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Hunting Behavior of Chimpanzees at Ngogo, Kibale National Park, Uganda

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Chimpanzees (Pan troglodytes) prey on a variety of vertebrates, mostly on red colobus (Procolobus spp.) where the two species are sympatric. Variation across population occurs in hunting frequency and success, in whether hunting is cooperative, i.e., payoffs to individual hunters increase with group size, and in the extent to which hunters coordinate their actions in space and time, and in the impact of hunting on red colobus populations. Also, hunting frequency varies over time within populations, for reasons that are unclear. We present new data on hunting by chimpanzees at Ngogo, Kibale National Park, Uganda, and combine them with earlier data (Mitani and Watts, 1999, Am. J. Phys. Anthropol. 109: 439–454) to examine hunting frequency and success, seasonality, and cooperation. The Ngogo community is the largest and has the most males of any known community. Chimpanzees there mostly hunt red colobus and are much more successful and make many more kills per hunt than at other sites; they kill 6–12% of the red colobus population annually. The number of kills and the offtake of meat per hunt increase with the number of hunters, but per capita meat intake is independent of hunting party size; this suggests that cheating occurs in large parties. Some behavioral cooperation occurs. Hunting success and estimated meat intake vary greatly among males, partly due to dominance rank effects. The high overall success rate leads to relatively high average per capita meat intake despite the large number of consumers. The frequency of hunts and of hunting patrols varies positively with the availability of ripe fruit; this is the first quantitative demonstration of a relationship between hunting frequency and the availability of other food, and implies that the chimpanzees hunt most when they can easily meet energy needs from other sources. We provide the first quantitative support for the argument that variation in canopy structure influences decisions to hunt red colobus because hunts are easier where the canopy is broken.
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International Journalof Primatology, Vol. 23, No.1, February 2002 ( c
°2002)
Hunting Behavior of Chimpanzees at Ngogo,
Kibale National Park, Uganda
David P. Watts1,3 and John C. Mitani2
Received May 17, 2000; accepted September 12, 2000
Chimpanzees ( Pan troglodytes) prey on a variety of vertebrates, mostly on
red colobus ( Procolobus spp.) where the two species are sympatric. Varia-
tion across population occurs in hunting frequency and success, in whether
hunting is cooperative, i.e., payoffs to individual hunters increase with group
size, and in the extent to which hunters coordinate their actions in space and
time, and in the impact of hunting on red colobus populations. Also, hunting
frequency varies over time within populations, for reasons that are unclear. We
present new data on hunting by chimpanzees at Ngogo, Kibale National Park,
Uganda, and combine them with earlier data (Mitani and Watts, 1999, Am.
J. Phys. Anthropol. 109: 439–454) to examine hunting frequency and success,
seasonality, and cooperation. The Ngogo community is the largest and has
the most males of any known community. Chimpanzees there mostly hunt red
colobus and are much more successful and make many more kills per hunt
than at other sites; they kill 6–12% of the red colobus population annually.
The number of kills and the offtake of meat per hunt increase with the number
of hunters, but per capita meat intake is independent of hunting party size; this
suggests that cheating occurs in large parties. Some behavioral cooperation
occurs. Hunting success and estimated meat intake vary greatly among males,
partly due to dominance rank effects. The high overall success rate leads to
relatively high average per capita meat intake despite the large number of
consumers. The frequency of hunts and of hunting patrols varies positively
with the availability of ripe fruit; this is the first quantitative demonstration of
a relationship between hunting frequency and the availability of other food,
and implies that the chimpanzees hunt most when they can easily meet energy
1Department of Anthropology, Yale University, P.O. Box 208277, New Haven, Connecticut
O6520-8277.
2Department of Anthropology, The University of Michigan, Ann Arbor, Michigan.
3To whom correspondence should be addressed; e-mail: david.watts@yale.edu.
1
0164-0291/02/0200-0001 C
°2002 Plenum Publishing Corporation
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2 Watts and Mitani
needs from other sources. We provide the first quantitative support for the
argument that variation in canopy structure influences decisions to hunt red
colobus because hunts are easier where the canopy is broken.
KEY WORDS: chimpanzees; predation; red colobus; cooperation; meat eating.
INTRODUCTION
Chimpanzees (Pan troglodytes) in all well-studied populations prey on a
variety of vertebrates (reviews: Wrangham and Bergman-Riss, 1990; Uehara,
1997). They prey most heavily on red colobus (Procolobus (Piliocolobus)
spp.) where the two species are sympatric (Boesch and Boesch, 1989, 2000;
Goodall, 1986; Hosaka et al., in press; Mitani and Watts, 1999; Uehara, 1997).
Ultimately, the fact that meat and bone marrow are rich sources of en-
ergy, protein, and other nutrients presumably explains why chimpanzees
hunt (Boesch, 1994a,b; Mitani and Watts, 2001; Stanford, 1996a,b; Takahata
et al., 1984; Teleki, 1973, 1981). However, hunting also has social impor-
tance, and the role of meat procurement and sharing in maintaining cooper-
ative social relationships between males (Nishida et al., 1992; Stanford, 1998;
Stanford et al., 1994b) may supersede the nutritional importance of hunting.
Hunting frequency and success rates, predator pursuit tactics, meat intake,
and other aspects of hunting and meat eating vary considerably across and
within populations, and often vary over time within communities (Boesch
and Boesch, 1989, 2000; Goodall, 1986; Stanford et al., 1994a; Uehara, 1997;
Wrangham and Bergmann-Riss, 1990). This variation is partly due to eco-
logical factors. For example, the taller, more continuous canopy at Tı than
at Gombe and the larger size of red colobus at Tı help to explain why Tı
chimpanzees rarely hunt by themselves and why they cooperate in pursu-
ing prey more often than Gombe chimpanzees do (Boesch, 1994b; Stanford
et al., 1994a).
However, many questions about variation in chimpanzee hunting be-
havior are unresolved. For example, disagreement exists about the extent to
which chimpanzees hunt cooperatively, partly because definitions of coop-
eration have varied. Most red colobus hunts are group activities (Boesch,
1994a,b; Boesch and Boesch, 1989, 2000; Mitani and Watts, 1999; Stanford,
1996, 1998; Uehara, 1997). Individuals can benefit by hunting in groups
if hunting success increases with the number of hunters and this leads to
greater per capita meat intake or net energy gain (Boesch, 1994b; Boesch
and Boesch, 2000; Packer and Ruttan, 1988; Scheel and Scheel, 1995). Such
payoffs could occur because hunters coordinate their actions in ways that
improve their capture success (Boesch and Boesch, 1989). However, group
members that pursue prey risk exploitation by others that refrain from
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Chimpanzee Hunting Behavior 3
pursuits but try to gain shares of captures (Packer and Ruttan, 1988). Packer
and Ruttan (1988) concluded from early Gombe data that the chimpanzees
hunted cooperatively because the percentage of hunts that led to captures
increased with the number of males present at hunts. However, success rates
were lower than predicted by a model of pure cooperation, which they took
as evidence of cheating. Some later data from Gombe (Stanford, 1996) also
showed that hunting success increased with increases in the number of adult
males present at hunts, and success increased with the number of hunters
at Tı (ibid.; Boesch and Boesch, 2000). However, Stanford (1996) argued
that Gombe chimpanzees did not hunt red colobus cooperatively because
per capita meat intake did not also increase with hunting party size. Positive
correlations between the number of males at hunts and hunting success were
not evident in other Gombe data (Busse, 1978), nor did success increase with
the number of hunters in Boesch’s (1994b) Gombe sample. A better criterion
for cooperation is that net per capita energy gain increases with group size,
at least up to some optimum, and is higher for groups than for solo hunters
(Boesch, 1994b; Scheel and Scheel, 1995). In the only study of chimpanzees
that included estimates of its net energy returns, Boesch (1994b) found that
it met this criterion at Tı, but not at Gombe. As defined by the extent to
which individual hunters coordinate their behavior in space and time, coop-
eration is also common at Tı, but not at Gombe (Boesch, 1994b; Boesch
and Boesch, 1989, 2000; Stanford, 1996).
Rates of predation by chimpanzees commonly vary over time (Boesch
and Boesch, 1989, 2000; Goodall, 1986; Mitani and Watts, 1999; Stanford,
1998; Stanford et al., 1994; Takahata et al., 1984; Uehara, 1997; Wrangham
and Bergman-Riss, 1990), and researchers have invoked both ecological and
social factors as explanatory variables. At Tı and Mahale, hunting is more
common during rainy seasons. These may be times of high fruit abundance
at Mahale (Takahata et al., 1984; Uehara, 1997). During the season of hunt-
ing at Tı, fruit abundance may initially be low, but subsequently increases
(Boesch, 1994a, 1996; Boesch and Boesch, 2000), and this also tends to be a
birth season for red colobus (Boesch and Boesch, 2000). In contrast, preda-
tion on red colobus at Gombe is more intense during dry seasons (Stanford
et al., 1994a). Stanford (1996) raised the possibility that Gombe chimpanzees
hunted more during the dry season because they needed energy, but noted
that the large mean size of parties was inconsistent with the assumption that
dry season fruit availability was low. He argued instead that variation in the
number of cycling females was probably more important (Stanford, 1996a,b;
Stanford et al., 1994a). However, quantitative data on fruit availability are
not available for these sites.
We present new data on hunting by chimpanzees at Ngogo, in Kibale
National Park, Uganda, collected during 11 months in 1998–1999, and add
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4 Watts and Mitani
them to data published by Mitani and Watts (1999). The Ngogo community
is unusually large, and the chimpanzees are more successful at hunting red
colobus and make more kills per hunt than chimpanzees elsewhere (Mitani
and Watts, 1999). We use our larger data set to update information on hunt-
ing frequency, prey choice, and hunt durations, to reexamine demographic
influences on hunting success, to examine the hypothesis that hunting is co-
operative in ecological terms, and to describe social cooperation. We also
examine individual variation in hunting, estimate total red colobus offtake
in numbers and in biomass, and update our estimates of the impact of pre-
dation on the red colobus population. We provide quantitative data on how
forest structure influences decisions to hunt red colobus and hunting success.
Finally, we analyze the relationships of hunting frequency and prey offtake
to variation in the availability of ripe fruit and provide quantitative evidence
that hunting frequency varies positively with the abundance of ripe fruit.
METHODS
The Ngogo study area covers about 30 km2of mixed mature and re-
generating forest transitional between lowland and montane evergreen for-
est and other, minor vegetation types (Butynski, 1990; Struhsaker, 1997).
Most chimpanzees in the Ngogo community, including all males, are well
habituated, but its exact composition is still uncertain. However, it is the
largest one to be documented (Mitani et al., 1999, in press; Mitani and Watts,
1999; Pepper et al., 1999; Watts, 1998, 2000a,b; Watts and Mitani, 2000): by
mid-1999, we had firmly identified 24 adult males and 15 adolescent males,
and estimated 47 adult and 9 adolescent females, 15 juveniles, and 34 infants,
for a total of 146 members.
We have opportunistically collected data on three kinds of predation
episodes (Table I; cf. Mitani and Watts, 1999). Occasionally we found chim-
panzees eating meat just after a successful hunt that we had not seen (meat-
eating episodes) or found individuals carrying carcasses (carcass-carrying
episodes). Hunts, during which we saw chimpanzees rush at prey, accounted
for most episodes (Table I). We define hunts as successful when the chim-
panzees captured 1 prey. For hunts, we recorded the prey species, the
number of kills, and the age–sex class of the prey. We also recorded the
time whence the first chimpanzee climbed toward the prey until the first kill
(hunt duration) for hunts of monkeys. We identified all chimpanzees present,
noted the identities of all that captured prey and that obtained some meat,
and noted all instances of meat sharing and theft (cf. Mitani and Watts, 1999).
The large size and spread of hunting parties, combined with constraints
on visibility in the canopy, precluded collection of systematic data on whether
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Chimpanzee Hunting Behavior 5
Table I. Frequency of predation episodes at Ngogo, listed by prey species
Species H #S ME CC Total Kills % Kills
Pennant’s red colobus, Procolobus 82 67 6 3 91 258 88.4
(Piliocolobus)pennantii
Black-and-white colobus, Colobus guereza 14 7 14 11 3.8
Red-tailed guenons, Cercopithecus ascanius 8 4 1 1 10 10 3.4
Mangabeys, Lophocebus albigena 3 1 3 1 0.3
Blue monkeys, Cercopithecus mitis 1 1 1 1 0.3
Red duiker, Cephalophus monticola 2 2 5 7 7 2.4
Blue duiker, Cephalophus callipyga 2 2 2 0.7
Bushpig, Potomochoerus porcus 1 0 1 1 1 0.3
Guinea fowl, Guttera pucherani 1 1 1 0.3
Total 111 80 16 4 131 292
Note. H: hunts; ME: meat-eating; CC: carcass-carrying; #S: number of successful hunts; Kills:
total number of observed kills.
all chimpanzees present pursued prey, on how much time individuals de-
voted to pursuits, and on whether individuals typically took certain roles
during hunts. The same problems prevented us from systematically collecting
data regarding behavioral coordination, though we did so opportunistically,
and on recording precise observations on how much meat all individuals
obtained. These uncertainties mean that our analyses of cooperation are
preliminary. Because of the observational constraints, we followed the con-
vention established by other students and used the number of adult males
present as our estimate of hunting party size (Stanford, 1996, 1998; Gombe:
Stanford et al., 1994a; Mahale: Hosaka et al., in press). The counts may over-
estimate the number of hunters because not all males necessarily pursue
prey, and we hope to get more precise counts of the number of pursuers in
the future.
There are two reasons why the total number of adult males present is
a reasonable estimate of hunting party size. First, bystanders that watch the
progress of hunts from positions on the ground can quickly become pursuers
when they see good opportunities to capture monkeys in the canopy or when
monkeys fall or are knocked to the ground (cf. Hosaka et al., 2000; Watts and
Mitani, in press). We suspect that certain males routinely stay on the ground
and use the wait-for-monkey-to-fall tactic also seen at Mahale by Hosaka
et al. (in press). Thus, absolute distinctions between bystanders and hunters
often do not exist. Second, whether cheaters exploit the efforts of pursuers
is a crucial issue with regard to whether individuals do better by hunting
in groups than by hunting alone, or by hunting in larger groups rather than
smaller ones (Packer and Ruttan, 1988). Using the total number of males as
the independent variable provides conservative tests of the hypotheses that
hunting success and prey offtake increase with the size of hunting parties.
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Success and offtake should not be positively related to total party size if
large parties contain relatively many cheaters (Packer and Ruttan, 1988).
For successful hunts of red colobus, we estimated the biomass of prey
harvested per hunt by multiplying the number of kills for a given age–sex
class by the estimated body mass of a member of that class, and then summing
across classes. Body masses were 11 kg for adult males, 6 kg for adult females
and for subadults, 3 kg for juveniles, and 1 kg for infants (Stanford, 1996;
Struhsaker, 1975; Struhsaker and Leland, 1987). Per capita meat availability
for adult male chimpanzees is the total biomass of prey killed divided by the
number of males present. We used Struhsaker and Leakey’s values for the
proportional representation of different age–sex classes in the red colobus
population to test the hypothesis that the chimpanzees preyed on classes
nonselectively (Struhsaker and Leakey, 1990). We used values of 2.04 red
colobus groups per km2(Mitani et al., 2000b) and 42 individuals per group
(T. Struhsaker, pers. comm.) to estimate the annual impact of chimpanzee
predation on the red colobus population.
During fieldwork in 1998–1999, we noted all visual encounters between
chimpanzees and red colobus; this yielded a total of 164 encounters, in 59
of which the chimpanzees hunted. We assigned the forest structure at en-
counter sites to one of three categories: (1) primary forest with a tall, contin-
uous canopy; (2) regenerating forest or swamp forest with a broken, mostly
low, canopy; and (3) forest with a tall canopy, but within about 100 m of
regenerating forest, open swamp forest, or grassland or bush with no tall
trees. Forest in the second and third categories offered the monkeys fewer
escape routes than tall primary forest did. We then used logistic regression
to examine the effect of forest structure on decisions to hunt, represented by
the categorical variable hunt/no hunt. We conducted a similar analysis for
the total sample of hunts (n=82) to examine the effects of forest structure
on hunting success, represented by the categorical variable success/no suc-
cess. We pooled data from forest structure categories 2 and 3 in the analysis
of hunting success because few hunts in these areas were unsuccessful.
We used data on phenology, tree size, and tree densities of the top
20 fruit species in the chimpanzees’ diet, as measured in 263 plots, each 5 ×
50 m, to compute an index of ripe fruit availability (ripe fruit score; RFS)
each month:
RFS =
20
X
i=1
pi·di·si
Where in pi=percentage of the ith tree species possessing ripe fruit
di=density of the ith tree species (trees/ha)
si=mean size of the ith tree species in cm DBH.
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Chimpanzee Hunting Behavior 7
We collected phenological data on the first 5 days of a month and centered
months around the midpoint of each sample for analytical purposes, under
the assumption that RFS values represent mean fruit availability during the
resulting interval. For example, the RFS value for April, 1999, applied to
behavioral data from March 19 through April 18.
We measured monthly frequencies of hunts and of hunting patrols as
the number of hunts or patrols per day per month, with months defined
as in the phenology sample. We included only days when we observed the
chimpanzees for 6 h, so that we could be reasonably sure we did not miss
hunts (cf. Boesch and Boesch, 2000). We then regressed the number of red
colobus hunts, the number of hunting patrols, and the number of kills per
month on RFS values to examine the relationship between hunting and fruit
availability. Data on hunting and phenology a available for 16 months in
1998–1999.
RESULTS
Prey Species and Pursuit Frequencies
The total sample is 131 predation episodes, 85% of which (111/131)
were hunts, involving 9 prey species (Table I), including 59 hunts, 43 of
which were successful, added during the 11-month 1998–1999 sample
(Period 2). Most hunts and kills were of red colobus (Table I). Black-and-
white colobus (Colobus guereza) were the secondmost frequently targeted
primate prey, followed by red-tailed guenons (Cercopithecus ascanius),
mangabeys (Lophocebus albigena), and blue monkeys (Cercopithecus mitis;
Table I). We saw two red duiker (Cephalophus monticola) hunts and 3 meat-
eating episodes in Period 2; combined with two earlier meat-eating episodes
there have been 7 predation episodes involving red duiker. We saw one un-
successful hunt of a juvenile bushpig (Potomochoerus porcus) in Period 2,
to add to an earlier meat-eating episode (Mitani and Watts, 1999). A suc-
cessful mangabey hunt and the guinea fowl (Guttera pucherani) meat-eating
episode in Period 2 (Table I) a the first records of predation on these species
at Ngogo.
We have probably underestimated the frequency of red-tailed guenon
hunts, which are opportunistic and quick. For example, T. Windfelder (pers.
comm.) saw a solitary adult male chimpanzee grab a red-tailed guenon that
had descended close to the forest floor in response to an attack on its group by
a crowned eagle (Stephanoetus coronatus). We once found a solitary male
chimpanzee eating a juvenile red-tailed guenon that he had presumably
just caught from a group foraging nearby. In contrast, all red colobus hunts
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8 Watts and Mitani
were by groups of males (cf. Mitani and Watts, 1999). We may also have
undercounted hunts of black-and-white colobus, which also are brief and
sometimes are solo hunts.
As at Gombe (Goodall, 1986) and Mahale (Uehara, 1997), duiker hunts
are opportunistic. Observed captures were by single individuals, though oth-
ers quickly joined them to beg for or to steal meat. We became aware of
duiker meat-eating episodes by following chimpanzees attracted to capture
sites. The were noisy events, but we are less likely to know of captures made
by isolated individuals, especially by adult females.
Hunting Success and Kills Per Red Colobus Hunt
Eighty-two percent of red colobus hunts were successful (Table I), in-
cluding 33 of 39 hunts (85%) in Period 2. This is far more than hunting
success at Tı, Mahale, and Gombe (Fig. 1; cf. Mitani and Watts, 1999). Suc-
cess rates for hunts of black-and-white colobus and of red-tailed guenons,
though lower, were still 50% (Table I) and comparable to values for red
colobus hunts elsewhere. We have seen too few hunts of other species to
Fig. 1. The percentage of red colobus hunts that were successful at Ngogo and at three
other research sites (successful), and the percentage of successful red colobus hunts
at which chimpanzees made multiple kills at Ngogo and at three other sites (multiple
kills).
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Chimpanzee Hunting Behavior 9
Fig. 2. The mean number of kills per successful red colobus hunt at Ngogo
and at three other sites.
estimate success rates. The chimpanzees made multiple kills in 87% (58/67)
of red colobus hunts (Fig. 1), including 90% (30/33) in Period 2. The mean
number of kills per hunt in Period 2 is 4.67 (SD =2.57, range =1–13), and
the overall mean is 3.93 (SD =2.33, range =1–13), which also is far higher
than at other sites (Fig. 2; cf. Mitani and Watts, 1999).
Hunt Durations
Red colobus hunts had a shorter mean duration in Period 2 than in
the earlier sample (13.4 vs. 21.4 min). The overall mean is 17.7 min (SD =
17.1 min, range =1–91 min; n=57 hunts), with a median of 15 min and a
mode at 6–10 min (Fig. 3). Nine completely observed hunts of other monkey
species had a mean duration of only 4.5 min (SD =2.0, range =1–7 min;
Fig. 3). It is significantly shorter than hunts of red colobus (Mann-Whitney
test: z=−3.61, n,m=9, 57, p<0.001), which reflects the fact that red
colobus usually did not flee from hunting chimpanzees, whereas other species
did. The chimpanzees often sat under red colobus groups for long periods
and repeatedly rushed at them. In 6 hunts, the chimpanzees made successful
initial attacks, ate meat for 20 min, then made 1 subsequent attacks in
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Fig. 3. Frequency distribution of the duration of monkey hunts at Ngogo.
which they killed more monkeys. In no case did the red colobus leave the
site after the first attack. In the 3 longest hunts of Period 2 (43, 45, and
63 min), the red colobus were initially isolated in one or two tall trees that the
chimpanzees could enter only at a single point, where male red colobus could
attack them. After long hesitation, one or two male chimpanzees entered
the trees and drove the monkeys toward the crown edges. More males then
entered the trees, and the chimpanzees started to catch monkeys either there,
in the canopies of neighboring trees into which the monkeys leapt, or on the
ground after the monkeys fell. In contrast, the chimpanzees pursued other
species only briefly and made kills quickly or not at all.
Hunting Success, Kills Per Hunt, and Hunting Party Size
The likelihood that the chimpanzees would make a kill during red
colobus hunts increased with the number of adult males present. We have
not seen single males hunt red colobus. Parties with 5 males hunted red
colobus only 3 times and captured prey in only one of them, while all hunts at
which 20 were present were successful (Fig. 4). Both the number of kills per
hunt and estimated offtake also increased significantly in association with the
number of adult males present (kills: F(1, 80) =69.49, r2=0.68, p<0.001;
Fig. 5; offtake: F(1, 65) =17.23, r2=0.21, n=67 hunts, p<0.001; Fig. 6).
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Chimpanzee Hunting Behavior 11
Fig. 4. The percentage of successful red colobus hunts by parties that
contained different numbers of adult male chimpanzees.
Males that joined large hunting parties had good chances to obtain some
meat: the number of males that obtained meat increased with the number of
males present (F(1, 65) =36.85, r2=0.36, n=67 hunts, p<0.001) and with
estimated meat offtake (F(1, 65) =77.75, r2=0.55, p<0.001; Fig. 7). Also,
Fig. 5. Relationship between the number of kills per red colobus hunt and the number
of adult male chimpanzees in the hunting party.
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Fig. 6. The relationship between estimated total prey offtake, in kg,
and the number of adult male chimpanzees in the hunting party, for
red colobus hunts.
the percentage of males that obtained meat is higher in large hunting parties
than in smaller ones (F(1, 65) =4.52, r2=0.05, p<0.05). However, the
relationship between per capita meat availability and the number of males
present, though positive, is nonsignificant (F(1, 65) =0.50, n=67 hunts,
p=0.48; Fig. 8). Our data do not permit adequate estimates of individual
Fig. 7. Relationship between the number of adult male chimpanzees
that obtained meat per hunt and the number present in the hunting
party.
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Chimpanzee Hunting Behavior 13
Fig. 8. Estimated mean per capita meat availability per hunt, in kg,
in relation to the number of adult males in the hunting party.
energy expenditure and intake during hunts, but this result makes it seem
unlikely that, on average, males had higher net energy gains in large hunting
parties than in smaller ones. It also suggests that some males in large parties
cheated.
Social Cooperation During Red Colobus Hunts
Males often took positions in trees surrounding those where red colobus
had taken refuge. This left them well placed to capture monkeys that leaped
into them as they fled attacks by other males. It also allowed pursuit from
several directions, and sometimes 2 males simultaneously pursued a sin-
gle prey. For example, CO chased a subadult red colobus that fled along a
large bough during one hunt, then leaped to a smaller branch. While CO
lunged at the monkey, PI ran along the bough from the opposite direction
and grabbed it. PI and CO then divided the carcass. Chimpanzees on the
ground often rushed ahead when red colobus fled through the canopy and
climbed to advantageous position along the monkeys’ route. They some-
times caused monkeys to fall by hitting them, pulling their tails, or shak-
ing branches to which they were clinging. Other males often sought ter-
restrial positions that afforded opportunities to capture them, and certain
males seemed especially likely to do so. We saw 19 red colobus kills on
the ground, including 16 of 19 adult males in the prey sample. Whether
this tactic pays off depends on the behavior of other chimpanzees in the
canopy.
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Age–Sex Classes of Successful Hunters
Period 2 data reinforce the earlier conclusion (Mitani and Watts, 1999)
that hunting is overwhelmingly an adult male activity. Of kills for which we
identified the killer, adult males accounted for 90% (235/261) and adolescent
males 8% (22/261). Adult females definitely killed two red colobus, and we
once saw several that were with their dependent offspring, but not with adult
males, eating a freshly killed red colobus. One adolescent female killed a
juvenile red colobus. An adult female also killed a blue duiker, though she
quickly lost it to adult males. One adult and one adolescent male captured
red duikers. Adult males were the first observed possessors and probable
captors of other red duiker carcasses, but we could not rule out theft.
Variation in Hunting Success Among Individual Males
The 24 males observed throughout all study periods were present at a
mean of 59 hunts of monkeys (SD =13.1, range =29–78), of which 49 ±11.3
were red colobus hunts (range =22–68). On average, each male killed a total
of 9.8 monkeys (SD =6.7, range =1–22), including 9.4 ±6.5 red colobus
(range =1–22). Individual males killed a mean of 0.18 ±0.11 red colobus
per hunt in which they participated (range =0.02–0.35; Table II). The wide
ranges show that the most successful males had success rates more than an
order of magnitude greater than the least successful.
Male dominance rank and age influenced this variation. Presence at
red colobus hunts (F(1, 22) =4.44, r2=0.13, p<0.05) and hunting success
(F(1, 22) =14.03, r2=0.36, p<0.001) both increased significantly with rank.
The most successful hunter, EL, that made many kills on the ground, occupied
ranks 2–4, and some other high-ranking males also had high success ratios
(e.g., LO, that became alpha male in 1999; Table II). However, the 4 males,
besides EL, that killed >0.30 red colobus per hunt covered a wide range of
ranks (Table II). Long-time alpha male MW had only the 15th highest suc-
cess ratio, though he regularly obtained meat from others and ate meat at
77% of hunts and meat-eating episodes at which he was present, the highest
proportion among males. The 3 lowest-ranking adults had the lowest hunting
success (Table II). Among them, ST and DZ were the smallest adults, and
ST had a deformed hand that impeded arboreal locomotion and might have
made pursuing red colobus particularly risky. The third, MZ, was apparently
the oldest male in the community and rarely pursued prey, though he was
highly successful at begging from others and obtained meat in 55% of hunts
he attended. Another noticeably old male, AY, had a low number of kills per
hunt, but a third (RU) was one of the most successful hunters (Table II).
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Chimpanzee Hunting Behavior 15
Table II. Hunting success of individual adult males
Availability
Male Hunts Success RC Success % Meat 1 2
MW 66 0.09 54 0.11 77.0 16.0 45.3
LO 59 0.24 48 0.29 60.0 35.3 26.8
BA 80 0.21 67 0.24 58.0 40.3 44.6
EL 63 0.29 52 0.35 67.7 48.6 43.2
CO 55 0.22 44 0.25 42.6 27.1 28.8
HA 66 0.20 55 0.20 55.7 29.8 37.4
BF 68 0.29 58 0.33 48.4 48.6 37.9
HO 72 0.14 60 0.18 38.7 24.3 33.7
PA 76 0.17 61 0.20 32.8 32.6 34.7
RU 58 0.26 49 0.31 60.7 43.1 30.8
MG 75 0.23 61 0.25 44.3 37.5 32.4
PI 57 0.30 50 0.34 42.1 45.8 29.1
DO 73 0.25 61 0.28 67.7 43.1 41.3
TY 48 0.08 38 0.11 24.2 11.0 14.2
AY 67 0.09 54 0.11 49.2 13.2 30.0
MO 78 0.30 70 0.33 39.2 59.6 37.0
OR 56 0.09 46 0.09 18.8 11.0 12.8
MI 43 0.14 36 0.14 14.6 11.0 6.8
BE 71 0.10 56 0.13 11.3 13.8 12.0
BS 31 0.10 24 0.13 40.0 12.5 16.8
GA 46 0.04 39 0.05 13.5 5.5 6.9
MZ 34 0.03 30 0.03 51.4 3.0 18.7
ST 55 0.02 45 0.02 4.0 2.8 4.1
DZ 60 0.05 40 0.04 4.7 5.5 3.0
Note. Males are listed in approximate order of dominance rank during Period 2 (some males
were tied, and several rank reversals occurred). Hunts =number of hunts of all species at
which a male was present; RC: number of red colobus hunts at which he was present; Success:
number of kills per hunt; % Meat: hunts and, for duiker, meat-eating episodes at which a male
obtained meat, as a percent of those at which he was present. Availability, 1: estimated annual
meat availability in kg, Version 1; Availability, 2: estimated annual meat availability, Version 2
(see text for explanation).
Age–Sex Classes of Red Colobus Prey
The chimpanzees preyed disproportionately on red colobus age–sex
classes (χ2=208, DF =4, p<0.001). Adult males are underrepresented,
and immatures overrepresented, relative to proportions in the population
(Table III). We saw proportionately more infant kills in Period 2 than earlier
(Mitani and Watts, 1999). We probably miss some infant kills, because some-
times captors run off with infants to avoid pressure to share meat, and our
records might have been more complete for Period 2. However, the chim-
panzees killed many infants during several hunts of large groups in Period 2,
and the greater representation of infants is probably real. For example, the
maximum number of kills in one hunt is 13, of which 8 were infants.
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16 Watts and Mitani
Table III. Age–sex class distribution of red colobus prey
Adult males Adult females Subadults Juveniles Infants Total
Period 2 17 29 24 29 52 151
(11.3) (19.2) (15.9) (19.2) (34.3)
All 20 48 38 74 78 258
(7.8) (18.6) (14.7) (28.7) (30.2)
Expected 35 86 16 99 22
Note. Period 2: data from October, 1998, through August, 1999; All: data from all observa-
tion periods. Values in parentheses are percentages of totals. Expected values are based on
demographic data in Struhsaker and Leakey (1990).
The increased representation of adult males in the Period 2 sample
(11 vs. 2% in earlier data; Mitani and Watts, 1999) is striking. Chimpanzee
hunters seemed to confront male red colobus directly and tried to dislodge
them from trees more often during Period 2.
Impact on the Red Colobus Population
Our estimate of the red colobus population in the 30 km2study area
is 2570, or 2150 for the 25 km2where the chimpanzees spend most of their
time. We saw one successful hunt per 6.6 observation days, or 55.2 per year,
in Period 2. At 4.67 kills per hunt, this yields 258 kills per year, or 10–12% of
the population killed per year. Combined data from all study periods gives
an estimated 45.1 successful hunts per year. At 3.93 kills per hunt, this yields
167 kills per year, or 6–8% of the population. Both estimates are higher than
the 3% we reported earlier, because the hunting rate and number of kills
per hunt were higher during Period 2 than earlier and because the estimated
density of red colobus groups was revised downward (Mitani et al., 2000b).
Amount of Meat Eaten by Chimpanzees
Given the age–sex distribution of kills, the weighted mean prey body
mass is 4 kg. We estimated the total biomass of red colobus captured by first
multiplying this value by 3.93 kills per hunt to give 15.7 kg captured per hunt.
Given 55 hunts per year and 82% success, the chimpanzees obtained about
708 kg annually. Similar assumptions give estimates of 11 successful hunts
of other monkey species and 7 of red duiker per year, plus occasional kills
of blue duiker and bushpig. The total annual biomass of prey may be about
850 kg. Adult males made 90% of red colobus kills; if this was 90% of prey
biomass (637 kg), they accounted for 26.6 kg per capita.
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Chimpanzee Hunting Behavior 17
We also used two other methods to estimate per capita availability of red
colobus meat for males more directly. First, we multiplied each adult male’s
hunting success rate by the mean prey biomass and by the number of hunts
at which he was present per year, then averaged across males (Version 1
in Table II). This gives a mean of 25.5 kg (SD =17.6 kg) captured per
male annually. Males shared some meat and lost some to theft, but most also
obtained some meat from others. Variation in hunting frequency and success
meant that individual males captured as much as about 60 kg and as little as
about 1 kg per year (Table III). Good hunters thus had a substantial annual
meat intake, even though they shared some of their meat.
In the second method (Version 2 in Table II), we first noted that means of
15.2 adult males and 12.5 other adults and adolescents were present per hunt,
but only 8.7 adult males and 12.1 individuals ate meat, excluding ones that
ate only dropped scraps. This gives 1.8 kg per male or 1.3 kg per consumer.
With 45 successful hunts per year, mean annual meat available per male is
M=Ã24
X
i=1
p(H)·S·45 ·1.8kg
!,24 =26.2kg(SD=13.6 kg),
Where in p(H)=the proportion of hunts at which a male was present, and
S=the proportion of hunts at which he obtained meat.
This is quite close to the estimate based on individual hunting success, though
estimates diverged for some individuals, e.g., MW and MZ (Table II). The
assumption that all meat-eaters got equal portions is unrealistic; males that
controlled carcasses would often have obtained more than was available per
capita. However, this method takes into account the fact that some males
were particularly successful at getting others to share with them, e.g., MW
and MZ (Table II).
The estimate of annual prey availability yields only about 6.3 kg of red
colobus and perhaps 7.7 kg total per individual in the community, excluding
infants, not all of which is edible. Adult males controlled most carcasses and
undoubtedly ate more than members of other age–sex classes did. Despite
uncertainty in our estimates, we can reasonably conclude that they obtained
a mean of 15–20 kg annually and that individuals got anywhere from 2 to
40 kg. Adolescent males generally gained little meat from adults and, except
when they made kills, got meat mostly by finding scraps under trees where
adults were feeding. Adult females were more successful at begging, but
most ate meat less often than most adult males did. On average, about 3
females ate meat per red colobus hunt; if each obtained 1.3 kg of meat
(almost certainly an overestimate), then the average female obtained only
about 3.7 kg annually. Those we saw most often at red colobus hunts might
have obtained closer to 10 kg.
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Influence of Forest Structure on Hunting Decisions and Success
The chimpanzees were significantly less likely to hunt red colobus on
encounters when they were in primary forest, far from areas with broken
canopies, than when they were in or near areas where the canopy was broken
and mostly low (χ2=42.80, DF =2, p<0.0001). They hunted in 64% of
encounters (27/42) in regenerating or swamp forest and 56% (19/34) of ones
near such forest or near grassland, but in only 15% of encounters (13/88)
in primary forest. Variation in the number of male chimpanzees present
also significantly influenced hunting decisions (Mitani and Watts, in press).
However, a multivariate logistic regression shows that canopy structure
had effects independently of those of male number (Wald statistic =21.91,
DF =2, p<0.0001).
Correspondingly, hunters were less likely to succeed in primary forest
(χ2=14.90, DF =1, p<0.001). Only 55% of hunts (12/22) in primary
forest were successful, whereas 92% (55/60) of hunts in or near regenerating
forest, swamp forest, grassland, or bush were successful. The chimpanzees
had difficulty catching red colobus when the monkeys fled, especially where
the canopy was high and continuous. All six unsuccessful hunts in Period 2
occurred in mature forest, and the monkeys fled in 5 of them. The 3 hunts with
the highest kill totals (n=13, 9, and 9) started when the red colobus were
in narrow strips of tall forest, on shallow slopes, bounded by open swamp
forest at the foot of the slope and grassland or low regenerating forest at
the top. The chimpanzees isolated the monkeys in several tall trees or, in
one case, chased them up and down the slope several times while cutting off
escapes to the sides.
Hunting Frequency, Hunting Patrols, and Variation in Fruit Availability
The number of hunts per day increased significantly as the amount
of ripe fruit, as measured by RFS, increased (F(1, 14) =9.22, r2=0.38,
p<0.01; Fig. 9(A)). The number of kills per month also showed a significant
positive relationship to RFS values (F(1, 14) =7.30, r2=0.30, p=0.05;
Fig. 9(B)).
In earlier data (Mitani and Watts, 1999), 41% of 49 hunts occurred
during patrols, on which the chimpanzees moved quietly and in single file,
sometimes for hours, while deliberately searching the canopy for monkeys.
Little or no other foraging occurred during the patrols. Patrols were even
more common in Period 2. Most (27/39) red colobus hunts happened during
patrols. The chimpanzees hunted black-and-white colobus, and killed 2, dur-
ing a patrol on which they later hunted red colobus and made 13 kills. On
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Chimpanzee Hunting Behavior 19
Fig. 9. (A) The number of monkey hunts per day, in rela-
tion to ripe fruit availability as estimated by the ripe fruit
score (RFS); (B) the number of kills per day in relation
to the RFS; (C) the number of hunting patrols per day,
in relation to the RFS.
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20 Watts and Mitani
another, they unsuccessfully hunted black-and-white colobus and manga-
beys. They also patrolled 5 times without finding monkeys. In all, 46%
(50/108) of hunts of monkeys involved patrols. The number of patrols per
observation day also increased significantly with the RFS (F(1, 14) =11.07,
r2=0.40, p=0.005; Fig. 9(C)).
The chimpanzees went on a 57-day binge from October until Decem-
ber, 1998, during which they hunted red colobus 17 times (15 of which were
successful), mangabeys twice, and black-and-white colobus twice and killed
69 red colobus, one mangabey, and one red duiker. Fifteen of the hunts
occurred during hunting patrols, and they made 3 other patrols without en-
countering monkeys. This binge coincided with a major fruit crop of Uvari-
opsis congensis, which had fruited earlier in 1998, during which fruiting peak
there also was a hunting binge (Mitani and Watts, 1999). Binges in 1995 and
1996 had also coincided with major fruit crops and with periods when the
chimpanzees formed large parties on most days (ibid.).
DISCUSSION
General Results
Our results corroborate earlier Ngogo data (Mitani and Watts, 1999).
The chimpanzees hunt 9 vertebrate species, but overwhelmingly focus on
red colobus. Hunting frequency at Ngogo is higher than at Kanyawara, also
in Kibale (R. Wrangham, pers. comm.), but lower than reported for Gombe
(Stanford et al., 1994a,b) and Tı during some periods (Boesch and Boesch
(1989, 2000) give a lower value for an earlier period). However, the success
rate for red colobus hunts at Ngogo and the proportion of hunts with multiple
kills far exceed values from other sites, and the mean number of kills per hunt
is more than twice the highest value reported elsewhere. The main reason for
these differences is presumably the extraordinarily large number of males
and the large size of hunting parties at Ngogo (Watts and Mitani, 1999).
Hunts of black-and-white colobus and of red-tailed guenons are much less
common, but success rates are comparable to those for red colobus hunts
at Tı (Boesch and Boesch, 1989), Gombe (Stanford et al., 1994a,b), and
Mahale (Uehara, 1997; Hosaka et al., in press).
Cooperative Hunting
The probability that the chimpanzees captured prey during red colobus
hunts, the number of kills, the amount of meat obtained, and the number
of males that got meat all increased with the number of males present.
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Hunting party size and the probability of success were also positively related
at Gombe (Packer and Ruttan, 1986; Stanford, 1996, 1998), and success in-
creased with the number of active hunters at Tı (Boesch and Boesch, 2000;
Stanford, 1996). Large hunting parties at Gombe were also more likely to
make multiple kills (Stanford et al., 1994a), and the amount of meat ob-
tained increased with hunting party size (Stanford, 1996). Mahale data have
not been analyzed in the same way, but successful hunting parties there a
significantly larger than unsuccessful ones (Hosaka et al., 1995).
Nevertheless, individuals did not necessarily gain by hunting red colobus
in large groups. The absence of solitary hunts suggests that hunting in groups
is a better tactic, though it prevents us from making direct comparisons.
However, per capita meat availability was independent of the number of
males per party and of the number that obtained meat, as Stanford (1996)
found at Gombe. We cannot rule out the possibility that net energy capture
is positively related to the number of hunters without more detailed data on
the behavior of individuals during hunts. Still, if the number of males present
at hunts is a valid estimate of the number of hunters—and we reiterate that
no clear distinction between bystanders and hunters exists at most hunts—
Ngogo data concur with those from Gombe (Boesch, 1994b), which show
that hunting in groups does not invariably pay for individuals. Males may
be more inclined to cheat when in large hunting parties than when in small
ones (cf. Packer and Ruttan, 1988).
However, per capita meat availability is independent of the number of
males per party and of the number that obtained meat. Data on the number
of captures and on meat offtake only measure gains from hunting, and we
need better data on costs and on the activities of individuals during hunts to
assess the relationship of net payoffs to hunting group size (Boesch, 1994b;
Creel and Creel, 1995). Even so, if the number of males present is a valid
estimator of hunting group size, Ngogo data are initially consistent with
others (Boesch, 1994b) showing that hunting in groups does not invariably
pay for individual chimpanzees.
Males may be more inclined to cheat when in large hunting parties
than in small ones (cf. Packer and Ruttan, 1988); we plan to focus on this
issue in the future. Certainly some cheating occurs, as witnessed by the high
frequency with which MZ obtained meat. In this case and others, social
factors might have influenced success at obtaining meat independently of
participation in hunts. For example, rank influenced how successful males
that had not made captures were at getting meat, and long-time alpha male
MW, which could easily steal meat and often begged successfully, had the
best access to meat despite its low hunting success (Table II). However,
begging for meat was not necessarily cheating. Males often pursued prey
and begged in the same hunt, and abandoned their own efforts to catch
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22 Watts and Mitani
monkeys to try to get meat from others that had caught some. The surviving
monkeys did not always take the resulting opportunity to escape. Hunts in
which the chimpanzees made 7, 9, or even 13 kills involved multiple attacks,
and males that did not succeed in gaining much meat from others after the
first attack were likely to initiate the second.
As the example of CO and PI shows, males sometimes showed coordi-
nation (Boesch and Boesch, 1989), i.e., concentrated similar actions on the
same prey and related their actions in space and time. Cases in which males
drove red colobus into trees where other chimpanzees were waiting looked
like collaboration (hunters direct complementary actions at the same prey;
Boesch and Boesch, 1989). The large number of captures of monkeys that
fell or were knocked to the ground also raise the issue of collaboration, es-
pecially given that certain males seemed likely to be either on the ground or
engaged in arboreal pursuits. However, distinguishing consistent role-taking
and other forms of collaboration from the simultaneous pursuit of individ-
ual tactics by multiple males (Goodall, 1986) is difficult and requires more
detailed data on individual behavior than we have.
Individual Variation in Hunting Behavior
New data corroborate the earlier finding that male hunting success var-
ied considerably (Mitani and Watts, 1999), as at other sites (Gombe: Goodall,
1986; Stanford, 1996; Tı: Boesch and Boesch, 2000). Some males, e.g., MG
and BF, seemed especially motivated to pursue monkeys (Table II). Variation
in motivation and success must depend partly on variation in characteristics
like age and body size. The significant relationship between dominance rank
and hunting success may have arisen because both covaried with features
like size, agility, willingness to take risks and skill at assessing when to do
so, and ability to coordinate the effects of one’s own behavior with those
of the behavior of others de Waal (1982) and Nishida (1983) have noted
similar characteristics of successful alpha males. But some highly motivated
and skilled males were not high-ranking; a few, e.g., MO and DO (Table II),
probably had not been so previously, nor will they become so.
Males were not equally able to gain meat from others. For example,
long-time alpha male MW, which could easily steal meat and often begged
successfully, had the best access to meat despite his low hunting success.
Meat Intake and Impact on the Red Colobus Population
Our estimates of annual per capita meat intake by males are lower than
those from Gombe (Stanford, 1996; Stanford et al., 1994a; Wrangham and
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Chimpanzee Hunting Behavior 23
Bergmann-Riss, 1990), but the difference is small despite the much higher
frequency of hunting at Gombe. This reflects the large size of hunting par-
ties at Ngogo, their high success rates, and the large number of monkeys
they kill per successful hunt, and it shows the important influence that de-
mographic variation can have on chimpanzee hunting behavior (Mitani and
Watts, 1999). Also, Stanford and colleagues (Stanford, 1996; Stanford et al.,
1994a) used higher prey body mass values, notably 9 kg for adult females
and subadults (which seems high for Kibale; Struhsaker, 1975). However,
estimated average meat intake by males at Tı was much higher, over a
4-year period, than estimated intake at Ngogo (Boesch and Boesch, 2000).
Proportionately more captures at Tı were of adults, especially adult males,
which accounts for some of this difference. More importantly, hunts were al-
most 5 times more frequent at Tı during this period than at Ngogo (Boesch
and Boesch, 2000). The population density of red colobus is higher at Tı
(Holenweg et al., 1996; Mitani et al., 2000b), and chimpanzees encounter red
colobus far more often there than at Ngogo (Boesch, 1994a; Boesch and
Boesch, 2000; Watts and Mitani, unpublished data).
The estimate of 6.5–12% of the red colobus population killed annually
is much lower than values of 16–40% reported for Gombe (Stanford, 1996,
1998; Wrangham and Bergmann-Riss, 1990). However, it is higher than es-
timates of 3.2–7.6% for Tı (Boesch and Boesch, 2000) and 3% at Mahale,
the estimated annual potential growth rate of the red colobus population
there (Ihobe and Uehara, 1999). Comparison of Ngogo census results from
different time periods show a long-term decline in the density of red colobus
groups in the study area, possibly because of predation by chimpanzees
(Mitani et al., 2000). Population density should also have declined, unless av-
erage group size has increased considerably; future research on red colobus
will address this issue.
Forest Structure and Hunting
Many observers at Gombe, starting with Wrangham (1975) and Teleki
(1975), have proposed that chimpanzees preferentially hunt red colobus in
areas with a broken canopy (Boesch, 1994b; Stanford, 1998; Stanford et al.,
1994b). Boesch (1994b) noted that the forest at Gombe is lower and more
open, with a more broken canopy, than at Tı and argued that capturing
red colobus is consequently more difficult at Tı unless the chimpanzees
cooperate. Ours are the first quantitative data to show that variation in forest
structure influences hunting decisions and to corroborate the impression that
capturing red colobus is more difficult in tall forest with a continuous canopy,
independently of other factors.
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24 Watts and Mitani
The large disparity in hunting success between areas where the canopy
is broken and those where it is not introduces a nuance into our earlier
argument about the impact of demographic variation on variation in hunting
success. On the one hand, the percentages of hunts in primary forest in
which the chimpanzees captured prey were about equal for Tı (Boesch
and Boesch, 1989, 2000) and Ngogo, despite the far larger number of males
at Ngogo. This highlights the importance of cooperation (in the sense of
coordination of activities among individuals; Boesch and Boesch, 1989) at
Ta¨ı. On the other, larger mean hunting party size at Ngogo meant that the
mean number of captures per successful hunt was almost 3 times that at Tı.
This fact, plus the strong positive relationships between hunting party size
and the number of kills per hunt at Ngogo, highlights the impact of variation
in the number of males per community on overall hunting success.
Hunting Seasonality
Ngogo data are also the first to establish a quantitative relationship be-
tween fruit availability and variation in hunting frequency and in the intensity
of predation on red colobus. Hunting frequency should vary inversely with
fruit abundance if meat is an alternative source of energy during fruit-poor
months. Conversely, if hunting is energetically expensive, but most important
for other nutritional reasons or for social reasons, it should be more common
when fruit is abundant. Wrangham and Bergmann-Riss (1990) found no sig-
nificant temporal variation in hunting frequency at Gombe between 1972
and 1975. Subsequent Gombe data (Stanford, 1996, 1998; Stanford et al.,
1994a,b) and data from other sites where researchers have stated that hunt-
ing frequency or predation pressure or both vary seasonally (Tı: Boesch and
Boesch, 1989; Mahale: Takahata et al., 1984; Hosaka, 1995; Uehara, 1997)
have not been accompanied by contemporaneous data on variation in fruit
availability. For example, in discussing Gombe data from 1982 through 1991,
Stanford and colleagues (Stanford, 1996, 1998; Stanford et al., 1994a,b) ar-
gued that fruit was scarce during dry seasons, as was the case in 1972 and
1973 (Wrangham, 1977). Dry seasons are predictably fruit-poor at Lop´e,
Gabon (Tutin et al., 1991) and offer less fruit than rainy seasons at Ndoki
(Kuroda et al., 1996), which may be true at Gombe. However, fruit produc-
tion in Kibale varies unpredictably at Kanyawara (Wrangham et al., 1996)
and independently of rainfall at Ngogo (Mitani and Watts, in press); thus
rainfall is not a universal proxy for fruit availability. Even at Lop´e, fruit pro-
duction is not significantly related to rainfall, and production varies greatly
from year to year (Tutin et al., 1991). Kibale data also show that some
tree species fruit asynchronously in different parts of the forest and that
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Chimpanzee Hunting Behavior 25
different chimpanzee communities in the same population can experience
quite different patterns of variation in food availability (Chapman et al.,
1997). We need similar quantitative data from other sites to determine if
hunting frequency and fruit availability are consistently related.
We do not know whether red colobus groups become more vulnerable
to attack during periods of peak fruit availability at Ngogo, e.g., because
they are predictably attracted to certain areas to feed or spend more time
in areas with broken canopy. Future research on red colobus behavior and
ecology should answer this question. Two lines of evidence suggest that the
chimpanzees do not hunt more frequently at these times simply because the
monkeys are more conspicuous. First, the increased frequency of hunting
patrols accounts for much of the increase in hunts. Second, the percent of
encounters that result in hunts also increases significantly with increases in
the availability of ripe fruit (Mitani and Watts, in press).
Rather than showing significant variation in hunting frequency, Gombe
data from 1982 to 1991 actually show that the mean number of red colobus
kills per month and per hunt and the proportion of encounters in which
the chimpanzees hunted were higher during the dry season (Stanford et al.,
1994a). Also, parties were larger, on average, during dry season months,
and hunting success increased with party size. Thus the chimpanzees did not
hunt more often during the dry season (except for a binge in 1990, when
47 hunts occurred within 68 days), but hunted more successfully and killed
more monkeys then. As Stanford (1996, 1998) notes, party size data were
inconsistent with the idea that meat is a fallback energy source when fruit
is scarce (Stanford et al., 1994a), given that party size varied inversely with
fruit availability in Wrangham’s (1977) Gombe sample, which is also the
case in Kibale (Wrangham et al., 1996; Chapman et al., 1997; Mitani et al.,
in press).
As Stanford et al. (1994b; Stanford, 1996, 1998) stressed, multiple fac-
tors explain why chimpanzees hunt and why they decide to hunt at some
encounters with red colobus, but not others (cf. Mitani and Watts, in press).
These almost certainly include nutritional factors, given the nutritional value
of meat, but energy may not be prominent among them. Hunting is energet-
ically expensive (Boesch, 1994b). Energy gains hunts can outweigh costs
(Boesch, 1994b), but this may often not be the case, at least for many
participants. Hunting may be nutritionally important because meat, mar-
row, and bones provide nutrients other than energy (Stanford, 1996, 1998;
Stanford et al., 1994a; Takahata et al., 1984). Suitable alternative sources of
such nutrients may be sufficiently common, though. For example, leaves are
a major source of protein (Wrangham et al., 1991), and oil palm fruit, which
are high in energy and lipids, are abundant during the dry season at Gombe
(Stanford, 1996). Hunting patrols, during which the chimpanzees may walk
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26 Watts and Mitani
for hours without feeding, add to the energetic cost of many hunts at Ngogo.
Hunting there seems to be most likely when the chimpanzees can easily
meet their daily energy needs because they have large fruit crops available.
Hunting patrols were most common at such times and typically started in
the afternoon, after the chimpanzees had eaten meals of fruit. Except for
any individuals still eating meat at dusk, they also had large fruit meals again
after patrols and hunts. They may hunt on encounter at any time, particu-
larly if they are in large parties (Mitani and Watts, in press.). But hunting
and hunting patrols are more likely when no risk of energy shortfalls exists.
We suspect that this is true for some other habitats, but only data on diet
and phenology can properly address the question.
ACKNOWLEDGMENTS
We thank The Uganda Wildlife Authority, Makerere University, and
Drs. John Kasenene and Gilbert Isibirye-Basuta for permission to work at
the Makerere University Biological Field Station. We thank Dr. Jeremiah
Lwanga for his expert management of field assistants at Ngogo and his
many contributions to research there. We are indebted to Godfrey Mbabazi,
Adolph Magoba, Lawrence Ndagezi, and Alfred Tumusiime for invaluable
assistance in the field and, in particular, for helping us to keep track of what
happens during hunts. C. Boesch provided valuable discussion of some of
the material presented here. J. Mitani was supported by NSF Presidential
Faculty Fellowship Award SBR-9253590 and by the L.S.B. Leakey Founda-
tion, and D. Watts was supported by the L.S.B. Leakey Foundation, Primate
Conservation, Inc., and Yale University. C. Boesch and one anonymous re-
viewer provided constructive criticism of an earlier version of this paper, and
we also thank C. Boesch for stimulating discussions of some of our results
and their comparative implications.
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