Aggression and conflict management at fusion in spider monkeys

Abstract

In social systems characterized by a high degree of fission-fusion dynamics, members of a large community are rarely all together, spending most of their time in smaller subgroups with flexible membership. Although fissioning into smaller subgroups is believed to reduce conflict among community members, fusions may create conflict among individuals from joining subgroups. Here, we present evidence for aggressive escalation at fusion and its mitigation by the use of embraces in wild spider monkeys (Ateles geoffroyi). Our findings provide the first systematic evidence for conflict management at fusion and may have implications for the function of human greetings.

Full text

Biol. Lett. (2007) 3, 147–149
doi:10.1098/rsbl.2007.0041
Published online 20 February 2007
Animal behaviour
Aggression and conflict
management at fusion
in spider monkeys
Filippo Aureli
1,
*
and Colleen M. Schaffner
2
1
Research Centre in Evolutionary Anthropology and Palaeoecology,
School of Biological and Earth Sciences,
Liverpool John Moores University, Liverpool L3 3AF, UK
2
Department of Psychology, University of Chester,
Chester CH1 4BJ, UK
*Author for correspondence ( f.aureli@ljmu.ac.uk).
In social systems characterized by a high degree
of fission–fusion dynamics, members of a large
community are rarely all together, spending most
of their time in smaller subgroups with flexible
membership. Although fissioning into smaller
subgroups is believed to reduce conflict among
community members, fusions may create conflict
among individuals from joining subgroups. Here,
we present evidence for aggressive escalation at
fusion and its mitigation by the use of embraces in
wild spider monkeys (Ateles geoffroyi). Our find-
ings provide the first systematic evidence for
conflict management at fusion and may have
implications for the function of human greetings.
Keywords: aggression; conflict; embrace;
fission–fusion; greeting; grooming
1. INTRODUCTION
Conflicts of interest between group members over
resources, travel decisions or allocation of time to
different activities are unavoidable, but they may com-
promise the cooperative benefits of group living,
especially when they escalate into aggression (Aureli
et al.2002). Group-living animals are therefore expected
to use various behavioural mechanisms to manage their
conflicts (Aureli & de Waal 2000; Wittig & Boesch
2003; Flack et al.2005). Most attention has been given
to post-conflict behaviour after aggressive interactions
(Arnold & Aureli 2006), but a more efficient means of
conflict management would be to prevent aggressive
escalation in the first place.
A possible way to reduce intragroup competition and
aggressive escalation is to adjust group size to local
resource availability (Janson 1988). The adjustment
can also occur within the same group resulting in the
temporary formation of smaller subgroups based on
fission–fusion dynamics (Kummer 1971; Wrangham
1979). Thus, whereas fissions into smaller subgroups
are likely to reduce conflicts over resources and
decisions, fusions may create such conflicts among
individuals from joining subgroups. If so, mechanisms
for mitigating the negative consequences of fusion are
likely to have evolved, but no previous study has
investigated this aspect of conflict management.
The aims of the present study were to document
whether fusions were characterized by aggressive escala-
tion and to examine whether post-fusion affiliative
interactions play a role in conflict management. We
carried out the study on wild spider monkeys (Ateles
geoffroyi ), one of the primate species with the highest
degree of fission–fusion dynamics (Symington 1990).
2. MATERIAL AND METHODS
Subjects of the study were the adult, subadult (thereafter labelled
together as (sub)adult) and juvenile members of two communities
(eastern and western) of spider monkeys living in the forest
surrounding the Punta Laguna lake, Yucatan peninsula, Mexico
(Ramos-Fernandez et al.2003). During the 2002–2003 study period,
the eastern community included 4–6 (sub)adult males, 5–7 (sub)
adult females and 0–4 juveniles, and the western community included
8–10 (sub)adult males, 12–14 (sub)adult females and 5–7 juveniles.
Although subgroups were usually separated by distances of
hundreds of metres, visibility constraints forced us to use a
conservative definition of subgroup. Individuals were not
considered to be in the followed subgroup if they were not observed
at a distance less than or equal to 30 m from at least one current
subgroup member for more than 30 min. The cut-off distance of
30 m for this chain rule was derived from previous data on the
same communities ( Ramos-Fernandez 2005). Fusion was recorded
when one or more individuals from another subgroup came within
30 m from any member of the followed subgroup.
Each subject was observed for approximately 160 h in the
eastern community and 40 h in the western community. All
occurrences of aggressive interactions including conspicuous
patterns, such as chases, physical contact and loud vocalizations,
were recorded along with ad libitum sampling of approaches within
an arm’s reach of another individual, grooming and embraces (i.e.
a monkey wraps one or two arms around another individual’s
back and/or performs pectoral sniff and cheek-to-cheek contact;
Schaffner & Aureli 2005).
Each subject was involved on average in 164 fusion events in
the eastern community and 31 in the western community. Analyses
were carried out at the individual level and compared the likelihood
of social interactions in the first 5 min following fusion with
baseline rates. Since preliminary analyses revealed that interactions
clustered just before fissions and after fusions, baseline periods for
each subject were obtained by excluding the 5 min preceding
fissions and the 5 min following fusions from the total time the
individual was observed in a subgroup. Hourly aggression rates
were calculated for the individual initiating the interaction. The
proportion of approaches that were followed by either embraces or
grooming was calculated for each subject that approached others in
the first 5 min following fusion and at baseline in at least three
separate occasions for more reliable estimates. When sample size
allowed, separate statistical analyses were carried out for the two
communities. As the results were highly consistent, we report the
results for the combined dataset of the two communities. Data that
were not normally distributed based on the Kolmogorov–Smirnov
test were square-root transformed. One-way repeated measures
ANOVA with correction for sphericity (followed by Bonferroni post
hoc tests) and paired t-tests with two-tailed probabilities were used
to compare hourly rates and proportions of approaches at the
individual level with an alpha level of 0.05.
3. RESULTS
The rates of aggressive interactions were affected by
fusion events (figure 1; F
2,52
Z15.95, pZ0.00004).
Post hoc tests revealed that during the 5 min following
fusion, spider monkeys behaved aggressively against
members of joining subgroups more often than
against members of the subgroup they were in before
the fusion ( pZ0.001) and at baseline ( pZ0.0001).
In contrast, rates of post-fusion aggression between
individuals that were in the same subgroup before
fusion did not differ from baseline rates ( pZ0.114).
The proportion of approaches followed by groom-
ing between individuals from joining subgroups
(meanGs.e.: 0.13G0.05) was significantly lower in
the first 5 post-fusion minutes than at baseline
(0.41G0.05; t
11
Z4.49, pZ0.001). In contrast, the
proportion of approaches followed by embraces
between such individuals was higher in the first 5
post-fusion minutes (0.55G0.10) than at baseline
Received 22 January 2007
Accepted 29 January 2007
147 This journal is q 2007 The Royal Society

(0.17G0.03; t
11
Z4.80, pZ0.001). This result was
confirmed as the 95% bootstrap confidence interval
(1000 replicates) for the mean of the difference
between the 5 min pre- and post-fusion embraces for
all individuals involved in fusions did not include zero
(0.50, 1.27). Embraces, like aggression, were usually
initiated only by one or two individuals per fusion
and thus only a minority of the potential dyads from
the joining subgroups were involved.
All 15 individuals for whom we recorded at least one
post-fusion embrace with a member of the joining
subgroup did not display or receive any aggressive
behaviour after the post-fusion embrace (in one case
there was aggression before the embrace). This is in
contrast with the mean hourly post-fusion aggression
rate of 0.56 (G0.11) for the same 15 individuals when
no embrace occurred. As post-fusion aggression rate
was highest in the first minute and post-fusion embraces
occurred on average within the first minute, we calcu-
lated the post-fusion aggression rate only for 2–5 min.
Even when using this conservative estimate, there was a
consistent decrease in post-fusion aggression rate after
embraces took place compared with when there was
no embrace (0.0G0.0 versus 0.21G0.05; t
14
Z4.23,
pZ0.001).
4. DISCUSSION
Our study shows that spider monkeys’ fusion events
are potentially risky situations in which aggression is
more likely to occur than at other times. Fusions did
not create situations in which aggression occurred
indiscriminately as a reflection of generalized tension
or increased number of group members because
aggressive interactions were selectively increased
between members of joining subgroups. Fusions were
also characterized by increased affiliation as members
from joining subgroups engaged in embraces, which
appeared to be one way to reduce the likelihood of
post-fusion aggression. These results were obtained
despite using a conservative definition of subgroup,
biasing against finding such effects of fusion events,
and a small number of subjects, predominantly
(sub)adults, for the analyses on affiliation. Although
increased levels of aggressive and affiliative
interactions after fusion were reported in other studies
(see below), this is the first systematic evidence for
post-fusion conflict management.
Previous studies described that chimpanzees (Pan
troglodytes) and spider monkeys display aggressive
behaviour when subgroups join one another (Klein
1974; Bauer 1975; Fedigan & Baxter 1984; Nishida
et al. 1999; Muller 2002). In chimpanzees, grooming
and other affiliative behaviour (no study focused
specifically on embraces or other brief friendly con-
tacts) seem more probable after fusion between the
members of joining subgroups (Bauer 1975; Nishida
et al. 1999; Okamoto et al. 2001). As in our study,
spider monkeys seem to exchange more embraces,
but not grooming, after fusion in the wild or at
reunion after separation in captivity (Klein & Klein
1971; van Roosemalen & Klein 1988; Schaffner &
Aureli 2005). Given that only a few individuals
exchange embraces during a given fusion event,
further research needs to focus on their possible
bond-testing function as has been shown for male
baboons’ (Papio populations) greetings in contexts
other than reunions ( Whitham & Maestripieri 2003).
Theprevalenceofembracesatfusionmaybe
related to the quick nature of the exchange relative to
grooming. Embraces can therefore be more effective in
appeasing or reassuring others when rapid action is
needed to reduce the likelihood of aggression at fusion.
Rapid contacts at reunion have been reported in other
species that experience high rates of fission and fusion,
such as spotted hyenas, Crocuta crocuta and bonobos,
Pan paniscus (East et al. 1993; Hohmann & Fruth
2000; G. Hohmann 2006 personal communication).
This is in contrast with the long and elaborate reunion
displays of capuchin monkeys (Cebus spp.) in which
separations are rare events (Matheson et al. 1996;
Manson & Perry 2004). Interestingly, humans
exchange rapid friendly contact, such as handshakes,
embraces, nose rubbing and kisses, when they are
reunited with familiar individuals and such greetings
have been interpreted as a ‘disclaimer of aggression’
(Firth 1972; Kendon & Ferber 1973). As human
societies are also characterized by frequent fissions
and fusions (Rodseth et al. 1991), our results suggest
that research on the potential function of such greet-
ings in reducing tension and facilitating tolerance at
reunions may contribute to the understanding of
human conflict management.
F.A. was supported by a HEFCE Promising Research
Fellowship. We thank the Wenner-Gren Foundation for
Anthropological Research, The British Academy and the
North of England Zoological Society for financial support
and Pronatura Peninsula de Yucatan, Jan Verpooten, Feder-
ica Amici, Eulogio Canul and Macedonio Canul for their
assistance.
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0
0.1
0.2
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j
oinin
g
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g
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Figure 1. Mean (Gs.e.) hourly rate of aggressive interactions
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