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Comparative locomotor behavior of chimpanzees and bonobos The influence of morphology on locomotion.

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AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY 91 :83-98 (1993)
Comparative Locomotor Behavior of Chimpanzees and Bonobos:
The influence of Morphology on Locomotion
DIANE M. DORAN
Department of Anatomical Sciences, SUNY at Stony Brook, Stony Brook,
New York 117948081
KEY WORDS
Pygmy chimpanzees, Positional behavior, Pan
paniscus, Pan troglodytes
ABSTRACT
Results from a 10 month study of adult male and female
bonobos (Pan paniscus) in the Lomako Forest, Zaire, and those from a 7
month study of adult male and female chimpanzees in the Tai Forest, Ivory
Coast (Pan troglodytes verus), were compared in order to determine whether
there are species differences in locomotor behavior and substrate use and, if
so, whether these differences support predictions made on the basis of interspecific morphological differences. Results indicate that bonobos are more
arboreal than chimpanzees and that male bonobos are more suspensory than
their chimpanzee counterpart. This would be predicted on the basis of male
bonobo’s longer and more narrow scapula. This particular finding is contrary
to the prediction that the bonobo is a “scaled reduced version of a chimpanzee”
with little or no positional behavior difference as had been suggested. This
study provides the behavioral data necessary to untangle contradictory interpretations of the morphological differences between chimpanzees and bonobos, and raises a previously discussed (Fleagle: Size and Scaling in Primate
Biology, pp. 1-19, 1985) but frequently overlooked point-that isometry in
allometric studies does not necessarily equate with behavioral equivalence.
Several researchers have demonstrated that bonobos and chimpanzees follow
the same scaling trends for many features, and are in some sense functionally
equivalent, since they manage to feed and reproduce. However, as reflected in
their morphologies, they do so through different types and frequencies of
locomotor behaviors. o 1993 Wiley-Liss, Inc.
Since the discovery of the bonobo (Panpaniscus; Schwarz, 1929), there has been considerable debate as to the nature and meaning of the morphological differences between
it and the chimpanzee (P. troglodytes). This
paper assesses how these morphological differences are correlated with differences in
behavior, by reviewing the morphological
differences between the species, considering
several proposed explanations for these differences, and finally, examining whether
these behavioral hypotheses can be supported by providing results from a comparative study of bonobo and chimpanzee locomotor behavior and substrate use.
0 1993 WILEY-LISS. INC
INTERSPECIFIC MORPHOLOGICAL
DIFFERENCES
The degree of size difference between
bonobos and chimpanzees has received particular attention. Coolidge (1933) described
a small specimen and estimated that bonobos were one-half the size of chimpanzees.
Ironically, as museum collections grew, it
became apparent that the early specimen
described by Coolidge (1933) was the smallest of all specimens collected.
Received July 13, 1992; accepted October 30, 1992
84
D.M. DORAN
More recently, with further study of additional specimens, some researchers have argued that the purported size difference is
small or nonexistent (Schultz, 1969; Zihlman and Cramer, 1978; Horn, 1979; McHenry and Corruccini, 1981; Susman and
Jungers, 19811, whereas others continue to
see the bonobo as a smaller, more gracile
animal (Pilbeam and Gould, 1974; Cousins,
1978; Johnson, 1981; Shea, 1983,1984).
Much of this confusion has resulted from
the failure t o distinguish among the three
geographically distinct chimpanzee subspecies which, in themselves differ significantly
in body size (Jungers and Susman, 1984). In
comparison with the three subspecies of
chimpanzees, the bonobo does not differ significantly in body weight from P. t. schweinfurthii, but is significantly lighter than P. t.
troglodytes (Jungers and Susman, 1984).
The sole available body weight for the third
subspecies of chimpanzee (P. t. uerus) places
it closer in size to P. t. schweiiafurthii than to
P. t. troglodytes (Jungers and Susman, 1984).
Beyond differences in body weight, bonobos and chimpanzees differ with respect to
skull shape and relative size (Coolidge,
1933; Weidenreich, 1941; Fenart and Deblock, 1973; Cramer, 1977; Shea, 19821,
teeth (Kmzey, 1971,1984; Johanson, 19741,
and postcrania.
Of particular interest with regard to potential differences in locomotor behavior are
the postcranial differences. These bonobo
differences include: a significantly lower intermembral index (Susman, 1979); a more
gracile upper body with a shorter clavicle
(Zihlman and Cramer, 19781, reduced chest
girth (Coolidge and Shea, 19821, and, at
least in males, a narrower scapula (Coolidge, 1933; Roberts, 1974; Jungers and Susman, 1984; Shea, 1986); a smaller pelvis
(Zihlman and Cramer, 1978), with a shorter
ilium than that of P. t. uerus or P. t. troglodytes, and a shorter pubis than any of the
three subspecies of chimpanzees (Jungers
and Susman, 1984); and hands with more
curved proximal and middle phalanges, and
metacarpals that display less well developed
dorsal ridges and thicker cortices in their
metacarpals and phalanges (Susman, 1979).
Finally, in marked contrast to chimpanzees,
bonobos are characterized by minimal sex-
ual dimorphism in their postcranial skeletons, in spite of significant sexual dimorphism in body weight (Zihlman and Cramer,
1978; Jungers and Susman, 1984).
In assessing how many of the morphological distinctions between the two species are
related to body size differences, Shea (1984,
1986) and Jungers and Susman (1984) find,
based on studies of ontogenetic allometry,
that with regard to chest girth, arm span,
scapula size and shape, and intralimb proportions of the two species, the ontogenetic
scaling trends exhibit considerable overlap.
As Shea (1984) notes, “the proportions observed in adult common chimpanzees are
those predicted if the growth patterns of the
pygmy chimpanzee are simply extended to
larger terminal size.” This is reminiscent of
Coolidge’s claim (1933, 1984) that the
bonobo is a “true paedomorph,” that is, a n
animal which retains juvenile characters in
the adult form.
PROPOSED INTERSPECIFIC
BEHAVIORAL DIFFERENCES
Several hypotheses, including the “suspensory” (Susman, 19791, the “best prototype of prehominid ancestors” (Zihlman
et al., 19781, the “scaled reduced common
chimpanzee” (McHenry and Corruccini,
19811, and the “true paedomorph (Coolidge,
1933) have been proposed to account for
these postcranial morphological differences.
Implicit in these hypotheses is the existence
of behavioral differences between the two
species which are associated with their morphological differences.
The “suspensory” hypothesis suggests
that these differences are specializations resulting from the bonobos’ isolation and adaptation to forest dwelling and arboreal existence (MacKinnon, 1978; Susman, 1979;
Horn, 1979; Johnson, 1981; Latimer et al.,
1981). Among the morphological evidence
cited to support this is the bonobos’ possibly
smaller body size, reduced postcranial sexual dimorphism, long and narrow scapula
(in males), and curved phalanges (Coolidge,
1933; Frechkop, 1935; Roberts, 1974; Susman, 1987). Behavioral predictions include
the bonobos more frequent use of arboreal
behaviors, particularly more armswinging,
in comparison with chimpanzees.
LOCOMOTOR BEHAVIOR OF CHIMPANZEES AND BONOBOS
A second, and mutually exclusive, explanation is that the bonobo is a “scaled reduced version” of a chimpanzee (Corruccini
and McHenry, 1979; McHenry and Corruccini, 1981) with morphological differences
between the two species due simply to body
size differences and the effects of allometry.
Behavioral predictions for this hypothesis
are that there are no positional behavior differences. In fact, McHenry and Corruccini
(1981) state that “in most respects the postcranium of Pan paniscus is an allometrically
scaled version of P. troglodytes and they
both retain essentially the same locomotor
and postural adaptations.”
It is known that animals with different
body sizes face different constraints within
the same habitat, and as a result are characterized by different locomotor and postural
profiles (Cartmill and Milton, 1977; Fleagle
and Mittermeier, 1980; Fleagle, 1985; Cant,
1989; Doran, 1992a, in press). In a given
arboreal habitat, larger animals leap less
and climb, bridge and use suspensory behavior more frequently than smaller animals. Therefore, in this “scaled down chimpanzee” hypothesis, the difference seen in
the behavior of bonobos and chimpanzees
should not be any greater than that between
chimpanzees with a comparable body size
difference.
Finally, Coolidge (1933, 1984) noted that
the adult bonobo is a “true paedomorph,”
because some of its adult proportions and
features are present in juvenile chimpanzees. Most important of these features in regards to predictable locomotor differences is
the long and narrow scapula. Historically, a
functional correlation has been drawn between narrow scapulae and arm-swinging
locomotion (Coolidge, 1933; Susman et al.,
1980). This has frequently been cited to predict that adult bonobos are more suspensory
than their chimpanzee counterpart (Coolidge, 1933; Frechkop, 1935; Roberts, 1974;
Susman et al., 1980).
Shea (1984, 1986) and Jungers and Susman (1984) have demonstrated that the
bonobo scapula scales ontogenetically with
that of chimpanzees. Thus the same morphological difference that distinguishes
bonobos from chimpanzees also serves to
distinguish immature and mature chimpan-
85
zees (and bonobos). Since ontogenetic
changes occur in primate locomotion (i.e.,
Goodall, 1968; Rose, 1977; Crompton, 1983;
Doran, 1992a), one could predict that bonobos are more similar to immature chimpanzees in their behavior than they are to adult
chimpanzees. Since immature chimpanzees
are more suspensory than mature chimpanzees (Doran, 1992a), one could also predict
that adult bonobos are more suspensory
than adult chimpanzees.
Predicted locomotor differences cannot be
presently supported or rejected by existing
locomotor data (but see Susman, 1984;
Hunt, 1992). In order to accurately assess
differences in arboreal behavior it is essential that bonobos and chimpanzees be studied in as similar habitat (i.e., lowland rain
forest) as possible. This paper presents new
data on the locomotor behavior and substrate use of bonobos in the Lomako Forest,
Zaire and compares it to recently published
data on chimpanzees from the Tai Forest,
Ivory Coast (Doran, in press) in order to test
whether there are predictable differences in
the locomotor behavior of forest dwelling
bonobos and chimpanzees.
Specific questions to be addressed include-
1. Are bonobos more arboreal than chimpanzees, and if so, in what way? Do they
engage in more frequent arm-swinging than
chimpanzees?
2. If there are locomotor differences between the two species, do they follow predictions based on body size differences alone, or
are there species differences in behavior
that cannot be explained by a possible body
size difference?
3. Are bonobos more similar to immature
rather than mature chimpanzees in their locomotor behavior?
METHODS
Study site and sampling methods:
Pan paniscus
Pan paniscus was observed for a total of
220 hours during 10 consecutive months,
from December 1986 through October 1987,
in the Lomako Forest, Zaire. For a complete
description of the study site see Badrian and
86
D.M. DORAN
Badrian (1977), Badrian and Malenky
(1984), and White (1986).
Results of locomotor behavior and substrate use are based on continuous focal animal locomotor bout sampling and include
the proportion of all bouts spent in each locomotor activity (% bouts-no distance), the
mean distance travelled per bout, and the
percent of bouts weighted by the distance
travelled per bout (% of bouts with distance).
The most accurate way to interpret locomotor bout data is from the percent of bouts
(with distance) (Doran, 1992b), and therefore all analyses of bonobos in this study are
based on bouts (with distance). The percentage of bouts (no distance) are included for
comparison with previous studies.
On each occasion that bonobos were contacted, they were followed for as long as possible. Arboreal andlor terrestrial travel was
recorded in 72.1% of all sightings (n = 111).
In 27.9% of all sightings, the animals were
lost immediately upon their first descent to
the ground, and thus, no travel was recorded.
During follows of bonobos, 45% of all
sightings with travel (n = 80) include sightings with terrestrial travel. I used two measures of terrestrial travel. Direct ground
travel, which accounts for 14.3% of total
ground distance followed, is when the chimpanzees are visible during travel, although
often this includes only glimpses of moving
animals. Indirect travel, which accounts for
85.7% of all ground follows, is when visual
contact is interrupted during the follow, and
resumed within 20 minutes. If the exact
route the chimpanzee took was not known,
the shortest distance between the starting
and ending points was measured.
Qualitative intersite comparisons of Pan
paniscus are based on observations and discussions with Drs. Kuroda and Furuichi
during a one week visit to the Wamba
(Zaire) study site in October 1987. For a description of the study site see Kano (1980,
1992), Kuroda (1980) and Furuichi (1987).
Study site and sampling methods: Pan
troglodytes verus
One nonprovisioned, habituated community of Pan troglodytes uerus was observed
for 430 hours during seven consecutive
months from March through September
1988 in the Tai National Park, Ivory Coast.
The Tai Forest was considered the most appropriate study site for comparison with
bonobos becuase it is a lowland rain forest
which shares many tree species in common
with the Lomako Forest. Since a primary
consideration of this study is to determine
whether there are differences in how arboreal the two species are, it was considered
essential that they be studied in environments that are as similar a s possible. Since
bonobos are restricted to lowland rain forest, it was determined that chimpanzees
should be studied in a lowland rain forest. In
addition, the chimpanzees of the Tai Forest
have been studied continuously since 1979
by Dr. C. Boesch and H. Boesch, and so represent the only existing community of well
habituated lowland rain forest dwelling
chimpanzees. For a complete description of
the study site and habituation process see
Boesch and Boesch (Boesch, 1978, 1991;
Boesch and Boesch, 1981,1983,1984,1989).
Focal animal sampling (Altmann, 1974)
was used and each focal animal was followed
for a n entire day, or a s long as possible
(mean length of focal animal sample = 259
min). Results reported here are from Doran
(199213) and are from one-minute instantaneous sampling (1msps).
Comparison of data from the two
separate sites
Several factors allow for direct comparisons of the results from the two sites. One
observer made all of the observations (of
both species) used in this study. Thus, all
categories of locomotor activity, substrate,
and social and maintenance activity are
identical a t the two study sites. Data obtained from concurrent instantaneous and
continuous locomotor bout (with distance)
sampling of chimpanzee locomotor behavior
do not yield different results (Doran, 1992b).
Thus, results obtained from instantaneous
sampling of chimpanzees and locomotor
bout sampling of bonobos are directly comparable.
However, there are differences a t the two
study sites that make direct comparisons
difficult. The primary difficulty was the uneven level of habituation a t the two study
87
LOCOMOTOR BEHAVIOR OF CHIMPANZEES AND BONOBOS
TABLE 1. A comparison of data from the Lomako Forest pygmy chimpanzee study
with data from the Tai Forest chimpanzee study
1. Dates of studyitotal time spent at each site
2. No. of sightings'
3. Total time of sightings
4. Mean length of sighting
5. Mean length of focal animal sighting
6. Distribution of sighting throughout day (9% total time)
P6
6-8
aio
10-12
12-14
14-16
16-18
7. Distributiodsex
Males
Females
I Jnknown
IA
Lomako
Tai
12/1/88-10/2/88
10 months
83
230 hr
168 rnin
18 rnin
(range = 2-102)
03/01/88-09/25/88
07 months
74
438 hr
355 min
259 min
(range = 16-660)
08
27
28
19
12
05
02
1,797 bouts
2,653 bouts
465 bouts
00
03
16
26
21
21
12
10,290 min
11,098 min
97 min
sighting is defined as any uninterrupted follow of the group in which visual contact is not lost for more than 10 minutes.
sites, which is clearly evident in a comparison of the data collected at the two sites (Table 1).The study of chimpanzees in the Tai
Forest yielded a greater number of total
hours of observation (in spite of the shorter
duration of the study), longer mean duration
per sighting and per focal animal follow, and
a more even distribution of sightings by sex
and throughout all hours of the day compared with the data collected in the bonobo
study (Table 1).
In addition, although Tai chimpanzees
could be followed during terrestrial travel
and accurate estimates of time spent on the
ground versus time spent arboreally could
be made, this was not the case with Lomako
bonobos. Since there were fewer difficulties
in observing the arboreal locomotion of
bonobos, all quantitative comparisons made
between bonobos and chimpanzees in this
study are based on arboreal behavior only.
Statistical analyses
G Tests of Independence (RXC contingency tables) were used to determine
whether the frequency of locomotor behavior
of one species is independent of the frequency of locomotor behavior used by the
other. The various statistics used are described in Sokal and Rohlf (1981). Significance values are from Rohlf and Sokal
(1981). Symbols for significance used include * = P < 0.05; ** = P < 0.01; *** =
P < 0.001; ns = not significant. All G values
have been corrected by Williams correction
factor in order to obtain a better approximation of the chi-square distribution. All tests
are two-tailed and performed on raw data.
RESULTS
Arboreal locomotor behavior of
Pan paniscus
There are no significant differences in the
frequency of locomotor bouts (with distance)
spent in each locomotor activity by adult
mothers with infants and adult females
without infants (Table 2; G = 9.0, ns).
There are, however, significant differences
in the arboreal locomotor behavior of adult
male and female bonobos (Table 2; G [males
vs. mothers] = 51.7***; G [males vs. females] = 52.0""").Males are less quadrupedal and use more quadrumanous climbing
and scrambling than females during arboreal locomotion. There is no significant sex
difference in the frequencies of suspensory
behavior, bipedalism, or leaping.
Both sexes show significant differences in
the frequencies of locomotor behaviors used
during feeding and travel (Table 3; G
[males] = 50.4"**;G [mothers] = 16.1**).
For both sexes, quadrupedalism is used
more frequently during feeding than during
arboreal travel.
88
D.M. DORAN
TABLE 2. Sex differences in the arboreal locomotor actiuitv
0uad
Adult males
26.1
% of bouts (with distance)
17.7
% of bouts (no distance)
4.3
Mean distance b o u t (steps)
n = 993 bouts
Mothers with infants
44.4
% of bouts (with distance)
26.7
% of bouts (no distance)
5.7
Mean distance b o u t (steps)
n = 468 bouts
Adult females (no infants)
%, of bouts (with distance)
39.2
% of bouts (no distance)
25.0
Mean distancehout (steps)
5.0
n = 1147 bouts
G ualues
Mothers with infants vs. adult females (no infants)
Adult males vs. mothers with infants
Adult males vs. adult females (no infants)
G
G
G
=
=
=
of
Pan uaniscus
Categories of locomotor activity’
Climb
Susu
Bioed
Leao
57.9
52.8
3.2
10.0
15.3
1.9
1.1
1.4
2.4
4.8
12.8
1.1
42.8
44.4
3.3
7.8
16.7
1.6
1.9
1.5
4.3
3.1
10.7
1.0
44.6
45.9
3.1
11.8
18.8
2.0
1.2
1.2
3.2
3.1
9.1
1.1
9.0 ns2
51.7***
52.0***
’Locomotor categories: quadrupedalism (quad),quadrumanous climbing and scrambling (climb),suspensory (susp), aided and unaided bipedalism (biped), leaping (leap).
’Significance levels: * = P < 0.05, ** = P < 0 01, *** = P < 0.01. ns = not significant.
nents, including travellascend (ascend), defined as travel in a vertical direction from
ground; travelhorizontal (travel), defined
as
arboreal travel in a horizontal direction
Travel
Feed
along
“arboreal highways”; traveVexit (exit),
Males Females Males Females
defined as exiting one tree and entering
Quadrupedalism
20.0
37.1
47.1
49.7
an adjacent one; travelfdescend (descend),
Climbkcram
64.3
43.3
40.8
41.7
defined as travel in a vertical direction
Suspensory
9.9
14.9
7.0
5.7
Bipedal
0.6
1.5
2.8
0.8
towards the ground; traveVto ground
Leaping
5.1
3.2
2.2
2.1
(ground), defined as the final 5-7 m during
n
691
850
152
156
G (male vs. female)
84.6***
2.2 ns
traveudescend during which the animal
50.4***
G (male feed vs. male travel)
moves
from an arboreal to a terrestrial sub16.1**
G (female feed vs. female travel)
strate.
Significance levels same as in Table 2.
In addition to subdivisions in the category
of travel, the locomotor category of quadruSex differences in arboreal locomotion oc- manous climbing and scrambling has been
cur during travel rather than during feeding divided into its 4 separate components.
(Table 3; G [feed] = 2.2, ns; G [travel] = These activities include: climbing (climb),
84.6***). Both males and females use large scrambling (scram.),tree-swaying (t. sway),
amounts of quadrupedalism and quadru- and bridging (bridge).
There are no significant sex differences in
manous climbing and scrambling during arboreal feeding. However, during arboreal locomotor activities used in ascending from
travel, males use considerably more quadru- the ground, descending during travel, or demanous climbing and scrambling and less scending to the ground (Table 4; G [asquadrupedalism than females. Males also cend] = 0.01, ns; G [descend] = 9.2, ns; G
use slightly more leaping and less suspen- [ground] = 0.03, ns). Both males and females use vertical climbing as the primary
sory behavior than females.
In order to pinpoint exactly what differ- activity during ascent from the ground. The
ences occur during arboreal travel locomo- most frequent means of descending to the
tion, the category of locomotion during ground is by climbing or sliding down a vertravel is further subdivided into five compo- tical trunk, however transferring from a
TABLE 3. A comparison of Pan paniscus arboreal
locomotron used during trauel uersus feeding, expressed as
percentages of distance traveled during bout sampling of
arboreal locomotor behauior
LOCOMOTOR BEHAVIOR OF CHIMPANZEES AND BONOBOS
89
TABLE 5. Frequency of subcategorces of quadrupedalcsm
and of quadrumanous climbing and scrambling
in Pan uaniscus
Percent
_ _
_ time
_ _ .
Males
Mothers
Quadrupedalism
Knuckle-walking
17.0
Palmigrade
83.0
n
176
G (males vs. mothers)
Quadrumanous climbinghcrambling
Quadrumanous climbing
86.3
Scrambling
13.7
n
513
G (males vs. mothers)
ns
=
13.0
87.0
121
0.77 n s
86.3
14.1
208
0.01 ns
not significant.
trunk to some other substrate and using body
weight to ridelcrash it to the ground (treesway) is also a common means of descent.
There are significant sex differences in
how bonobos travel horizontally and exit
from one tree to another (Table 4; G
[exit] = 20.9**; G [travel] = 19.3**).During
horizontal arboreal travel, males use less
quadrupedalism and more quadrumanous
climbing, tree-swaying, and leaping than females. When crossing a discontinuity in arboreal substrate (exit), although both males
and females use bridging most frequently,
females use this conservative means of
crossing more frequently than males. Females also use more suspensory behavior
when crossing a discontinuity, whereas
males use much more leaping than females.
There are no sex differences in the
type of quadrupedalism or quadrumanous
climbing/scrambling used during arboreal
locomotor activity (Table 5; G [quadrupedalism] = 0.77, ns; G [climbk.cramblel = 0.01,
ns). Eighty-five percent of all arboreal quadrupedal activity for both males and females
is palmigrade rather than knuckle-walking.
Of all quadrumanous climbing and scrambling locomotor bouts, both males and females spend 86% of bouts (with distance) in
climbing, and only 14%in scrambling activities.
Arboreal substrate use of Pan paniscus
There are significant differences in the
substrate use of bonobo males, mothers, and
adult females without infants (Table 6; G
[males vs. mothers] = 33.9***; G [males vs.
~
90
D.M. DORAN
TABLE 6. Pan paniscus substrate use during arboreal locomotion
Adult males
% of bouts (with distance)
24.7
12.4
% of bouts (no distance)
15.6
8.0
Mean distancelsubstrate
4.6
4.5
n = 963 bouts
Mothers with infants
I
of bouts (with distance)
15.5
19.6
% of bouts (no distance)
10.3
12.1
Mean distancdsubstrate
5.2
5.6
n = 453 bouts
Adult females (no infants)
I
of bouts (with distance)
13.9
20.3
% of bouts (no distance)
9.0
12.2
Mean distancehbstrate
5.2
5.6
n = 1,116
G values (% of bouts [with distance] converted to raw nos.)
Mothers with infants vs. adult females (no infants)
G = 19.3***
Adult males
vs. mothers with infants
G = 33.9***
Adult males
vs. adult females (no infants)
G = 117.1***
32.3
33.6
2.8
14.1
31.6
1.3
16.5
11.2
4.3
40.0
34.6
4.0
9.8
30.7
1.1
15.0
12.1
4.3
47.1
39.7
4.0
10.7
32.7
8.0
6.4
4.3
1.1
'Substrates are as follows: trunk ( T R b s t o u t , primary members of the tree; bough (BOLsecondary elements that range 15-20 cm in diameter;
branch ( B R t t e r t i a r y supports that range from 2-15 cm in diameter; Lane (LI).
Significance levels are the same as in Table 2
females] = 117.1"""; G [mothers vs. females] = 19.3"""). Adult females without
infants differ from adult mothers in using
branches more frequently, and lianes less
frequently. Both adult mothers and females
differ in substrate use from males in using
trunks less frequently and boughs and
branches more frequently. Foliage use is approximately equal to or slightly higher in
males.
Relationship between arboreal substrate
use and locomotor activity of Pan
paniscus
For all substrates, except branches, there
are no sex differences in the type of locomotor activity performed on a given substrate
(Table 7; G [trunk] = 1.1 ns; G [bough] =
2.3, ns; G [branch] = 12.9"; G [foliage] =
7.6 ns; G [lianel = 3.2, ns). Quadrumanous
climbing is the most frequently occurring activity on trunks and lianes. Palmigrade quadrupedalism is the predominant activity
on boughs. Quadrumanous climbing and
scrambling is used most frequently in foliage.
Adult male and female bonobos do however, differ in the type of activities performed on branches (Table 7; G = 12.9").
Females are more frequently quadrupedal
on branches than are males. Males climb
and scramble, leap, arm-swing, and walk bipedally (aided) more frequently on branches
than females do.
Ranging behavior of Pan paniscus:
Arboreal and terrestrial travel
Arboreal travel, defined here as above
ground travel between feeding or resting
sites, contributes a significant portion of
Lomako Forest bonobo ranging. The exact
proportion of overall distance travelled arboreally versus terrestrially cannot be ascertained at this time for reasons cited
above.
However, 86% of all sightings that included travel of any kind (n = 80) included
some arboreal travel. The mean arboreal
distance travelled per arboreal sighting is
234 m, ranging from 6 to 1,169 m (n = 69).
Fifty-two percent of arboreal travel sightings were of a distance of <ZOO m. Twentynine percent of these sightings were of distances ranging between 200 and 400 m, and
19% of these sightings were for distances of
>400 m. Two sightings included group arboreal travel of >1km.
The mean distance of terrestrial travel
per sighting, based on combined direct and
indirect sightings, is 359.3 m, ranging from
01 to 1,700 m (n = 45). Nearly all observed
terrestrial travel (99.7% of bouts with dis-
LOCOMOTOR BEHAVIOR OF CHIMPANZEES AND BONOBOS
91
TABLE 7. Percentage of male and female locomotor bouts (with distance) spent in each
locomotor activity in Pan paniscus, related to locomotor types
Substrate
Quad
Categories of locomotor activities
susp
Biped
Climb
Leap
n
~
Trunk
Males
Females
G = 1.1ns
Bough
Males
Females
G = 2.3 ns
Branch
Males
Females
G = 12.9*
Foliage
Males
Females
G = 7.6
Liane
Males
Females
G = 3.2 ns
-
97.3
98.9
2.0
1.2
150
101
78.0
85.5
12.7
7.1
3.8
4.7
1.4
0.5
4.1
2.2
77
136
49.0
57.0
27.0
24.7
16.9
15.8
2.9
0.4
4.2
2.1
324
443
0.2
0.2
68.9
61.1
17.8
27.1
-
13.1
11.5
304
365
3.7
7.9
88.2
84.1
6.1
5.7
2.0
1.9
0.3
108
71
Significance levels are the same as in Table 2
tance) is quadrupedal knuckle-walking. Unaided bipedalism accounts for the remaining
0.3% of bouts.
Ranging behavior of Pan troglodytes
verus: Arboreal and terrestrial travel
Arboreal andor terrestrial travel was
recorded during 89.9% of sightings (n =
79). Although adult Pan troglodytes uerus
spent considerable time arboreally (Doran,
1992b), travel between feeding and resting
sites was exclusively terrestrial. During the
7 month study, no adult arboreal travel comparable to that seen in bonobos was recorded.
Eighty-eight percent of the arboreal locomotion during travel recorded for Tai chimpanzees was strictly ascending into feeding
or resting trees, or descending from them to
the ground (n = 6 5 , l msps). The remainder
of adult arboreal travel (n = 7 , l msps) was
spent exiting one feeding or resting tree, and
entering a smaller adjacent tree, either to
rest, or to use in descending to the ground.
The mean horizontal distance travelled during these 8 episodes of arboreal travel was
53.0 m, ranging from 25 to 96 m.
The only incident of arboreal travel between feeding or resting sites at Tai involved a juvenile male (Sartre). On August
16, 1988, he and his mother (Salome) and
her infant were grooming in a tree. The
mother (with infant) descended to the
ground, and slowly knuckle-walked 120 m to
another tree, and then ascended into it. The
juvenile did not descend to the ground, but,
instead travelled along an arboreal route to
join his mother. In 430 hrs of observation,
this was the single episode of arboreal travel
between (non-adjacent) feeding sites.
lnterspecific differences in arboreal
locomotor behavior and substrate use
Both male and female bonobos use more
quadrupedalism and less quadrumanous
climbing and scrambling than their chimpanzee counterparts (Table 8; G [males] =
33.2***; G [mothers] = 11.8"). The species
difference is more pronounced between
males, with P. paniscus males additionally
using more suspensory and leaping behavior
than male P. troglodytes.
For both sexes, there is a significant species difference in the type of arboreal quadrupedalism used (Table 9; G [males] =
14.1***;G [mothers] = 25.6***). Both male
and female bonobos use far more palmigrade quadrupedalism and less knucklewalking quadrupedalism than Tai chimpanzees. In fact, approximately 85% of bonobo
arboreal quadrupedalism is palmigrade,
whereas only 3 0 4 0 % of chimpanzee arbo-
92
D.M. DOFUN
TABLE 8. Interspecific differences in chimpanzee
arboreal locomotor behavior
Pan paniscus
Males Mothers'
Pan troglodytes
Males Mothers'
44.4
11.6
30.3
26.1
Quadrupedalisrn
59.8
42.8
76.7
Quad. climb
57.9
Suspensory
10.0
7.8
5.8
7.4
1.9
5.8
0.8
Biped a1
1.1
1.6
Leap
4.8
3.1
n
993
468
103
122
G Values
Pan paniscus males vs. mothers
51.7"""
17.7**
P. troglodytes males vs. mothers
Males: P. paniscus vs. P. troglodytes 33.2*"*
Mothers: P. paniscus vs. P. troglodytes 11.8"
'Since there were no adult females without infants in the Tai community studied, interspecific comparisons between females are restricted
to mothers
Significance levels are the same a s in Table 2.
TABLE 9. A comparison of Pan paniscus and Pan
troelodvtes arboreal auadruoedalism
Arboreal quadrupedalism i%)
Pan paniscus
Pan troglodytes
Males Mothers Males Mothers
72.7
57.1
17.0
13.0
Knuckle-walking
42.9
87.0
27.3
Palmigrade
83.0
quadrupedalism
n
176
121
11
35
G (males vs. mothers)
0.77 ns
0.88 ns
G (Interspecific difference: males)
14.1*""
G (Interspecific difference: females)
25.6***
Significance levels are the same as in Table 2.
real quadrupedalism is palmigrade. In addition, within each species, females are more
palmigrade in their arboreal quadrupedalism than the males are.
There are neither sex differences within
each species nor interspecific differences in
the type of quadrumanous climbing and
scrambling used (G [males] = 0.01, ns; G
[mothers] = 2.14, ns). For both sexes, quadrumanous climbing is a far more frequent
activity than scrambling.
There are significant interspecific differences in substrate use for both males and
females (Table 10; G [males] = 11.8"; G
Lmothers] = 38.6"""). Male and female Pan
paniscus both use trunks less often and
boughs and branches more often than male
and female Tai chimpanzees. Foliage use is
approximately equal in the two species. Lianes are used less frequently by bonobo
mothers than chimpanzee mothers.
TABLE 10. Interspecific differences in chimpanzee
arboreal substrate use
Pan paniscus
Males
Mothers
Pan troglodytes
Males
Mothers
Substrate
Trunk
24.7
15.5
34.9
34.4
Bough
12.4
9.7
7.4
19.6
Branch
32.3
40.0
18.4
22.9
9.8
18.4
10.7
Foliage
14.1
15.0
18.4
24.6
Liane
16.5
n
963
453
103
122
G values
Pan paniscus males vs. mothers
G = 51.7%""
P. troglodytes males vs. mothers
G = 4.2 ns
Males: P. paniscus vs. P. troglodytes G = 11.8"
Mothers: P. paniscus vs. P. troglodytes G = 38.6"""
Significance levels are the same as in Table 2
There are no interspecific differences in
the frequency of activity performed on
trunks, branches, foliage, or lianes (Table
11;G [trunk] = 0.34, ns; G [branch] = 7.24,
ns; G [foliage] = 4.61, ns; G [lianel = 4.50,
ns). For both species, quadrumanous climbing is the most frequently used activity on
vertical trunks and lianes. Quadrupedalism
accounts for 26% of all locomotor activities
occurring on trunks at Tai. However, this
is because horizontal (treefalls) and vertical trunks are included in the same category. Quadrupedalism occurs on horizontal
trunks. The results of trunk use are not statistically different at the two sites. Quadrumanous climbing and scrambling is used
most frequently on branches, although both
bonobos and chimpanzees use climbing,
scrambling, and suspensory behavior as
well on branches. In foliage, quadrumanous
climbing and scrambling and suspensory behavior are frequent activities for both species.
However, there is an interspecific difference in bough use (Table 11; G = 21.5""").
Bonobos are most frequently quadrupedal
(and particularly palmigrade quadrupedal)
when on boughs, whereas chimpanzees use
little quadrupedalism and far more quadrumanous climbing, scrambling and aided bipedalism.
DISCUSSION
Pan paniscus locomotor and ranging
behavior
Data are presently insufficient to determine exactly what proportion of bonobo
LOCOMOTOR BEHAVIOR OF CHIMPANZEES AND BONOBOS
93
TABLE 11. Interspecific differences in the relationship between arboreal substrate and locomotor activity performed
% of combined adult male and female
locomotor bouts spent in each locomotor activity
Substrate
Trunk
P. paniscus
P. troglodytes
G = 0.34, ns
Bough
P. paniscus
P. troglodytes
G = 21.5***
Branch
P. paniscus
P. troglodytes
G = 7.24, ns
Foliage
P . paniscus
P. troglodytes
G = 4.61, n s
Liane
P. paniscus
P. troglodytes
G = 4.50. n s
.
Climb
s us p
Biped
3.5
26.0
94.6
74.0
-
0.4
-
1.5
-
-
260
77
82.6
31.2
9.4
56.2
-
0.9
12.5
2.8
213
-
53.6
46.8
25.6
21.3
16.3
19.1
1.4
8.5
3.0
4.2
766
47
0.3
64.6
75.0
22.9
20.8
-
12.3
4.2
-
669
24
93.9
100.0
-
2.1
-
-
2.9
-
4.2
Leap
__
Quad
1.1
n
16
145
47
Significance levels same as in Table 2.
TABLE 12. Comparison of this study of Pan paniscus
arboreal locomotor behavior with Susman’s study at
the same site: percentages of arboreal locomotor bouts
snent in each activity
__...
Present study
(combined male and
female adults)
9% bouts
%bouts
(with
(no
distance) distance)
Susman (1984)
(all age classes)
%bouts
(no
distance)
Quadrupedalism
35.2
22.5
Climb/scram
49.4
48.2
Suspensory
10.3
17.1
Bipedal
1.3
1.2
Leap
3.8
10.8
n
2,608
2,608
G values
G (bouts with distance vs. pervious work)
G (bouts no distance vs. urevious work)
31
31
21
10
6
1,700
65.0***
290.9***
Significancelevels same as in Table 2
travel is made arboreally versus terrestrially. It is most likely that, like chimpanzees,
bonobos’ primary means of travel between
feeding and resting sites is knuckle-walking
quadrupedalism. However, data presented
here indicate that bonobos can, and do,
travel substantial distances arboreally.
There are significant sex differences in
the frequencies of locomotor behaviors used
(particularly on branches) during arboreal
horizontal travel and in exiting from one
tree to an adjacent one. Females use more
quadrupedalism and less climbing, treeswaying and leaping than males during horizontal arboreal travel. Results of males decreased quadrupedalism and increased
climbing and scrambling are in accordance
with predictions based on body size. As
males and females travel through the same
“arboreal highways”, the smaller females
encounter more substrates (relative to
males) that are sturdy enough to support
them during quadrupedalism. The larger
males find fewer substrates that support
them during quadrupedalism, and this
probably accounts for their greater incidence of climbing and scrambling in comparison with females. However, contrary to predictions, when exiting trees, males leap
more frequently and bridge less frequently
than females. Cartmill and Milton (1977) argue that larger animals should be more cautious (and thus leap less) than smaller animals. This was not the case in this study.
One confounding factor here is that the
smaller animals (females) are accompanied
by their offspring and, not unreasonably,
choose the more cautious method of bridging
(reaching out and pulling foliage from the
adjacent tree to form a bridge of foliage for
her offspring t o cross on). She then swings
across on the foliage, which accounts for her
94
D.M. DORAN
higher incidence of suspensory behavior
than males.
This sex difference in exiting trees partially supports Kano's (1983) observation
that males engage more frequently than females in high risk activities such as annswinging and diving. In this study a higher
incidence of suspensory behavior was not
noted.
There is a sex difference in the frequency
of substrate use as well. Males use trunks
(and climb) more frequently than females.
On two separate occasions during follows of
adult female bonobos, a male exited the
feeding tree and descended to the ground.
The females travelled arboreally to a feeding
tree. A few minutes after their arrival, the
same lone male arrived on the ground,
climbed up the trunk, and joined the females
in the feeding tree.
It seems possible that males may travel
terrestrially relatively more frequently than
females. This sex difference in travel would
not be unprecedented since there is evidence
that orangutan males travel more frequently on the ground than females (Galdikas and Teleki, 1981; Sugardjito and van
Hoof, 1986). Further study is necessary to
determine if it occurs on any regular basis,
and if so, to what extent.
Pan paniscus locomotor behavior:
Comparisons with previously published
work
Table 12 compares the results of this
study with those from earlier work of Susman (1984). The two studies are similar in
that both were conducted at the same study
site on the same communities of animals.
This study was conducted 4 years after the
completion of Susman's study. The categories of behavior and substrate are identical.
There are two major differences in the
sampling methods used in the two studies.
The first is that although locomotor bout
sampling was used in both studies, this
study reports all results based on locomotor
bouts (with distance), whereas Susman's results are based on the percentage of bouts
(no distance) spent in a given activity. The
second difference in the two studies is that
this study reports the results of adults only,
whereas Susman (1984) combines all age
and sex classes.
For purposes of comparison, male and female data from this study are combined into
one category. In addition, results from both
locomotor bouts (with distance) and bouts
(no distance) are included in order to more
accurately assess the difference in the results of the two studies.
There are significant differences in the result of this study with those of Susman (Table 12; G [bouts with distance] = 65.0""";
G [bouts no distance] = 290.9"""). The results of the present study indicate less suspensory behavior, bipedalism and leaping,
and a considerable increase in the amount of
climbing and scrambling.
When the results of this study are considered without weighting bouts with distance,
the differences in the two studies, at least in
regard to the frequency of suspensory behavior, are less marked. However, it has
since been demonstrated that failure to
weight bouts with distance distorts results
by overestimating activities that occur relatively frequently, but have a short mean distance per bout (Doran, 1992b).This suggests
that the higher frequency of suspensory behavior reported in earlier studies is due, in
part, to the sampling method used.
In addition, immature bonobos engage
in more frequent suspensory and less frequent quadrupedal behavior than their
adult counterparts (Doran, 1992a). The
higher incidence of suspensory behavior reported by Susman may also be explained, in
part, by his combining immature and adult
data.
However, all dissimilarities in the results
of the two studies cannot be attributed
solely to differences in sampling methods.
Susman (1984) noted a difference in the locomotor behavior of the bonobos of the Lomako Forest through time, as a result of
their increased habituation to observers. He
reported t,hhat the frequency of bipedalism
and leaping and diving decreased, and that
of quadrumanous climbing and scrambling
increased through time (Susman et al.,
1980; Susman, 1984). These same trends
are evident in a comparison of this study
with his earlier results, indicating that further habituation has occurred.
LOCOMOTOR BEHAVIOR OF CHIMPANZEES AND BONOBOS
No quantitative description of the locomotor behavior of bonobos has been made at
other sites. However, from qualitative descriptions (Horn, 1976; Kano, 1983; pers.
obs.), it appears that there are some intersite behavioral differences. The most striking intersite difference is that at the Wamba
(Zaire) study site, bonobos are reported to
travel exclusively on the ground (per. com.
Kuroda; pers. obs.). This difference is probably related to 3 factors: habitat differences,
differences in the degree of habituation,
and the possible effects of artificial provisioning.
Although the Lomako and Wamba study
sites are only separated geographically by a
distance of approximately 300 km, there are
major habitat differences at the two sites.
The Lomako site is situated at a distance of
35 km from the nearest road or village, and
consists largely of undisturbed primary forest. The Wamba study site is comprised of
approximately 100 square km, which surrounds and includes the 5 hamlets of the
village of Wamba (Kano and Mulavwa,
1984). There is more secondary forest at
Wamba than in the Lomako, and the area
has been influenced greatly by human inhabitants, as is evidenced by the relative
paucity of wildlife in comparison with the
Lomako. Thus, more primary forest with resultant different forest structure (more continuous arboreal routes) and possibly higher
density of predators (due to undisturbed nature of forest with less human intervention)
in the Lomako Forest may have some influence on the greater incidence of arboreal
travel in the Lomako Forest compared with
Wamba.
The bonobos of the Lomako Forest have
never been artificially provisioned. Neither
are they fully habituated to the presence of
human observers, whereas the bonobos at
Wamba are fully habituated to human observers. Kuroda reported (pers. com.) that
before habituation at Wamba, bonobos fled
human observers by travelling in the trees.
This suggests that the distances (of up to
1,200 m) travelled arboreally by Lomako
bonobos are an artifact of the lack of habituation. I suggest that the length of the distances travelled refute that argument to
some extent, since although travelling arbo-
95
really for short distances (<200 m) might be
a means for the animals to distance themselves from the observer before descending
to the ground, there were several sightings
in which the animals travelled arboreally
greater distances. Travel frequently occurred at a leisurely pace, after animals fed
and rested in one tree, and then continued
on to another tree. In one instance, this continued for 12 hrs, during which the animals
fed and travelled arboreally (1,200 m) without descending to the ground.
Provisioning may also have an effect on
terrestriality. On the morning that I observed at Wamba, bundles of sugar cane
were distributed under the night nests and
the bonobos descended immediately to the
ground and fed on the sugar cane, rested at
the same site, and eventually moved off on
the ground. It is not surprising that with
food available on the ground the bonobos descended upon leaving their night nests.
These differences in forest structure, degree
of habituation, and effects of provisioning
make it difficult to interpret the differences
in ranging at the two sites. Clearly, these
intersite differences cannot be resolved
without further study. However, it clearly
points out a sharp contrast in the behavior
of bonobos and chimpanzees. Whether habituated or not, no equivalent behavior for
forest dwelling chimpanzees has been reported.
lnterspecific differences in arboreal
locomotor behavior
There are species differences in chimpanzee and bonobo ranging behavior, arboreal
locomotion, substrate choice, and type of locomotor activity performed on boughs. Both
species (most likely) range between feeding
and resting sites primarily by knuckle-walking quadrupedalism. However, arboreal
travel is a more significant component of
bonobo travel than of chimpanzee travel.
This is as predicted on the basis of species
differences in scapular shape.
In comparisons of arboreal locomotor behavior, bonobos (both males and females)
use more quadrupedalism and less quadrumanous climbing and scrambling than their
chimpanzee counterparts. This trend is seen
96
D.M. DORAN
for both species in comparisons of male and
female behavior, with the smaller females
using more quadrupedalism and less climbing and scrambling than the larger males.
At first glance this supports the idea that
this interspecific difference is a result of
body-size differences between the two species. However, there is also a striking qualitative difference in quadrupedalism between species that is not present between
the sexes. Bonobos not only use more arboreal quadrupedalism than chimpanzees do,
but it is primarily palmigrade in nature,
whereas chimpanzees almost always
knuckle-walk, when walking quadrupedally.
This difference is most noticeable on boughs,
where bonobos use palmigrade quadrupedalism and chimpanzees use climbing,
scrambling and aided bipedalism. This “reluctance” to use palmigrade quadrupedalism may influence the overall reduced usage
of arboreal quadrupedalism by chimpanzees
since knuckle-walking would, on average,
require a larger substrate than palmigrade
quadrupedalism does. No clear explanation
presents itself to explain why this species
difference should occur, although it suggests
potential anatomical differences in the wrist
and hand of the two species.
An additional, species difference in arboreal locomotor behavior that cannot be explained on the basis of possible body size
difference is the increased suspensory behavior of male bonobos in comparison with
male chimpanzees. Based on body-size predictions the opposite should be true. Instead, as predicted by Susman (19791, increased suspensory behavior is probably
correlated with the bonobo’s longer and
more narrow scapula and more curved phalanges.
In addition to differences in overall frequencies of locomotor activities, there are
interspecific differences in substrate choice:
chimpanzees use trunks more frequently;
bonobos use boughs and branches more frequently. Since chimpanzee arboreal travel
consists primarily of ascending into or descending from feeding or resting trees, it is
not surprising that both male and female
chimpanzees use trunks relatively more frequently (in comparison with other substrates) than bonobos.
CONCLUSIONS
My data support the prediction, based on
scapular shape and degree of curvature of
the phalanges, that bonobos are more suspensory than chimpanzees. It is clear that
arboreal travel is a significant component
of bonobo overall travel, and that no equivalent chimpanzee behavior has yet been
reported. In addition, during arboreal locomotion, male bonobos arm-swing more frequently than male chimpanzees. This could
not have been predicted on the basis of body
size, since, if there is a body-size difference
between P. t. uerus and P. paniscus (which
has not yet been determined), then one
would predict that (larger) chimpanzees
would be more suspensory during arboreal
locomotion than (smaller) bonobos. Clearly,
the suggestion that the bonobo is a “scaled
reduced version” of a chimpanzee with little
or no positional behavior difference can be
rejected. Although I presented no data on
the ontogeny of locomotor behavior in this
study, previously reported results indicate
that the arboreal locomotor behavior of
adult bonobos resembles that of immature
rather than mature chimpanzees (Doran,
1992a),which thus supports Coolidge (1933)
in his description of paedomorphism. The
results of this study also support earlier predictions of Coolidge (19331, Roberts (19741,
and Susman (1987)that associated with the
bonobo’s longer and more narrow scapula
are predictable behavioral differences of increased arboreality and suspensory behavior.
ACKNOWLEDGMENTS
I would like to thank Drs. John Fleagle,
Charles Janson, William Jungers, Michael
Rose, Randall Susman, and Russell Tuttle
for their insightful comments on this manuscript. In addition, I would especially like to
acknowledge the debt I owe Drs. John Fleagle and Dr. Randall Susman for their guidance during my graduate training. I am
grateful to the Ministry of Scientific Research and the Station d’Ecologie Tropicale
and its director, Dr. Henri Dosso, for permission to conduct research in the Ivory Coast. I
thank the Centre Suisse de Recherche Scientifique and its director, M. Peter Lehman,
LOCOMOTOR BEHAVIOR OF CHIMPANZEES AND BONOBOS
for logistic support during my stay in the
Ivory Coast. Special thanks to Dr. Christophe and Hedwige Boesch for sharing their
friendship, guidance, and the chimpanzees
of Tai with me. My field work at Tai would
have been less profitable and far less enjoyable without the assistance and friendship
of my field assistant, M. Gregoire Nohon. 1
a m grateful to the government of Zaire and
its Institut de Recherche Scientifique for
permission to study in the Lomako Forest.
Special thanks go to Drs. Nancy ThompsonHandler, Richard Malenky, and Annette
Lanjouw for my introduction to field work
and to the pygmy chimpanzees. In addition,
I am most grateful to Dr. Takayoshi Kano
for permission to visit Wamba, and to Drs.
Suehisa Kuroda and Takeshi Furuichi for
gracious hospitality and lively discussions
during my stay. I gratefully acknowledge
the Louis B. Leakey Foundation, the Wenner Gren Foundation, and the National Science Foundation (to R. L. Susman) for their
generous financial support.
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