducted of
chimpanzee bipedalism, Hunt (1994, 1996) documented 97 instances of bipedalism in Mahale National Park chimpanzees in 701 hr of observation
(0.14 bouts/hr), more than 80% of which occurred in
ing overhead limbs for support, to standing on one leg
while suspending the other leg in space, to extended-lean
standing, in which bipedal standing transitioned into horizontal arm-leg suspension as the animal reached for more
distant fruits. Bipedalism was used as part of a behavioral
repertoire that integrated brachiation, four-limbed suspension, and forelimb-supported standing for effective smallfruit foraging. These observations suggest that under certain
ecological conditions, arboreal bipedalism can be an important posture for wild chimpanzees, and may have been an
important behavioral precursor to full terrestrial bipedalism. Am J Phys Anthropol 119:87–91, 2002.
2002 Wiley-Liss, Inc.
a feeding context. Bipedalism in Hunt’s (1994, 1996)
study was as commonly seen in trees as on the
ground, and was used especially when foraging for
small-diameter fruits such as figs. Nearly all of his
bipedalism data were based on postural, as opposed
to locomotor, bipedalism.
In this paper, I report on the extensive use of
arboreal bipedalism by a previously little-known
chimpanzee population. During one 2-week period
in May 2001, the Ruhija chimpanzees of Bwindi
Impenetrable National Park were bipedal at a rate
exceeding that published for any other population of
free-ranging chimpanzees.
Grant sponsor: National Geographic Society; Grant sponsor:
C.I.E.S. (Fulbright) Foundation; Grant sponsor: L.S.B. Leakey Foundation; Grant sponsor: Wenner-Gren Foundation; Grant sponsor: University of Southern California.
*Correspondence to: Craig B. Stanford, Jane Goodall Research Center, Department of Anthropology, University of Southern California,
Los Angeles, CA 90089-0032. E-mail:
Received 2 August 2001; accepted 26 November 2001.
DOI 10.1002/ajpa.10050
Published online in Wiley InterScience (www.interscience.wiley.
The chimpanzees (Pan troglodytes schweinfurthii)
of Bwindi Impenetrable National Park in southwestern Uganda (S 01° 05⬘, E 29° 38⬘) have been studied
since 1996 as part of the Bwindi Impenetrable Great
Ape Project, a field study of the sympatric ecology of
chimpanzees and mountain gorillas (Gorilla gorilla
beringei). Bwindi Impenetrable National Park (331
km2) is one of the largest remaining tracts of East
African afro-montane forest still in existence, and
has a high level of floral and faunal species endemism. The Ruhija chimpanzee community occupies
a home range of unknown size in the northeastern
sector of the park, within an elevational gradient
from approximately 2,000 –2,350 m. Although the
study subjects cannot be followed on the ground at
close range, they tolerate observers within 30 m
while in feeding trees. Twenty individually identified members of the study community were observed
during May 2001 as they fed in an adjacent pair of
large strangler figs (Ficus natalensis, Moraceae): 5
adult males, 1 subadult male, 5 adult females, 5
infants, and 4 juveniles.
Data were collected on the 9 days on which the
chimpanzees fed in the Ficus. Observations were
made with binoculars, and data were collected in
15-min scan sampling whenever animals were visible between 0715–1430 hr, with critical events noted
ad libitum. Bipedalism was defined, following Hunt
(1994), as a posture in which more than 50% of the
body’s weight was judged to be supported by the
hind limbs. If the forelimbs lent support or stability,
as by grasping an overhead branch, the posture was
characterized as assisted bipedalism. In practice,
the diversity and gradations of bipedal and semibipedal postures sometimes made it difficult to judge
at what point most of the weight support had been
shifted to the legs. Since all observed instances of
bipedalism were arm-assisted (no unassisted bipedalism standing or walking was seen), all bipedal
bouts involved some degree of support from the upper body. Other variants on bipedalism were noted
descriptively and later categorized on the basis of
Hunt (1994).
A total of 38.5 hr of observational data was collected on 20 individuals, during which 78 instances
of bipedalism by 9 individuals were recorded, an
average of 0.49 bouts per observation hour for the
entire community. All bipedalism occurred arboreally on the larger limbs of the Ficus natalensis, and
all instances occurred in a fig-fruit feeding context.
Individuals varied widely in their tendency to be
bipedal (Fig. 1), as well as in the average duration of
their bipedal bouts. One male (KD) was bipedal at
least 24 times in 20.25 contact hours. The sample
included 4 adult males and 4 adult females; males
were bipedal significantly more often than females
(Table 1).
The overall mean duration of bipedal bouts was
13.7 sec (⫾1.6 sec). Females exhibited a longer mean
duration of bipedal bouts, though not significantly so
(Fig. 2, P ⬎ 0.05). This longer duration was due
mainly to one adult female (MA) who engaged in
several lengthy bipedal feeding bouts of up to 65 sec.
The females stood bipedally while carrying dependent infants, either on their backs or clinging to
their chests. On three occasions, pairs of chimpanzees stood bipedally side-by-side while foraging from
the same cluster of figs.
Perhaps the most noteworthy aspect of these observations of bipedalism was not its frequency, but
its continuity with other forms of arboreal posture
and locomotion. Although some bipedal bouts consisted simply of the animals standing upright on a
limb while reaching up for overhead fruits, in at
least 45 of 78 instances (58%), bipedalism was used
either in conjunction with, or graded into, armhanging and foot-hanging postures that allowed a
greater reach during fruit-foraging. For instance, a
chimpanzee would, after a short period standing
bipedally with both feet planted on the limb and one
hand lending support overhead, reach further out
into space for more fruit. As it did so, its body would
slowly lean from vertical toward horizontal, and at
some point the arm on the branch above would shift
from a supporting role to a brachiator role. This
allowed the animal to swing across to an adjacent
limb, whereupon it might resume a bipedal posture.
Alternatively, the two-legged bipedal posture might
give way to one leg on the substrate and one leg
lifted in the air, allowing the chimpanzee greater
reaching distance with its arms.
As in Hunt (1994), foraging for small-diameter
fruits (in this case, figs) seemed to promote bipedal
posture. No data were collected on substrate diameter or angle, but this was clearly also a major determinant of bipedal foraging. Bipedalism was observed only on the largest terminal branches of the
Ficus, perhaps because only these limbs provided
both access to preferred ripe figs and a large-diameter substrate for standing upright.
Whether the extensive use of bipedalism noted at
Bwindi is characteristic of the chimpanzees there or
was a rare aberration is unknown. Certainly many
primates engage in brief bouts of arboreal bipedalism while foraging. If Bwindi chimpanzees are more
bipedal than other chimpanzee populations, it is
unknown whether this is due to aspects of the forest
structure at Bwindi that promote arboreal bipedal
foraging, or is a culturally inherited locomotor tradition.
Some evidence suggests that a special foraging
substrate may have accounted for the level of bipedalism observed. The large Ficus natalensis that
was the main feeding site for these data was growing on a steep hillside, leaning at an approximately
45° angle from the vertical. Because of this, the large
Fig. 1. Bipedalism bouts observed in individual chimpanzees. FD, KM, MA, and AF are adult females; FR, KD, KU, and MB are
adult males; JU and YW are immatures.
TABLE 1. Frequency of bipedalism by age-sex class
during May 2001
Total bipedal bouts
% 15-min scans,
Adult male
Adult female
outer branches of the fig were nearly horizontal,
providing arboreal substrates that were both strong
enough to be stood upon and also with fruits reachable on the terminal branches. Had the tree been
standing vertically, these branches would have been
angled closer to the vertical and would not have
been substrates on which bipedalism could have
been easily used. The unusual angle of growth created a web of branches that were nearly horizontal
and in some cases parallel to each other, allowing
chimpanzees to stand on one limb and reach upward
to the next for support and for fruit. Since limited
observations of the same animals in other tree
crowns produced no evidence of bipedalism, the level
of bipedalism reported in this paper may have been
promoted by the unusual angle of growth of the
One explanation for bipedalism in wild chimpanzees has been injuries to the arms or hands. Following a polio epidemic in the 1960s in Gombe National
Park, Tanzania, some chimpanzees who suffered paralysis to the upper body began to walk bipedally
(Goodall, 1986). Snare injuries to the hands are a
common problem for chimpanzees in Ugandan forests, and this might explain a high incidence of
bipedalism. However, among the Ruhija chimpanzees only one animal, the adult male (KU) who accounted for 4 of the 78 bouts of arboreal bipedalism,
possessed a severe snare injury. It seems unlikely,
therefore, that injury accounts for the frequency of
bipedalism among Bwindi chimpanzees.
Despite widespread anecdotal mention of the degree of bipedalism in bonobos, P. paniscus (e.g.,
Kano, 1992; de Waal and Lanting, 1997), there are
no published data indicating that wild bonobos are
more bipedal than wild chimpanzees. Doran and
Fig. 2.
Mean duration of bipedalism bouts, males vs. females. Immatures were omitted due to small sample size.
Hunt (1994) compared locomotor data on the two
species and found that bonobos were more arboreal
than wild chimpanzees, but their analysis did not
include bipedalism data. Videan and McGrew (2001)
showed that among captive populations of both species, there was no statistically significant difference
in the degree of bipedalism. Assertions that bonobos
are more bipedal than chimpanzees should therefore
be regarded with caution. It has long been thought
that savanna-dwelling chimpanzee populations may
be highly bipedal. Data from Bwindi chimpanzees,
like those of Hunt (1994), suggest that forest-living
chimpanzees may be as bipedal, although on different substrates, as those living in more open landscapes.
Chimpanzees have long been a model for the evolution of human behavior, and chimpanzee locomotion has long been used as evidence in debates about
the locomotor patterns of the earliest hominids.
Washburn (1963, 1968), for example, saw terrestrial
knuckle-walking as the way our immediate prehominid ancestors must have traveled, and advocated early terrestriality on emerging savannas as
the behavioral ecological shift that preadapted prehominids to bipedalism. Bipedalism and its variants
may, however, have emerged from an arboreal hab-
itus that placed an evolutionary premium on foraging benefits that accrued to apes during small-object
feeding (Tuttle, 1981; Rose, 1984; Hunt, 1994). Further studies of bipedal posture and locomotion in
free-ranging great apes can address this question as
a key complement to the fossil record.
Research in the Bwindi Impenetrable National
Park was carried out with the permission of the
Uganda Wildlife Authority, the Ugandan National
Council for Science and Technology, and the Institute for Tropical Forest Conservation. I gratefully
acknowledge Dr. Alastair McNeilage, Caleb Mgambaneza, John Bosco Nkurunungi, and Gervase Tumwebaze for their assistance.
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