Group decision-making in chacma baboons: leadership, order and communication during movement.

Group decision-making in chacma baboons:
leadership, order and communication during
movement
Sueur
Sueur BMC Ecology 2011, 11:26
http://www.biomedcentral.com/1472-6785/11/26 (20 October 2011)

RESEARCH ARTICLE Open Access
Group decision-making in chacma baboons:
leadership, order and communication during
movement
Cédric Sueur1,2,3,4
Abstract
Background: Group coordination is one of the greatest challenges facing animals living in groups. Obligatory
trade-offs faced by group members can potentially lead to phenomena at the group level such as the emergence
of a leader, consistent structure in the organization of individuals when moving, and the use of visual or acoustic
communication. This paper describes the study of collective decision-making at the time of departure (i.e.
initiation) for movements of two groups of wild chacma baboons (Papio ursinus). One group was composed of 11
individuals, whilst the other consisted of about 100 individuals.
Results: Results for both groups showed that adult males initiated more movements even if the leadership was
also distributed to adult females and young individuals. Baboons then joined a movement according to a specific
order: adult males and adult females were at the front and the back of the group, sub-adults were at the back and
juveniles were located in the central part of the progression. In the two groups, vocalisations, especially loud calls,
were more frequently emitted just before the initiation of a group movement, but the frequency of these
vocalisations did not influence the success of an initiation in any way.
Conclusion: The emergence of a leadership biased towards male group members might be related to their
dominance rank and to the fact that they have the highest nutrient requirements in the group. Loud calls are
probably not used as recruitment signals but more as a cue concerning the motivation to move, therefore
enhancing coordination between group members.
Background
Animals living in groups have to synchronise their activ-
ities and coordinate their movements in order to remain
cohesive [1-4]. Group members therefore have to reach
consensus about the time and the direction to move col-
lectively [5]. However, animals in heterogeneous groups
differ in their nutrient requirements, in the information
they have about the environment or in their ability to
monopolise resources. These differences lead to conflicts
of interest between group members, which could result in
different strategies emerging about the best way to collec-
tively decide where and when to move [6]. These strategies
could be summarised and observed via three different
explorations: 1) who leads the groups and at what
frequency, 2) how individuals are organized during a
group movement and 3) what form of communication, if
any, is used at the start of group movements, or during
them.
The term “leadership” commonly refers to individuals
initiating movements or to individuals who change the
direction of movement and are followed by the rest of the
group [7-10]. Even if more and more studies show that
this leadership is distributed among all group members,
some individuals are seen to initiate movements more
often, or are more often located at the front of the pro-
gression than their conspecifics [7,8,10-12]. Several the-
ories have been proposed to explain this emergence of
leadership, and they can be separated into two models: the
conflict model and the integrative (or voluntary) model
[13]. In the first model, the leader forces its conspecifics to
follow him. In the second model, the emergence of a lea-
der within the group “may in some cases be preferred over
Correspondence: cedric.sueur@iphc.cnrs.fr
1Department of Ecology and Evolutionary Biology, Princeton University,
Princeton, USA
Full list of author information is available at the end of the article
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egalitarian arrangements [...] as a solution to the key chal-
lenges of life in social groups, such as conflicts over
resources, coordination failures and free-riding in coopera-
tive relationships” according to Hooper and collaborators
[13]. In this last model, the leader does not impose a deci-
sion on its conspecifics: the latter “decide to let one indivi-
dual decide”. Several individuals or social factors influence
the leadership [3,6]. Old individuals are presumed to have
a better knowledge of the environment and may therefore
lead the group to rare food resources. Many instances
have been described in which aged individuals led the
group more often than other group members (for instance,
see rhesus macaques, Macaca mulatta: [14]). The most
striking example however is most probably the matriarch
in elephants [15]. Dominance is also a factor affecting lea-
dership. In the mountain gorilla (Gorilla gorilla berengei),
the silverback walks quickly in the direction of the future
movement and the other group members follow him [16].
There is however an increase in frequency of grunts
emitted by females before the initiation of the silverback
[17]. Similar initiations by dominants have also been
described in wolves (Canis lupus, [18]) and in mongooses
(Helogale parvula, [19]). However, high-ranking indivi-
duals are often those with the highest body mass, and thus
the highest nutrient requirements. The different needs of
individuals also have an impact on initiation frequency.
Lactating or pregnant females decide more often than the
other group members in baboons [20] or in zebras (Equus
burchellii, [21]). A study on the emergence of leadership
based on nutrient requirements showed that the individual
having the highest needs becomes the leader in about 80%
of collective movements. This decision-making system is
viable: all individuals are satisfied and can also meet their
energetic needs throughout the day, even if they rarely
make the decision for the group to move [22]. For
instance, the body mass of adult chacma baboon males is
around 29 kg whilst females weigh between 16 and 20 kg
[23,24]. However, lactating females need about 200% more
protein and water than non-lactating females [25].
Following is possibly as important as leading [2,26]. Cer-
tain advantages could be gained from the rank held by
individuals within the progression (in this article, the term
“rank” indicates the position of an individual within the
progression order, and is not related to dominance hierar-
chy in any way). For instance, dominant males or lactating
females are at the front of the progression, where they
have better access to food resources and can decide which
direction to take (African buffalos, Syncerus caffer, [27];
zebras, [28]; yellow baboons, Papio cynocephalus, [29];
chimpanzees, Pan troglodytes, [30]). On the other hand,
juveniles or some females could also be in the central part
of the progression simply because these ranks offer the
best protection against predators [31-33]. Some authors
suggest that this type of order within the progression of
primates should be attributed to their high cognitive abil-
ities [34,35]: “ In a species as intelligent as the baboon,
almost any form of social behaviour, spatial or otherwise,
could understandably exhibit considerable variation arising
from the baboon’s ability to adapt features of his world”
[29]. However, recent research in primates shows that the
complex patterns we observed during collective move-
ments can be explained by self-organised processes that
have also been described in species such as insects and
ungulates [36-39]. There is no real necessity for each
group member to know its conspecifics individually or to
have a global view of the collective phenomenon [4,36,40]:
an estimation of the number of individuals already moving
and/or the number of individuals still resting is sufficient
to produce complex collective phenomena. These “rules of
thumb” are based on physiological needs and social rela-
tionships [4,40], and they have explained departure laten-
cies, order and associations of individuals during
movements in macaques [41] and lemurs [42]. Whilst for-
mer studies tend to prove that the whole group decides to
move only through the initiator’s act (leadership hypoth-
esis, see [42] for details), an increasing number of recent
studies now show that following behaviour depends on
mimetism (or social facilitation [43] i.e., the probability of
an individual to display a behaviour depends on the num-
ber of individuals already performing this behaviour
[4,40]) and that the initiator is not the only decision-
maker for a group movement [37,38,41,44-48].
Collective decision-making cannot be achieved if group
members do not exchange information about their state
or their motivations. This communication does not need
to be complex and can simply rely on local interactions
[6,9,37]. The most basic signal is walking in a specific
direction, showing that the individual is now motivated to
go in that particular direction [49]. Even if vocalisations
are used, studies on macaques [10,50] and chacma
baboons [9] showed that this simple visual signal is
enough to initiate a movement and to make all group
members join it. The same conclusion is reached in the
case of non primate species such as geese (Anser domesti-
cus, [51]), cattle (Bos Taurus, [52]) and sheep (Ovis aries,
[53]). Authors have however observed that acoustic sig-
nals, such as a ‘coo’ vocalization in Japanese macaques
(Macaca fuscata, [54]) and ‘trill’ in the white-faced capu-
chin monkey (Cebus capucinus, [34]) are also given before
the departure of groups. The acoustic signal is maybe
more advantageous in primates than a visual signal, as it
has the advantage of propagating the information over a
longer distance without being affected by physical barriers
[55]. Authors reported that in baboons, vocalisations such
as grunts or loud calls (’wahoos’) may play a role in the
cohesion of group members [56-60]. Although all indivi-
duals emit grunts, loud calls are only emitted by males
and depend on the dominance rank with high-ranking
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individuals showing a greater probability to emit grunts
[59,60]. However, these signals are not displayed or do not
have influence on collective movements in some groups of
baboons [9].
In this study, I attempt to identify collective decision-
making mechanisms in two groups of free-ranging
chacma baboons, namely one small group of 11 indivi-
duals and one large group of about 100 individuals. First,
the emergence of leadership, organization of individuals
and communication had never been studied at the same
time on a same group. Secondly, no previous study has
assessed how group size might affect these three variables
during group movements. I assess in turn the distribution
of leadership in both groups, the organization of group
members after departure, and communication used by
individuals to reach a consensus. I expect adult males to
be the main leaders of the group, mainly because of their
higher nutrient requirements or their dominance rank
[22,61]. For progression order, adult males and sub-
adults should rather be located at the front and the back
of the progression (the distribution of ranks should fol-
low a parabolic curve, for instance) whilst adult females
and juveniles should be found more in the middle of the
progression (the distribution of ranks should follow an
inverse parabolic curve) for a better protection against
predators. If there is a possibility of vocalizations improv-
ing coordination between members, I suggest that vocali-
zations, and particularly loud calls, should be particularly
emitted before the initiation of a movement and in the
large group. The results of the two groups will be com-
pared in order to assess whether or not the mechanisms
underlying decision-making differ according to group
size.
Methods
Study site and subjects
Data on chacma baboons were scored at the Wildcliff Nat-
ure Reserve, Western Cape, South Africa (33.959997°N,
21.034478°E) from May to July 2009 for the large group
and from May to July 2010 for the small group. The
reserve is a mountain wilderness reserve consisting of
deep ravines with afro-mountain forest, rocky mountain
tops and high fynbos meadows. An invasive plant, the
black wattle, and a grassy meadow can also be found on
the reserve. Direct or indirect cues showed that leopards
and other small carnivores were present in the reserve.
Three groups of chacma baboons populate this reserve
and its surroundings: a large group, a small group and a
third group entirely composed of males. At the time of the
study, the large group consisted of between 95 and 105
individuals (about 9.1% were adult males, 37% were adult
females without babies (< 1 year), 5.6% were adult females
with babies, 16.5% were sub-adults (4-6 years old) and
31.8% were juveniles (1-3 years old)). The small group
consisted of 11 individuals: one adult male, five adult
females (without babies), two sub-adults and four juve-
niles. The compositions of these two groups are therefore
comparable. The two groups were already habituated to
the presence of human beings. Group members could not
be identified individually, only classes of sex and age were
identifiable (see definitions of classes below). A previous
study showed that movement of these baboons through-
out the day are relative to foraging and finding resources
[62].
Definitions
An ‘initiation’ was defined as the movement of an indivi-
dual in one direction over a distance of at least 20 m
beyond the periphery of the group, when no other initia-
tion had been made in the same direction within the 15
previous minutes (as used in previous studies on primates:
[7,9,10,26,63]). An individual performing this behaviour is
called the ‘initiator’. All departing movements in the same
direction as the initiator within the 15 minutes following
the initiation were considered to be following movement,
and the individuals performing these behaviours for dis-
tances of 20 meters or more were defined as followers (as
defined in previous studies [7,9,10,26,63]). If no group
members followed the initiator within 15 minutes, the
phenomenon was defined as a ‘failed initiation’. I scored
the number of followers for each initiation and analysed it.
I did not define a threshold to qualify an initiation as ‘suc-
cessful’. Indeed, it is difficult to predict exactly how many
followers are necessary to make an initiation successful for
an initiator, even if previous research has revealed a
threshold of three followers in capuchins [45] and maca-
ques [50] when an initiator stops to emit recruitment sig-
nals. In baboons, Stueckle and Zinner [9] set the threshold
at five following individuals to qualify an initiation as
successful.
The order of progression during departure (lag between
the initiation and the adhesion of the last follower) was
calculated. The initiator was ranked 1, the first follower
was ranked 2 and the rank of the jth follower was ranked j
+1. When two individuals joined the movement at the
same time, I considered the two individuals to have an
identical rank. The joining time for all participants is
defined as the time between the departure of the initiator
and the departure of the last follower. The departure
latency of a follower j is the time between the departure of
follower j and the departure of the previous follower j - 1.
Data collection
Accompanied by a field assistant, I scored the location
of the baboon group from dawn (about 7:00) to dusk
(about 17:00) throughout the study period.
Initiations were only retained if at least 75% of indivi-
duals were visible and no agonistic interaction was
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observed [9,10,26]. 64 movements were recorded over
23 observation days for the large group, compared to 61
movements over 25 observation days in the small group.
Morning departures account for 14 movements in the
large group and 10 in the small group respectively. Each
movement is considered as an independent event (see
definition above for the time between two movements)
since it is defined as a combination of several variables
that are different from one movement to another
[7,9,10,26,63,64].
I split individuals into five categories: adult males, adult
females without infant, adult females with infant, sub-
adults and juveniles. I recorded the behaviour of indivi-
duals simultaneously because the focal individual sampling
method is not appropriate for studying collective decisions
[9,10,49]. Data were collected using Cyber Tracker 3.0
(Cyber Tracker Conservation, Bellville, SA) with a PDA
Asus 620 and a Palm Treo 750. During movement depar-
tures, I scored the rank, the time of departure and the
individual category (see above) of each individual joining a
movement. The field assistant used continuous sampling
to score the vocalisations associated with movements
(loud calls and grunts) both during movement and outside
the moving context. Only the frequencies of vocalisations
at the group level were analysed in this study. It was often
impossible to identify the emitters of vocalisations within
the larger group.
Data analyses
Leadership: I compared the absolute frequency of initia-
tions (i.e., number of initiations per category) between
categories and between groups using a Chi square test.
The corrected number of initiations was defined as the
number of initiations observed per category divided by the
ratio of individuals of this category within the group (for
instance, 9.1% of adult males for the large group, see
above). This corrected number of initiations per category
was then divided by the sum of all corrected numbers of
initiations for all categories in order to obtain a relative
frequency of initiations or relative leadership. The shape of
the relative leadership distribution was analysed using a
curve estimation test (curve estimation regression statis-
tics) in order to measure how this leadership varies
between categories. The ranking of categories from the
highest to the lowest leadership rate made it possible to
test the shape of the distribution; see [46,65] for similar
analyses. The relative frequency per category was then
compared to the mean body mass of each category using a
curve estimation test. Animals were not weighted, mean-
ing that body mass was estimated on the basis of the mean
documented mass for each general age-sex category
[23,24]. Even if the measure is not exact, it provides infor-
mation about the relation between the leadership rate and
the body mass. A comparison was carried out between the
different categories for the number of followers and the
joining time for all participants via Mann-Whitney testing
in the small group and using a Kruskal-Wallis test in the
large group. I also analysed the distribution (absolute fre-
quency and cumulative relative frequency) of the number
of followers for each group using curve estimation tests.
Distribution analysis is useful here to assess whether the
group is either cohesive or clustered in different sub-
groups.
Joining and order during progression: A survival analysis
was carried out on departure latencies; the shape of the
inverse cumulative distribution of departure latencies was
analysed using curve estimation tests. A shape following
an exponential law means that the probability of indivi-
duals joining the movement is not dependent on time, but
rather on the number of individuals that already joined
the movement, whilst a shape following a linear curve
means that the probability is time-dependent. The shape
of the mean departure latencies distribution per rank was
analysed using the same test. The joining process is sug-
gested to be mimetic if this distribution is parabolic [40].
I calculated the mean order of progression for each cate-
gory of individuals to be the frequency at which one cate-
gory occupies each rank j, divided by the number of
movements for which I observed each rank. I then cor-
rected this number by the proportion of individuals in
each category. I only retained ranks for which at least five
movements were observed. Considering this, the highest
analysed rank is 91 for the large group, and 11 for the
small group. The homogeneity of the absolute rank distri-
bution was analysed using a Chi square test. The relative
frequency distribution per rank and for each category of
individuals was analysed using curve estimation tests [66].
Different functions (parabolic, inverse parabolic, linear and
cubic) were tested and only the best-fitting function was
retained to explain the observed distribution (based on R2.
R2 are adjusted according to the number of free para-
meters [67-69]). A linear distribution of ranks indicates
that a category has the same probability to occupy all
ranks. A parabolic distribution (U-shape) indicates that a
category has more probability to be in the front and at the
back of the movement. On the contrary, an inverse para-
bolic distribution (n-shape) suggests that a category is
located in the middle of the progression rather than in the
front or at the back. I also compared these different distri-
butions between groups using the same test.
Communication: Using the Chi square test, I first ana-
lysed if the frequency of vocalisations differed between
moving context and stationary context, and whether the
emission of vocalisations was followed by an initiation or
not. Individuals took 15 minutes to consider a vocalisation
linked to a subsequent initiation (in agreement with defini-
tions above). I then assessed whether vocalisations were
more frequent during morning departures, in comparison
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with other movements, using a Mann-Whitney test. I also
analysed if vocalisations are emitted more before or after
an initiation using a Wilcoxon test. Their frequency was
then correlated with the number of followers and the join-
ing time of all participants using a Spearman rank correla-
tion test. I did not test this correlation for joining time in
the large group, since group members rarely moved all
together, and the number of values with a similar number
of followers was too weak to make the statistical testing of
this correlation possible. This was not the case for the
small group, since the majority of movements concerned
the entire group. Lastly I compared the frequency of voca-
lisations per movement between both groups using a
Mann-Whitney test.
The significance level was set at 0.05. All tests were
two-tailed. I carried out statistical tests using SPSS 10.00
(SPSS Inc., Chicago, IL, USA). Means are ± SE.
Results
Leadership
Leadership is not equally distributed between age and sex
categories in the small group of chacma baboons (c2 =
362, df = 2, P < 0.00001). The adult male initiated more
than adult females, who initiated more than sub-adults
and juveniles. Juveniles never initiated movements and
sub-adults only did so once. This distribution of leader-
ship (categories are ranked from the highest leadership
rate to the slowest one) follows an exponential law (R2 =
0.99, F1,2 = 304, P = 0.003, y = 3.225e
-1.482x, Figure 1a).
There is no difference in the number of followers
between movements initiated by the adult male and
those initiated by adult females (Mann-Whitney test: Z =
-0.870, Nmale = 27, Nfemale = 32, Mmale = 10.62 ± 0.37,
Mfemale = 10.06 ± 0.52, P = 0.385). I obtained the same
result for the joining time of all participants (Mann-
Whitney test: Z = -0.200, Nmale = 27, Nfemale = 32, Mmale
= 381.67 ± 78.54, Mfemale = 374.35 ± 72.10, P = 0.841).
Only three failed start attempts were observed (4.9% of
cases, two attempts by adult females, one attempt by an
adult male) and one movement with one follower (1.6%,
in adult females). 88.5% of movements included all group
members and 4.9% included ten individuals, showing that
the group is cohesive [41,44]. Indeed, a survival analysis
showed that the inverse cumulative distribution is non
linear and follows a sigmoid function with a threshold
equal to the number of group members (R2 = 0.90, F1,9 =
70, P = 0.00005, y =
0.95
( x
11
)30 , Figure 2a).
In the large group, leadership is not equally distribu-
ted between age and sex categories (c2 = 527, df = 3, P
< 0.00001) and follows an exponential law (R2 = 0.99,
F1,2 = 1706, P = 0.001, y = 2.4874e
-1.245x, Figure 1a).
The adult females with infants initiated fewer move-
ments than females without infants (c2 = 20, df = 1, P <
0.00001). No difference was observed in the number of
followers between the different categories of individuals
(K-W test: U = 4.206, N = 64, P = 0.240). As far as the
distribution of the number of followers is concerned, all
cases were observed, with 11 failed initiations (17.2% of
initiations) and 53 initiations (86.8%) attracting 2 to 101
followers. I then categorised the number of followers by
intervals of 20 (1-20, 21-40, 41-60, 61-80, 81-105) and
found that the frequency of collective movements is the
same between each of these categories (K-W test: U =
7.260, N = 53, P = 0.123). Indeed, a survival analysis
showed that the inverse cumulative distribution is linear
(R2 = 0.95, F1,103 = 694, P < 0.00001, y = -0.0071x +
0.6991, Figure 2b). All sub-group sizes during move-
ments were observed at the same frequency.
Both groups have the same leadership distribution (c2 =
2.3, df = 3, P = 0.551, Figure 1a). The higher the mean
body mass of categories, the more they are seen to lead
(R2 = 0.83, F1,6 = 171, P = 0.002, y = 0.291x - 0.1898,
Figure 1b).
Joining and order during progression
Survival analysis in the small group showed that the
inverse cumulative distribution of departure latencies of
joiners follows an exponential law (R2 = 0.91, F1,95 = 908,
P < 0.0001, y = 0.3553e-0.011x, Figure 3a) better than a lin-
ear one (R2 = 0.44, F1,95 = 75.58, P < 0.0001, y = -0.0013x
+ 0.3252), meaning that the probability to join the move-
ment is constant per time unit and equals 0.011
[38,41,45,70]. Departure latency distribution according to
the rank of followers follows a parabolic curve (R2 = 0.75,
F1,8 = 26, P = 0.001, y = 1.6515x
2 - 24.761x + 112.04,
Figure 1 Leadership in the two groups of baboons (a) Relative
frequency of initiations per category of individuals in the small
group (black bars) and in the large group (white bars) of chacma
baboons. The dotted line is the leadership distribution according to
the category and follows an exponential function. (b) Relative
leadership frequency according to the mean body mass of each
category of individuals. Lozenges are observed data. The line
represents the linear relationship between the two variables.
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Figure 3b). I calculated the observation frequencies per
rank for each category of individuals. The distribution of
absolute frequency according to rank during progression
is not homogenous for the adult male (c2 = 100, df = 9, P
< 0.0001), but follows a cubic function (Table 1, Figure
4a). The male was more often located at the front (specifi-
cally at the first rank) or at the back of the progression
than in the middle. I obtained the same result for adult
females. The distribution is not homogeneous (c2 = 28, df
= 10, P = 0.002) but follows a parabolic law (Table 1, Fig-
ure 4b). Adult females occupied positions at the front and
back of the progression more than in the middle. Sub-
adults also had specific ranks: the distribution is not
homogenous (c2 = 33, df = 10, P = 0.0003) but follows a
cubic function (Table 1, Figure 4c), meaning that sub-
adults were rarely observed at the front of the progression
but were observed at the back of the progression in the
majority of cases. Lastly, the distribution of juvenile ranks
is not homogenous either (c2 = 64, df = 9, P < 0.0001)
and follows an inverse parabolic law (or cubic, Table 1,
Figure 4d). Contrary to other group members, juveniles
were located in the central part of the progression.
The large group seems to be organized in the same way
as the small group. Indeed, I observed the same rule
underlying the joining and order of group members. The
inverse cumulative distribution of departure latencies of
joiners follows an exponential law (R2 = 0.90, F1,153 =
1269, P < 0.0001, y = 0.1383e-0.009x, Figure 3c) better than
a linear one (R2 = 0.25, F1,153 = 52.28, P < 0.0001 y =
-0.0004x + 0.1402): the probability of joining the move-
ment is constant per time unit and equals 0.009. The dis-
tribution of departure latencies according to the rank of
joiners follows a parabolic curve (R2 = 0.43, F1,9 = 6.6, P =
0.030, y = 0.0067x2 - 0.7489x + 33.081, Figure 3d). Adult
males were more often observed at the front or back of
the progression: the frequency distribution per rank is not
homogeneous (c2 = 309, df = 66, P < 0.00001) and follows
a parabolic function (Table 2, Figure 5a). The frequency
distribution per rank in adult females is not homogeneous
either (c2 = 512, df = 89, P < 0.00001) and also follows a
parabolic function (Table 2, Figure 5b). However, among
adult females, the distribution for females without infants
is heterogeneous (c2 = 432, df = 82, P < 0.0001) and fol-
lows a parabolic law (R2 = 0.18, F1,89 = 22, P < 0.0001, y =
2E-06x2 - 0.0001x + 0.0106) whilst the distribution for
females with infants is homogeneous (c2 = 26, df = 63, P =
1.00). Sub-adults occupied certain ranks more than others
(c2 = 137, df = 84, P < 0.0001) but no function fits with
the distribution, even the cubic curve (Table 2, Figure 5c).
The distribution in juveniles is not homogeneous (c2 =
304, df = 80, P < 0.0001) and follows an inverse parabolic
curve (Table 2, Figure 5d).
The inverse cumulative distribution of departure laten-
cies in the small group is correlated to that of the large
group (R2 = 0.86, F1,808 = 5290, P < 0.0001, 1.2222x +
0.0127). The exponential coefficient of these distributions
(y = 0.3553e-0.011x for the small group; y = 0.1383e-0.009x
for the large group; see above and Figure 3a, c) corre-
sponds to the mean probability to join a movement, and
is 0.009 for the members of the small group and 0.011
for members of the large group process. The functions
obtained in the four categories of individuals for the
small group were then transposed to the large group for
the calculation of simulated frequencies per rank for each
category. I then compared these simulated frequencies to
the frequencies obtained in the large group. Results
showed that the distributions are correlated (R2 = 0.68,
F1,354 = 936, P < 0.0001, y = 0.9706x + 0.0165) showing
that members of both groups used similar rules of pro-
gression during collective movements.
Communication
The small group did not display more movements
with loud calls (44.2%) than without loud calls (55.8%)
Figure 2 Frequency of initiations according to the number of
followers. Relative frequency (bars) and survival analysis (line) in (a)
the small group and (b) the large group.
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(c2 = 0.5, df = 1, P = 0.484). Similarly, movements with
grunts (41.3%) were no more numerous than those with-
out grunts (58.7%) (c2 = 1.5, df = 1, P = 0.238). However,
a movement consistently followed the emission of loud
calls (100% of cases), whilst only 87.1% of grunt emissions
were followed by a movement (the difference observed
with/without grunts is significant: c2 = 17, df = 1, P <
0.00001). There were more loud calls (Mann-Whitney
test: Z = -2.319, Nmorning = 10, Nother = 51, Mmorning =
5.44 ± 2.47, Mother = 1.68 ± 0.65, P = 0.038) and more
grunts (Mann-Whitney test: Z = -2.952, Nmorning = 10,
Nother = 51, Mmorning = 31.33 ± 21.65, Mother = 2.84 ±
0.66, P = 0.003) during the morning departures compared
to other movements. There were also more loud calls
(Wilcoxon test: Z = -3.359, Nbefore = Nafter = 61, Mbefore =
2.96 ± 0.74, Mafter = 0.15 ± 0.10, P = 0.001) and more
grunts (Wilcoxon test: Z = -3.642, Nbefore = Nafter = 61,
Mbefore = 8.51 ± 2.20, Mafter = 0.52 ± 0.31, P = 0.0003)
prior to initiations than after initiations. Grunts do not
appear to be more numerous prior to loud call emissions
than after them (Wilcoxon test: Z = -0.827, Nbefore = Nafter
= 12, Mbefore = 19.58 ± 16.71, Mafter = 4.83 ± 1.62, P =
0.408). The number of loud calls is not correlated to the
success of the movement, either in terms of number of fol-
lowers (Spearman rank correlation: rs = -0.119, N = 61,
P = 0.400) or joining time for all participants (Spearman
rank correlation: rs = -0.243, N = 57, P = 0.160). The same
result is obtained for the relationship between the number
of grunts and the number of followers (Spearman rank
correlation: rs = 0.005, N = 61, P = 0.972) and for the
Figure 3 Distributions of departure latencies. Survival analysis of departure latencies in (a) the small group and (c) the large group. Mean
departure latency according to the rank of progression in (b) the small group and (d) the large group. Lozenges are observed data. The line is
the relationship between the two variables and follows a parabolic function.
Table 1 Curve estimations of the distribution of individual frequency per rank for each age-sex category in the small
group of chacma baboons
Category Function R2 F P equation
Linear 0.19 2.19 0.172 y = -0.0175x + 0.1926
Adult male Parabolic 0.68 19.71 0.002 y = 0.0104x2 - 0.1429x + 0.4642
Cubic 0.86 54.96 < 0.0001 y = -0.0025x3 + 0.0547x2 - 0.3648x + 0.7329
Linear 0.03 0.279 0.610 y = -0.0079x + 0.4902
Adult females Parabolic 0.41 141.24 < 0.0001 y = 0.0101x2 - 0.1294x + 0.7534
Cubic 0.41 6.34 0.033 y = 3E-05x3 + 0.0096x2 - 0.1268x + 0.7503
Linear 0.60 13.69 0.005 y = 0.0244x + 0.0303
Sub-adults Parabolic 0.61 14.09 0.005 y = 0.0009x2 + 0.0133x + 0.0544
Cubic 0.82 39.43 0.0001 y = 0.0019x3 - 0.034x2 + 0.1885x - 0.1577
Linear 0.01 0.03 0.868 y = 0.0035x + 0.2448
Juveniles Parabolic 0.88 71.02 < 0.0001 y = -0.021x2 + 0.255x - 0.3001
Cubic 0.88 68.09 < 0.0001 y = 9E-05x3 - 0.0226x2 + 0.2635x - 0.3103
Results in bold indicate the best-fitting function explaining the distribution. R2 are adjusted according to the number of free parameters.
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Figure 4 Relative frequency of observations according to the rank of progression for the small group and per category of individuals.
Lozenges are observed data. The dotted line represents the distribution of frequencies under the hypothesis that there is no specific order
(homogeneous distribution). The continuous line represents the observed relationship between the two variables.
Table 2 Curve estimations of the distribution of individual frequency per rank for each age-sex category in the large
group of chacma baboons
Category Function R2 F P equation
Linear 0.04 4.42 0.038 y = -3E-05x + 0.0119
Adult males Parabolic 0.19 23.43 < 0.0001 y = 9E-06x2 - 0.0009x + 0.026
Cubic 0.08 9.35 0.003 y = -2E-08x3 + 1E-05x2 - 0.001x + 0.0268
Linear 0.14 16.17 0.0001 y = 4E-05x + 0.0072
Adult females Parabolic 0.32 46.35 < 0.0001 y = 2E-06x2 - 0.0001x + 0.0105
Cubic 0.21 28.50 < 0.0001 y = -8E-09x3 + 3E-06x2 - 0.0002x + 0.011
Linear 0.00 0.40 0.529 y = 2E-05x + 0.0079
Sub-adults Parabolic 0.02 1.45 0.231 y = 2E-06x2 - 0.0001x + 0.0101
Cubic 0.03 2.64 0.108 y = 1E-07x3 - 1E-05x2 + 0.0004x + 0.0058
Linear 0.013 0.50 0.479 y = -2E-05x + 0.0093
Juveniles Parabolic 0.25 35.21 < 0.0001 y = -3E-06x2 + 0.0003x + 0.0038
Cubic 0.17 21.67 < 0.0001 y = 9E-08x3 - 2E-05x2 + 0.0008x - 0.0005
Results in bold indicate the best-fitting function explaining the distribution. R2 are adjusted according to the number of free parameters.
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joining time of all participants (Spearman rank correlation:
rs = 0.101, N = 57, P = 0.562).
In the large group, movements with loud calls are no
more numerous (56%) than those without loud calls
(44%) (c2 = 0.7, df = 1, P = 0.396). Similarly, the group
did not display more movements with grunts (62%) than
without grunts (48%) (c2 = 2.8, df = 1, P = 0.090). Emis-
sions of loud calls are more often followed by a move-
ment (71.4%) than by no movement at all (28.6%) (c2 =
12.8, df = 1, P = 0.0003). However, emissions of grunts
are no more frequently followed by a movement (67.8%)
than by no movement at all (32.2%) even if there is a
statistical tendency (c2 = 3.6, df = 1, P = 0.059). The fre-
quency of loud calls is higher when followed by a move-
ment (Mann-Whitney test: Z = -2.331, Nwithmovement =
61, Nwithoutmovement = 20, Mwithmovement = 2.66 ± 0.61,
Mwithoutmovement = 1.10 ± 0.57, P = 0.02), whilst there is
no difference concerning the frequency of grunts
(Mann-Whitney test: Z = -0.309, Nwithmovement = 61,
Nwithoutmovement = 20, Mwithmovement = 2.38 ± 0.77,
Mwithoutmovement = 5.5 ± 4.16, P = 0.759). There are
more loud calls during morning departures than during
other movements (Mann-Whitney test: Z = -2.706,
Nmorning = 14, Nother = 50, Mmorning = 4.09 ± 1.48,
Mother = 2.25 ± 0.66, P = 0.007) but not more grunts
(Mann-Whitney test: Z = -0.470, Nmorning = 14, Nother =
50, Mmorning = 1.55 ± 0.91, Mother = 2.26 ± 0.66, P =
0.962). There are more loud calls (Wilcoxon test: Z =
-2.652, Nbefore = Nafter = 64, Mbefore = 3.32 ± 0.65, Mafter
= 1.67 ± 0.67, P = 0.008) and more grunts (Wilcoxon
test: Z = -2.019, Nbefore = Nafter = 64, Mbefore = 2.81 ±
0.68, Mafter = 1.13 ± 0.38, P = 0.044) prior to initiations
Figure 5 Relative frequency of observations according to the rank of progression for the large group and per category of individuals.
Lozenges are observed data. The dotted line represents the distribution of frequencies under the hypothesis that there is no specific order
(homogeneous distribution). The continuous line represents the observed relationship between the two variables.
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than after initiations. Grunts are no more numerous
prior to the emission of loud calls than after it (Wil-
coxon test: Z = -1.686, Nbefore = Nafter = 12, Mbefore =
4.83 ± 1.67, Mafter = 1.25 ± 0.51, P = 0.092). The num-
ber of loud calls (Spearman rank correlation: rs =
-0.167, N = 64, P = 0.361) and the number of grunts
(Spearman rank correlation: rs = -0.076, N = 64, P =
0.678) are not correlated to the number of followers.
Direct comparison of the two groups in terms of abso-
lute frequencies of loud calls and grunts per movement
did not reveal any difference between groups for loud calls
(Mann-Whitney test: Z = -0.995, Nsmall = 61, Nlarge = 64,
Msmall = 2.41 ± 0.67, Mlarge = 2.66 ± 0.61, P = 0.320) but
showed that the small group emitted more grunts per
movement than the large group (Mann-Whitney test: Z =
-2.345, Nsmall = 61, Nlarge = 64, Msmall = 4.09 ± 0.87,
Mlarge = 2.38 ± 0.77, P = 0.019).
Discussion
This study is the first to simultaneously study leadership,
progression order and communication during move-
ments in two different-sized groups of baboons. What-
ever the group size, group members seem to show the
same rules of decision making. Even if we need to con-
firm this result in other groups, the latter is very surpris-
ing given existing theoretical literature which suggests
that small and large groups should display different
mechanisms of communication and organization during
movements [4,71]. This study showed that in both
groups, adult males are more prone to lead the group,
even if adult females and sub-adults can also initiate
movements. However, there is no difference between
individuals of different sex or age as far as the success of
initiations is concerned. During progression, individuals
seem to join the movement through mimetism or social
amplification [37,38,41,45,70]. The parabolic shape of the
distribution of latencies is typically the signature of a
mimetic process where an individual displayed a beha-
viour according to the number of individuals already per-
forming it [38,41,72]. If joining a movement followed an
independence process (individuals are not influenced by
their conspecifics) or a leadership process (the probability
to join the movement only depend on the movement
initiator), the distribution of latencies would more follow
a linear function [40,42]. However, joining order is not
random, with adult males and adult females being located
more often at the back or front of the movement, whilst
juveniles are in the centre of the progression. Lastly,
baboons seem to use vocalisations on a context-depen-
dent basis: grunts and loud calls were emitted more often
before the initiations of movement than after an initia-
tion or within other contexts. The results are similar in
both groups, despite the difference in group size.
Literature often associates leadership, or the frequent
initiation of movements, with either dominance of group
members or the philopatric sex [3,9,10,45,73]. Few studies
have showed the relationship between leadership and
nutrient requirements [22,74-76]. In this study, adult
males initiated more movements than adult females, sub-
adults or juveniles in both of the groups studied. The
same result was found in other studies on baboons [9,11].
However, with the exception of juveniles in the small
group, all individuals seem to be able to initiate a move-
ment and are followed to the same extent as adult males.
Moreover, the exponential distribution of relative leader-
ship obtained in both groups is corrected by the number
of individuals per category. If the leadership is not cor-
rected by this ratio, it is therefore approximately the same
for adult males and adult females (48% vs. 48% in the
small group; 47% vs. 40% in the large group). Adult males
and adult females initiated the same number of initiations
(at the group level), but as the ratio of adult males is lower
than that of adult females, the relative leadership is higher
for males (at the individual level, one adult male initiated
more often movements than one adult female). This
means that the leadership is distributed among all group
members even if it is biased amongst adult males. The
advantages of leading a group for an individual are that he
or she can not only decide on movement direction but
also be the first to access resources [11]. Adult males
might also be at the front of the progression because they
are larger than other group members and therefore play a
role in defending the group from predators or competitor
groups. Leading the group might therefore be explained
by dominance of individuals - high dominance may allow
an individual to decide on direction and have access to
resources - or could be explained by the needs of indivi-
duals - individuals with the highest needs decide which
direction will be taken by the group. These two factors -
dominance and needs - are often correlated, and authors
mainly conclude that leadership is influenced by domi-
nance. However, a theoretical study shows that a biased
leadership in favour of adult males could emerge from dif-
ferences in nutrient requirement between group members
[22]. This theoretical study illustrates leadership distribu-
tion following the same exponential distribution as that
found not only in this current study but also in another
study on baboons [9]. Adult males might be more prone
to lead because they have higher nutritional needs, or in
other words, because other group members as juveniles,
have lower nutritional demands. However, this assumption
needs to be confirmed by evaluating the independent abil-
ity of dominance rank and body mass to predict the prob-
ability of initiating a movement across individuals: if body
mass significantly predicts the outcome, but dominance
rank does not, one could definitively state that the
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nutrient-need hypothesis outperforms the dominance
hypothesis.
Adult males are more often at the front and back of the
progression, and particularly occupy the first ranks of the
progression. Adult females occupy the same ranks but to
a lesser extent. Sub-adults are found more at the back of
the group whilst juveniles are in the central part of the
progression. These results have already been found in
other groups of baboons [29,31,32] and other groups of
primates [14,30]. The main ecological reason suggested
to explain this specific order is the protection of juveniles
against potential predators and other groups of baboons.
Authors have often reported that the high cognitive skills
of primates, and particularly of baboons, might allow
them to adapt their behaviours (here, the order during
progression) to the risk level of their environment
[29,34,77]. However, this specific order during progres-
sion has also been described in ungulates [27,28,78] and
social carnivores [79]. In groups of plains zebras, females
(especially those lactating) are more often at the front of
movements than sub-adults or juveniles [28]. In African
buffalos, juveniles and sub-adults do not decide about
movements [27]. Females are also at the front of the
movements in a herd of cattle [78]. In social carnivores,
females are more often observed at the front of the
movement, and juveniles are seldom observed at the
front in species such as coatis (Nasua narica), mon-
gooses, lions (Panthera leo), cap hunting dogs (Lycaon
pictus) or spotted hyenas (Crocuta crocuta) [79]. Another
explanation is that simple and local rules could lead to
the emergence of these complex patterns during move-
ments [36,37,41,45]. Sueur and colleagues [26,41,70]
showed that similar patterns of organization can be
explained in macaques and lemurs by their use of
mimetic rules based on physiological needs, and mimet-
ism based on social relationships between group mem-
bers. I also found a mimetic process in this study
showing that an amplification rule could also be applied
in the two studied groups. Both studied groups displayed
the same rules for order during progression, irrespective
of size. However, the differences between categories for
the order of progression are less pronounced in large
groups. There is more noise, firstly because of the large
group size (the fluctuations and variations of a collective
phenomenon increase with the number of individuals
implied in the phenomenon [80]), but probably also due
to the existence of sub-groups. Indeed, large troops of
baboons are often composed of different sub-groups.
These sub-groups are of different sizes. Composed of
adult males, females and juveniles, they have their own
organization [81-83] leading to multiple combinations at
the group level.
Vocal communication is not necessary between mem-
bers of either group for movement decisions [6,9], nor is
this acoustic communication necessary for an initiator to
be followed by its conspecifics. In this context, the ‘visual
contact’ between individuals appears to be enough for
group coordination [4,9,49] and communication would
be only local, which is reminiscent of self-organized pro-
cesses [36,37,41]. However, this study shows that vocali-
zations, especially loud calls, are emitted more in the
context of collective movements. These loud calls are
also more frequently emitted before the initiation of a
movement than after it. This result is reminiscent of the
vocalisations emitted in female gorillas before the silver-
back initiated a movement [16], but also the group cere-
mony preceding initiations in social carnivores [79] or
the pre-departure processes also described in ungulates
or macaques [10,27,52]. However, it is impossible to con-
clude from this study that the frequency of these loud
calls influences decision-making for an initiation or
affects its success. Other studies on baboons have also
failed to show a direct effect of vocalisations on the
recruitment process [9]. Instead of having a direct effect
on the joining process, such as recruiting partners, these
vocalizations might only be used to signal the motivation
to move, and lead in some way to a better coordination
of individuals [6,50,55]. Under the global communication
hypothesis, I expected the large group to emit more voca-
lisations than the small one. However, I found that the
small group emitted the same number of loud calls per
movement and more grunts than the large group. This
seems to show that local communication was used more
than global communication between individuals within
the large group. Global communication might not be so
efficient when the group size becomes too large, as Con-
radt and Roper [71] have already stipulated. Indeed, as
the group size increases, the mean distance between two
individuals also increases (and the number of visual or
acoustic obstacles) and the probability of information
transmission decreases. This leads the communication to
be more local than global [4]. Another explanation might
be that members of the small group emitted more vocali-
sations because the cohesiveness in this group is stronger
and the reasons to remain cohesive are more numerous
than in the large one. There is still a lot to be done if
we wish to gain a clear understanding of coordination
differences between small and large groups, so further
studies are necessary in order to assess coordination
mechanisms.
Conclusion
This study provides a clarification of mechanisms under-
lying the emergence of leadership and the complex pat-
terns of organization during movements in baboons.
However, these rules can also be applied in other species,
primates or non primates. Physiological differences
between individuals, coupled to social amplification
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based on social relationships, seem to be crucial factors
affecting collective decisions. However, further experi-
mental and theoretical studies are necessary before we
can disentangle the influences of the different variables
affecting the patterns of collective movements.
Acknowledgements
The author is grateful to A. Brotz, L. Durand and P. Pebsworth for their help
on the field, and thanks J. Munro and M. Pelé for their help on this
manuscript and English editing. This study was funded by the Fyssen
foundation, the Franco-American commission and the Japan Society for the
Promotion of Science.
Author details
1Department of Ecology and Evolutionary Biology, Princeton University,
Princeton, USA. 2Centre National de la Recherche Scientifique, Département
Ecologie, Physiologie et Ethologie, Strasbourg, France. 3Université de
Strasbourg, Institut Pluridisciplinaire Hubert Curien, Strasbourg, France. 4Unit
of Social Ecology, free University of Brussels, Brussels, Belgium.
Authors’ contributions
CS performed the experiments. CS analyzed the data. CS wrote the paper.
Received: 23 June 2011 Accepted: 20 October 2011
Published: 20 October 2011
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doi:10.1186/1472-6785-11-26
Cite this article as: Sueur: Group decision-making in chacma baboons:
leadership, order and communication during movement. BMC Ecology
2011 11:26.
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