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Conflict management by hamadryas baboons (Papio hamadryas hamadryas) during crowding a tension-reduction strategy.

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American Journal of Primatology 68:993–1006 (2006)
RESEARCH ARTICLE
Conflict Management by Hamadryas Baboons
(Papio hamadryas hamadryas) During Crowding:
A Tension-Reduction Strategy
P.G. JUDGE1,2, N.S. GRIFFATON2, AND A.M. FINCKE2
1
Psychology Department, Bucknell University, Lewisburg, Pennsylvania
2
Program in Animal Behavior, Bucknell University, Lewisburg, Pennsylvania
Primates change a variety of behavioral responses during short-term
exposure to crowding. Under crowded conditions, rates of aggression,
submissive behavior, and affiliative behavior may increase or decrease.
Different patterns of change among these three categories of response have
been interpreted as various coping ‘‘strategies’’ for managing the increased
risk of conflict under crowded conditions. Grooming is of particular interest
because this behavior is known to have a calming influence on the recipient
and could be used to manage conflict under tense situations. A captive group
of nine hamadryas baboons (Papio hamadryas hamadryas) composed of two
harems was observed under short-term crowding to determine whether this
species adopts conflict-management strategies similar to those described in
other primates. The aggression, submission, affiliation, and displacement
activities of the six adults in the group (two males and four females) were
recorded, and behaviors in their small indoor quarters was compared with
baseline behaviors in the outdoor section of their enclosure, which had over
10 times more space. Repeated-measures analysis of variance (ANOVA) was
used to compare behavioral rates during crowding with baseline rates
recorded immediately after crowding and during matched controls collected
at the same time of day as the crowding sessions. Aggression and submission
rates did not change significantly across conditions. Huddling together and
proximity increased during crowding, and females increased grooming of
their harem male during crowding. Displacement activities (e.g., pacing and
self-grooming) increased during crowding, but scratching, an indicator of
anxiety in primates, did not. The pattern of behavior exhibited by this group
conforms to an active ‘‘tension-reduction’’ strategy in which animals
successfully reduce the higher risk of aggression during crowding.
Am. J. Primatol. 68:993–1006, 2006. c 2006 Wiley-Liss, Inc.
Key words: aggression; crowding; grooming; conflict management;
Papio hamadryas hamadryas
Correspondence to: Peter G. Judge, Psychology Department and Program in Animal Behavior,
Bucknell University, Lewisburg, PA 17837. E-mail: pjudge@bucknell.edu
Received 28 June 2005; revised 8 December 2005; revision accepted 2 January 2006
DOI 10.1002/ajp.20290
Published online 4 August 2006 in Wiley InterScience (www.interscience.wiley.com).
r 2006 Wiley-Liss, Inc.
994 / Judge et al.
INTRODUCTION
Primates have developed behavioral mechanisms to manage conflict within
their groups in order to reduce the costs of aggressive competition for resources and
gain the benefits of group living. Reconciliation, in which opponents make friendly
contact shortly after a fight, is one such mechanism. This response has been shown
to restore disrupted relationships, reduce the likelihood of continued aggression,
and reduce tension caused by the conflict (see review by Aureli et al. [2002]). While
reconciliation is an effective means of diminishing the costs of aggression after it
has occurred, primates may also take proactive measures to reduce the risk of
aggressive outbreaks. For example, previous studies have shown that grooming and
other forms of conciliatory and tension-reducing social behavior increase in captive
primate groups during the period immediately prior to a regularly scheduled
feeding [de Waal, 1987; Koyama & Dunbar, 1996; Mayagoitia et al., 1993]. The
authors concluded that the animals anticipated aggressive conflict and affiliated
with likely competitors to manage conflict and gain tolerance during feeding.
Studying primates living under various levels of crowding may also be a
means of investigating whether animals modify their behavior to manage conflict.
Historically, the increased competition for resources under crowded conditions
has been thought to produce high rates of harmful unrestrained aggression (see
review by Judge [2000]). However, de Waal [1989] argued that this view was
oversimplified for primates because they have developed many behavioral
mechanisms to manage conflicts in their groups. Instead, he proposed a ‘‘coping
model’’ and suggested that primates living under crowded conditions would
increase the rate at which they performed conflict-reducing responses in order to
minimize the amount of aggression. de Waal [1989] based the model on a study of
a captive chimpanzee (Pan troglodytes) group that was restricted to its indoor
quarters for several months each winter [Nieuwenhuijsen & de Waal, 1982]. Mild
forms of aggression (e.g., stylized threats) increased under crowding, while
unrestrained heavy aggression (e.g., bites and chases) did not. Submissive
displays to appease potential aggressors also increased under crowding. de Waal
[1989] particularly emphasized an increase in allogrooming under crowded
conditions because this behavior is known to have a calming influence on the
recipient [Aureli et al., 1999; Boccia et al., 1989] and is considered to be a tensionreduction mechanism [Schino et al., 1988; Terry, 1970]. At high density, animals
may groom more as a proactive means of reducing social tension.
Judge and de Waal [1997] tested the coping model on different rhesus
macaque (Macaca mulatta) groups that had lived for generations under a wide
range of population densities, and the results provided some support for the
model. As density increased, the animals groomed each other more. In addition,
females directed the increases in grooming toward nonkin females. Since the most
serious aggression often occurs between matrilines in matrifocal societies, such as
those of rhesus macaques [Ehardt & Bernstein, 1986; Gygax et al., 1997; Samuels
& Henrickson, 1983], the authors suggested that the animals may have been
strategically directing their grooming toward individuals that posed the highest
risk for escalated aggression. As predicted, submissive behavior (particularly the
bared-teeth display, which is a formal indicator of subordinate status in macaques
[de Waal & Luttrell, 1985]) increased as density increased. Mild aggression
increased with higher density, and, in contrast to the model, heavy aggression
also increased among adult females. Although the animals appeared to be
adopting a conflict reduction strategy through increased grooming, the tactic was
not entirely effective in reducing aggression.
Am. J. Primatol. DOI 10.1002/ajp
Baboon Conflict Management and Crowding / 995
Similar studies conducted to assess coping responses under crowded
conditions have found that animals appear to use a variety of behavioral
‘‘strategies’’ to manage conflict. A strategy can be defined as a specific pattern of
change among particular behavioral categories that imply a particular function.
For example, the increases in grooming and submission accompanied by no
increase in heavy aggression found by Nieuwenhuijsen and de Waal [1982] might
be described as a ‘‘tension-reduction’’ strategy in which animals take proactive
measures that are effective in reducing conflict. However, a ‘‘conflict avoidance’’
strategy was reported for a group of rhesus monkeys during a short-term
crowding experiment in which density was manipulated by a temporary reduction
of the available space [Judge & de Waal, 1993]. Mild aggression increased to
threaten animals away, but there was no increase in heavy aggression. The
animals avoided each other more often and increased their performance of formal
submissive bared-teeth displays. They also increased huddling with their kin and
remained immobile. Finally, contrary to a tension-reduction strategy, social
grooming significantly decreased during crowding. The pattern of change across
many behavioral categories appeared to serve the function of avoiding any
interactions, and thus conflicts, during crowding. Virtually the same pattern of
behavioral changes was observed in long-tail macaques (Macaca fascicularis)
during a similar short-term crowding experiment [Aureli et al., 1995], which
perhaps justifies the labeling of this pattern as a distinctive strategy. A somewhat
different strategy was described for chimpanzees that were subjected to shortterm crowding [Aureli & de Waal, 1997]. The chimpanzees exhibited significant
decreases in aggression, submission, and grooming. The pattern of change was
interpreted as an ‘‘inhibition strategy’’ in which animals suppressed all behavior
to reduce the possibility of conflict.
The strategies mentioned above support the concept of a coping model, since
the primates altered their behavior under crowded conditions in ways that may
have managed conflict. The described avoidance and inhibition strategies were
responses to crowding that did not reflect more proactive measures taken to
reduce conflict, such as increases in grooming, that de Waal [1989] originally
proposed. de Waal suggested that the behavioral changes under high density
predicted by a tension-reduction strategy might take long periods to develop and
could only be observed in groups that had lived together under crowded
conditions for many years, possibly even generations. During periods of shortterm crowding, animals may not have time to develop the more-active strategy
and instead take immediate measures to reduce conflict by avoiding others and
inhibiting interaction. Further, it may not be warranted to speak of particular
unique strategies without conducting more tests on more species. Investigators
may be observing idiosyncratic changes that are dependent on the species studied,
the experimental conditions, and the length of crowding.
The purpose of this study was to conduct a short-term crowding experiment
on a captive group of hamadryas baboons (Papio hamadryas), a species that had
not been tested in a crowding experiment, to determine whether they exhibited
patterns of behavioral change that conformed to any of the strategies mentioned
above. In addition, we examined whether the unique social system of hamadryas
baboons would affect conflict management between particular partners. Hamadryas baboons live in multi-tiered societies in which the most basic social unit is
a harem consisting of a male leader and his females [Kummer, 1984]. Males exert
a rather tyrannical presence over their females, and offspring and females are
coerced into bonding strongly with their male. Several harems associate to form
clans in which males are generally intolerant of each other but cooperate to
Am. J. Primatol. DOI 10.1002/ajp
996 / Judge et al.
compete against other clans. Since the observed hamadryas group was composed
of two harems, we tested for differential changes in behavior between males and
their harem females during crowding. Previous studies of conflict management
have indicated that animals strategically direct conflict reduction behavior toward
the individuals with whom they are most likely to come into conflict. Since
hamadryas females are likely to receive aggression from their harem males
[Abegglen, 1984], we predicted that conflict-reduction behavior between these
partners would increase during crowding.
Finally, displacement activities, such as scratching, are positively correlated
to anxiety in nonhuman primates [Maestripieri et al., 1992; Schino et al., 1996],
and they have been used productively as behavioral indicators of emotion in
studies of nonhuman primates [e.g., Aureli et al., 1989; Castles & Whiten, 1998;
Judge & Mullen, 2005]. In the short-term crowding study of chimpanzees
mentioned above [Aureli & de Waal, 1997], animals increased their rates of
scratching and yawning under crowding, indicating an increase in tension despite
the behavioral inhibition strategy employed. We recorded rates of displacement
activities in this study to determine whether the crowding manipulation affected
emotional states or the performance of conflict management responses controlled
levels of social tension.
MATERIALS AND METHODS
Subjects and Housing
The subjects of this study were a group of nine hamadryas baboons. The
group was established in 1968 at Bucknell University in Lewisburg, Pennsylvania, and consisted of two adult males, four adult females, and three of their
juvenile offspring (3–14 months old). The adult females were born into the group
from a common ancestor. The adult males were introduced 2.5 years prior to the
study. After the introduction of the males, two harems were established in which
each male acquired two different adult females. Dominance/submission interactions indicated that one male was clearly dominant over the other.
The group history differed from wild groups in that all females were related
and were raised together since birth. In the wild, daughters typically disperse,
join males from non-natal harems, and establish long-term relationships with the
other females in the new harem. Thus, the females in this study may have been
more strongly bonded than those in naturally-formed groups, which may have
increased their tolerance levels during crowding. However, the females were
fairly distantly related. The kinship classes represented among them were sister,
niece, cousin, and second cousin, and there were no mother–daughter pairs. In
addition, in all cases the female ancestor(s) that linked two individuals in the
matriline were deceased, which further reduced familial relations.
The enclosure consisted of a 9 11 4.5 m outdoor compound with adjoining
9 6 2.25 m indoor quarters. The outdoor section was constructed from
concrete block and chain link fencing, and contained a gravel substrate. Perches,
climbing structures, and swings were provided to promote species-typical
locomotion, and enrichment devices were continually available. The indoor
quarters were constructed from concrete block, chain link fencing, and a poured
concrete floor. The indoor quarters consisted of three interconnected 3 6 2.5 m
compartments, each of which had a doorway that led to the outdoor compound.
Food and water were available ad libitum.
Am. J. Primatol. DOI 10.1002/ajp
Baboon Conflict Management and Crowding / 997
Data Collection Procedures
Rates of behavior were assessed while the animals were in their outdoor
compound (baseline condition) and compared with rates of behavior that occurred
when the animals were locked into one of the three interconnecting compartments of their indoor quarters (crowding condition). Considering the twodimensional floor space available, the crowded condition represented a 5.5-fold
reduction in space. Considering the three-dimensional space available, crowding
represented an 11-fold reduction in space. The animals were accustomed to being
locked inside for maintenance and repairs to their outdoor compound. They
would also be locked into their heated indoor quarters for weeks or months at a
time each winter due to cold weather. They usually had access to all three of the
interconnected indoor compartments during these periods, but would frequently
be locked into a single compartment for cleaning of the other two. Even though
the animals typically had access to both the indoor and outdoor sections of their
enclosure on a day-to-day basis, we considered the use of just the outdoor
compound as a valid baseline because the group preferred the outdoors, and all
members would usually stay outside when the doors to their indoor quarters were
open. In addition, the animals were locked outside on a daily basis for indoor
cleaning and would remain locked outside for most of the day while the indoor
quarters dried.
Immediately after the animals were locked inside, we conducted a 15-min
habituation period with the observer standing at an indoor observation post. The
observation post afforded an unrestricted view of the animals through a
1.9 1.0 m doorway constructed of 2.5-cm-wide metal bars. After habituation
the observer would conduct a 10-min focal sample on each of the six adults in a
randomized order. Immediately after the data were collected the animals were
released back into their outdoor compound, which resulted in approximately
75 min of crowding. Eleven such crowding sessions were conducted over a 34-day
period, for a total of 1.83 hr of crowding data for each animal. Crowding sessions
were conducted in the early afternoon at approximately 1300 hr.
Two types of baseline data were collected while the animals were locked
outside in their outdoor compound: time-matched baseline and post-crowding
baseline. Time-matched baseline observations were conducted in the outdoor
compound at the same time of day as the crowding sessions (1300 hr). Time
matching was included in the design to control for possible changes in behavior
related to circadian changes in activity throughout the day. The 11 time-matched
baseline sessions were typically interspersed between crowding days on an
alternating basis. Exceptions occurred due to inclement weather, since we
sometimes postponed scheduled baseline sessions to control for weather and
temperature differences between baseline and crowding sessions. Post-crowding
baseline observations were conducted immediately after crowding sessions when
the animals were released into their outdoor compound. During each type of
baseline session, a 10-min focal animal sample was collected on each adult animal
in a randomized order. Only nine post-crowding sessions were conducted, because
heavy rains occurred following two crowding sessions. The animals tended to
aggregate under shelters and become immobile during heavy rain, so we
considered observations under these conditions uncharacteristic of baseline
levels of behavior, and possibly a confound when compared to behavior during
crowding due to the enforced closeness produced by the rain. Thus, the postconflict baseline data were based on 1.5 hr of data for each focal subject. We
collected both time-matched and post-crowding baseline data to conduct a more
Am. J. Primatol. DOI 10.1002/ajp
998 / Judge et al.
comprehensive test of the effects of crowding. If crowding alone affected behavior,
we would expect similar rates of behavior during both baseline conditions. On the
other hand, if being crowded influenced interactions immediately afterward, this
aftereffect of crowding would produce differences in the two baseline conditions.
Behavioral responses collected during focal animal samples were grouped
into four general categories: aggression, submission, affiliation, and displacement.
Aggression was divided into heavy aggression (bite and chase) and mild
aggression (threat and rough behavior). The definitions used to score individual
behavioral responses are listed in Table I. Submissive behavior included
hindquarter presentation, avoidance, screams, crouching, and baring teeth. We
analyzed only submissive responses that occurred outside of aggressive contexts
(i.e., not following the aggressive responses scored) because these would provide
an index of submission levels that was relatively independent of the levels of
aggression that occurred during crowding. We analyzed hindquarter presentation
separately because this response has been identified as a specialized gesture used
to preempt aggression in baboons [Hausfater & Takacs, 1987], and as such may
have had additional significance during crowding. Affiliative behavior included
grooms, huddles, and proximity. Grooms and huddles were analyzed separately
because these responses have been shown to vary independently with particular
behavioral strategies associated with crowding. Proximity was analyzed as a
manipulation check to verify that animals were actually crowded while locked
inside, since proximity would be expected to increase under conditions of higher
density. Displacement activities were recorded as another manipulation check
because these responses have been shown to be indicators of anxiety and tension
[Maestripieri et al., 1992]. We assumed that if animals were experiencing the
subjective experience of crowding, these emotional indicators would increase
during crowding. The displacement activities scored were pace, scratch, selfgroom, and yawn. Scratching was analyzed separately because this response has
received the most corroboration as an anxiety indicator [Schino et al., 1996]. The
other displacement activities were pooled for analysis, which is a common practice
in studies of primate emotion [e.g., Castles & Whiten, 1998].
Analysis
We used repeated-measures analyses of variance (ANOVAs) to test for
differences in the mean hourly rates of behavior performed and received by
subjects across the three observation conditions. When we analyzed the sex of
partners (males with females or females with females), we conducted separate
ANOVAs for each combination rather than a single two-way ANOVA (sex of
partner condition) in order to maximize the small sample size for the
male–female test (n 5 6) compared to the female–female test (n 5 4). Post hoc
tests following significant ANOVAs were conducted using paired t-tests. All tests
were conducted as two-tailed tests with the alpha level set at Po0.05. In two
tests, Mauchly’s test indicated that the sphericity assumption was violated, and
the Greenhouse-Geisser correction was used to assess the probability of obtaining
those results.
RESULTS
Rates of aggression did not differ across the three conditions (F2,10 5 0.36,
P 5 0.71; Fig. 1a). Too few instances of heavy aggression (bites and chases)
occurred for meaningful analysis (n 5 6), but these were distributed uniformly
across conditions (Fig. 1a). Since most aggression was initiated by or received
Am. J. Primatol. DOI 10.1002/ajp
Baboon Conflict Management and Crowding / 999
TABLE I. Definitions of Behavioral Responses Scored
Response
Definition
I. Aggressive behavior
Bite
A strong grip of the skin/limb of another with the teeth which is almost
always accompanied by a scream.
Chase
Rapid pursuit past the location the recipient maintained at the start of
the interaction.
Threat
A variety of interactions containing any of the facial, vocal, or motor
components listed below either alone or more typically in combination.
Facial gestures could include mouth open wide, thrusting the head
toward the target, glaring, and raised eyebrows displaying the white
portions of the eyelids. Vocalizations could include loud low-pitched
guttural grunts or screams. Motor components could include charges
and lunges toward another animal that did not exceed the recepient’s
location at the time the action began. Threats could also include
manual contact.
Rough behavior Mild physical contact that did not contain any of the facial or vocal
components involved in a threat. Rough behavior included a variety of
contacts such as grabbing, kicking, pulling, pushing, poking, slapping,
pulling hair, butting, and shoving.
II. Submissive behavior
Avoid
Walking or running more than one step from another animal within 15
seconds of an approach.
Bare-teeth
A facial expression in which the teeth were bared by tightly pulling back
the muscles of the face.
Crouch
Lowering the chest and/or head to a position just above the ground by
bending the forelimbs and/or hind limbs.
Present
Orientation of the hindquarters toward another animal, usually
accompanied by lowering the forelimbs, lifting of the tail, or looking
back over the shoulder.
Scream
A loud high-pitched vocalization.
III. Affiliative behavior
Groom
Manipulation, brushing or licking of the fur of another animal with one
or both hands for at least 15 seconds. Grooming bouts ended with
interuptions of more than 15 seconds or by moving a distance of
over 0.5 m.
Huddle
Stationary contact, other than grooming, with one or more other
individuals for more than 15 seconds. Bouts ended with interruptions
of more than 15 seconds or by moving a distance of
over 0.5 m.
Proximity
Approach to within 0.5 m of another animal for at least 15 seconds
without performing any other category of affiliative behavior.
IV. Displacement activities
Pace
Sustained locomotion of at least 15 seconds in a repetitive pattern within
a prescribed area.
Scratch
Quick repetitive touches on any part of the body with the hand or foot.
Self-groom
Grooming as defined above but directed toward oneself. Self-groom also
included nail biting and wound licking. Bouts were punctuated by
interruptions of more than 15 seconds.
Yawn
Opening of the jaws with a downward thrust of the lower mandible.
Am. J. Primatol. DOI 10.1002/ajp
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a. Aggression
b. Submission
Mild
8
3
Heavy
Mean hourly rate
Mean hourly rate
6
2
1
4
2
0
0
Baseline
Baseline
Crowding Post-crowding
c. Hindquarter presentation
Crowding Post-crowding
d. Grooming
5
5
Alpha male
4
Others
Mean hourly rate
Mean hourly rate
4
3
2
1
2
1
0
0
Baseline
Baseline
Crowding Post-crowding
e. Huddling
f. Proximity
20
40
15
30
Mean hourly rate
Mean hourly rate
3
10
5
Crowding Post-crowding
20
10
0
0
Baseline
Crowding Post-crowding
Baseline
Crowding Post-crowding
Fig. 1. Mean hourly rates (1SE) of the six categories of social behavior recorded under the three
observation conditions. For aggression the proportion of heavy vs. mild aggression is indicated. For
hindquarter presentation the proportion with the alpha male vs. other group members is indicated.
Am. J. Primatol. DOI 10.1002/ajp
Baboon Conflict Management and Crowding / 1001
from the alpha male (82.1%: 46 of 56 instances), the lack of change across
conditions occurred because the alpha male did not react to crowding with
aggression. Most submissive responses (avoid, scream, crouch, and bare teeth)
involved cases in which one animal avoided another (92%: 113 of 123), and
submissive responses did not differ across conditions (F2,10 5 1.74, P 5 0.22;
Fig. 1b). The rate at which animals presented their hindquarters to other animals
increased during crowding, although the result was not statistically significant
(F2,10 5 2.25, P 5 0.16; Fig. 1c). Further, most of these hindquarter presentations
were made to the alpha male (91%: 58 of 64; Fig. 1c).
Grooming increased during crowding relative to both baseline conditions
(Fig. 1d), but the difference across conditions was not statistically significant
(F2,10 5 2.11, P 5 0.24). However, an analysis of the sex of grooming partners
indicated that grooming changed across conditions between males and females
(F2,10 5 5.64, P 5 0.023) and not between females (F2,6 5 0.49, P 5 0.63; Fig. 2).
Male–female grooming rates were significantly greater during crowding than
during baseline conditions (t(5) 5 3.06, P 5 0.03), but none of the other conditions
differed significantly from each other (baseline/post-crowding: t(5) 5 2.44,
P 5 0.06; crowding/post-crowding: t(5) 5 1.23, P 5 0.28). Since females initiated
all male–female grooming and directed almost all of that behavior toward their
harem male (96%: 48 of 50 cases), the increase in grooming during crowding was
the result of the females’ increased grooming of their harem male. It is interesting
to note that the two exceptions of females grooming their own male were cases in
which a female in the harem of the lower-ranking male groomed the alpha male
during crowding.
Huddling significantly differed across crowding conditions (F2,10 5 17.42,
P 5 0.001), with more huddling occurring during crowding (Fig. 1e). Partner
analyses indicated that the adult males did not huddle with each other
throughout the study, and when they huddled with females they only huddled
with those in their harems. Although there was a general increase in huddling
between males and females during crowding (Fig. 3), the change across conditions
was not statistically significant (F2,10 5 1.64, P 5 0.24). Differences in huddling
between adult females differed significantly across conditions (F2,6 5 15.40,
P 5 0.03), with significantly more huddling occurring during crowding than
Mean hourly rate per partner
1.5
1
Female-female
Female-male
0.5
0
Baseline
Crowding Post-crowding
Fig. 2. Mean hourly rates per partner (1SE) of grooming under the three observation conditions,
indicating rates of female–female and female–male grooming.
Am. J. Primatol. DOI 10.1002/ajp
1002 / Judge et al.
during baseline or post-crowding, and no difference between baseline and postcrowding (crowding/baseline: t(3) 5 3.15, P 5 0.05; crowding/post-crowding:
t(3) 5 5.22, P 5 0.02; baseline/post-crowding: t(3) 5 2.29, P 5 0.11; Fig. 3). The
rate at which animals came into proximity to each other differed significantly
across crowding conditions (F2,10 5 21.24, Po0.001; Fig. 1f), with significantly
more proximity during crowding than during baseline or post-crowding, and no
difference between baseline and post-crowding (crowding/baseline: t(5) 5 5.30,
P 5 0.004; crowding/post-crowding: t(5) 5 4.58, P 5 0.006; baseline/post-crowding:
t(5) 5 0.89, P 5 0.42). Thus, the spatial manipulation did appear to produce the
desired crowding effect.
Scratching appeared to be unaffected by crowding (F2,10 5 0.74, P 5 0.50;
Fig. 4a). However, the pooled rate of the other displacement activities (pace, selfgroom, and yawn) significantly differed across conditions (F2,10 5 11.94,
P 5 0.003), with significantly more displacement activities occurring during
crowding than during baseline or post-crowding, and no difference between
baseline and post-crowding (crowding/baseline: t(5) 5 4.57, P 5 0.006; crowding/
post-crowding: t(5) 5 3.43, P 5 0.02; baseline/post-crowding: t(5) 5 0.94, P 5 0.39;
Fig. 4b).
Mean hourly rate per partner
2.5
2
1.5
Female-female
1
Female-male
0.5
0
Baseline
Crowding Post-crowding
Fig. 3. Mean hourly rates per partner (1SE) of huddling under the three observation conditions,
indicating rates of female–female and female–male huddling.
b. Displacement activities
a. Scratch
30
8
6
Mean hourly rate
Mean hourly rate
25
20
15
10
4
2
5
0
0
Baseline
Crowding Post-crowding
Baseline
Crowding Post-crowding
Fig. 4. Mean hourly rates (1SE) of (a) scratching and (b) other displacement activities under the
three observation conditions.
Am. J. Primatol. DOI 10.1002/ajp
Baboon Conflict Management and Crowding / 1003
DISCUSSION
In contrast to similar studies on macaques and baboons [Alexander & Roth,
1971; Aureli et al., 1995; Elton & Anderson, 1977; Judge & de Waal, 1993],
the hamadryas baboons in this study showed no difference in aggression
across crowding conditions. If anything, inspection of the means indicated a
nonsignificant decrease in absolute levels of aggression during crowding (Fig. 1a).
In addition, there was no change in levels of general submissive responses
(e.g., avoid), which have often been shown to increase during short-term
crowding [Alexander & Roth, 1971; Elton & Anderson, 1977; Judge & de Waal,
1993; Nieuwenhuijsen & de Waal, 1982]. Hindquarter presentation, a specialized
gesture used to preempt aggression in baboons [Hausfater & Takacs, 1987],
did not change significantly across conditions (P 5 0.16), but the frequency
with which animals directed this display toward the alpha male nearly
doubled during crowding (Fig. 1c). A similar increase in a formal submissive
display, the bared-teeth display of macaques, has been observed during
crowding in rhesus macaques [Judge & de Waal, 1993], and females
have been shown to selectively increase the response to adult males
at higher density [Judge & de Waal, 1997]. Crowding appeared to
have little influence on aggressive and submissive behaviors, and perhaps
motivated the animals to formally indicate their submissive status to the alpha
male.
Females increased huddling with other females during crowding–a response
observed in similar studies [Aureli et al., 1995; Judge & de Waal, 1993]. Such
increases in friendly behavior during tense situations may help relieve social
tension [de Waal, 1989]. Perhaps most significantly, adult females groomed their
harem males more often during crowding. Grooming is used to reduce social
tension in primate groups [Schino et al., 1988; Terry, 1970] and it has a calming
influence on the recipient [Aureli et al., 1999; Boccia et al., 1989]. Further, male
harem leaders are the most aggressive adults in hamadryas baboon groups,
particularly in captivity [Kummer & Kurt, 1965], and they are despotic rulers
that are likely to aggressively herd their females and punish them with speciestypical neck bites [Abegglen, 1984]. By placating their leader males with
grooming, females may reduce the increased possibility of aggression from males
during crowding. Calming the males may also reduce the chances of the males
fighting with each other, in effect mediating potential conflict that might have
negative repercussions for the females themselves. As in previous studies of
conflict management [de Waal, 1987; Judge & de Waal, 1997; Koyama & Dunbar,
1996; Mayagoitia et al., 1993], the animals appeared to strategically direct
affiliative behavior toward the animals with which they were most likely to come
into conflict.
The overall pattern of change observed across all behavior categories
could be interpreted as a ‘‘tension-reduction’’ strategy similar to the proactive
conflict reduction strategy originally proposed by de Waal [1989] but
never observed in its entirety. The animals may have increased grooming to
calm others, and increased friendly affiliative behavior to reduce social tension.
The result was to manage aggression and keep it at the same levels
as in noncrowded conditions. Not only is this study the first to observe some of
the behavioral changes predicted to occur with the use of a proactive
conflict-reduction strategy, it is also unique because the changes occurred
during a short-term crowding study. Although de Waal [1989] suggested that
animals might have to live for long periods at high density before they develop
Am. J. Primatol. DOI 10.1002/ajp
1004 / Judge et al.
this style of behavior, the baboons in this study adopted the behavioral pattern
during brief bouts of crowding. Perhaps the predictions also apply to short-term
situations, or the animals were so accustomed to being locked into their indoor
quarters over the years that they learned to utilize the strategy quickly and
opportunistically.
An alternate explanation for the lack of change in aggression could be that
the 11-fold reduction in space produced by the crowding manipulation was not
extreme enough to produce the increases in aggression observed in similar
studies. The lack of increase in scratching rates during crowding may lend
support to this explanation because scratching is an indicator of emotional
arousal in nonhuman primates [Maestripieri et al., 1992; Schino et al., 1996], and
one could argue that if scratching did not increase, the manipulation did not
produce the subjective experience of crowding. On the other hand, the number of
times the animals came into proximity significantly increased during crowding,
indicating that they were physically closer together. Further, displacement
activities such as yawning, self-grooming, and pacing increased during crowding,
indicating some form of emotional reaction to the manipulation. Finally, one
could also argue that scratching rates remained low during crowding because the
behavioral mechanisms employed by the animals during crowding were effective
enough to reduce social tension.
Another indication that the crowding manipulation was valid is that
rates of social behavior during crowding were always either slightly or markedly
different from those during the two different types of baseline observations,
which were always remarkably similar to each other (Fig. 1). If the crowding
manipulation did not have an influence, and we were effectively sampling
baseline behavior during all three conditions, one would not expect
such consistency with the behavioral rate during crowding always being the
unique extreme compared to the baselines. The close similarity of the two
baseline conditions also indicates that there was no ‘‘aftereffect of crowding’’ such
that the animals would have behaved differently during the post-crowding
condition compared to the time-matched baseline on days when they were not
crowded.
The species-typical behavior of hamadryas baboons should also be considered
in interpretations of the current results. Harems are closely knit units in which
the females are strongly bonded to their males. Females are typically coerced into
staying in close proximity to their male [Abegglen, 1984], thus producing what
might be considered enforced crowding around him. The tension-reduction
strategy that was adopted during short-term crowding could be similar to
behavioral modifications the females have developed during tense situations near
their males. The tendencies of hamadryas to gather at night in large groups on
the face of a sleeping cliff, and for harems to share small outcroppings [Kummer,
1984] may also predispose this species to make rapid behavioral adjustments to
temporary crowding that are effective in controlling or inhibiting aggression.
Nevertheless, the results support the concept of the coping model [de Waal, 1989],
which predicts that primates will develop behavioral responses to manage conflict
in their groups, and increase performance of the responses when they are at risk
for increased aggression. In addition, most conflict research and studies of the
mechanisms developed by primates to control aggression and live in cohesive
social groups have focused on events following aggression. The results of this
study suggest that animals anticipate aggressive disruptions and use conflictreduction behavior routinely outside of aggressive contexts to manage aggression
before it begins.
Am. J. Primatol. DOI 10.1002/ajp
Baboon Conflict Management and Crowding / 1005
ACKNOWLEDGMENTS
We thank Mary Gavitt for providing technical assistance. The primate facility
was supported by Bucknell University.
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