Reaching a Consensus: Terminology and Concepts Used in Coordination and Decision-Making Research.

Reaching a Consensus: Terminology
and Concepts Used in Coordination
and Decision-Making Research
Lennart W. Pyritz & Andrew J. King &
Cédric Sueur & Claudia Fichtel
Received: 17 January 2011 /Accepted: 15 March 2011
# The Author(s) 2011. This article is published with open access at Springerlink.com
Abstract Research on coordination and decision-making in humans and nonhuman
primates has increased considerably throughout the last decade. However,
terminology has been used inconsistently, hampering the broader integration of
results from different studies. In this short article, we provide a glossary containing
the central terms of coordination and decision-making research. The glossary is
based on previous definitions that have been critically revised and annotated by the
participants of the symposium “Where next? Coordination and decision-making in
primate groups” at the XXIIIth Congress of the International Primatological Society
(IPS) in Kyoto, Japan. We discuss a number of conceptual and methodological
issues and highlight consequences for their implementation. In summary, we
recommend that future studies on coordination and decision-making in animal
groups do not use the terms “combined decision” and “democratic/despotic
decision-making.” This will avoid ambiguity as well as anthropocentric
connotations. Further, we demonstrate the importance of 1) taxon-specific
definitions of coordination parameters (initiation, leadership, followership,
termination), 2) differentiation between coordination research on individual-
level process and group-level outcome, 3) analyses of collective action processes
including initiation and termination, and 4) operationalization of successful group
Int J Primatol
DOI 10.1007/s10764-011-9524-9
L. W. Pyritz (*) : C. Fichtel
Behavioral Ecology & Sociobiology Unit, German Primate Center; and CRC Evolution of Social
Behaviour, University of Göttingen, 37077 Göttingen, Germany
e-mail: LennartPyritz@gmx.net
A. J. King
Structure & Motion Laboratory, Royal Veterinary College, University of London, Hertfordshire AL9
7DY, UK
C. Sueur
Primate Research Institute, Kyoto University, Inuyama 484–8506 Aichi, Japan
C. Sueur
Unit of Social Ecology, Free University of Brussels, 1050 Brussels, Belgium

movements in the field to collect meaningful and comparable data across
different species.
Keywords Animal groups . Coordination . Decision-making . Terminology
Introduction
The number of studies on group coordination, leadership, and decision-making in
humans and nonhuman primates has increased considerably during the last decade
(Conradt and Roper 2005; Conradt and List 2009; Fichtel et al. 2011; King et al.
2009; Petit and Bon 2010). However, integration of results from different studies has
been impeded by ambiguous terminology and conceptual/methodological short-
comings (see Jacobs 2010; Petit and Bon 2010; Pyritz et al. 2010 for recent
discussions). We discussed these inconsistencies with participants during the
symposium “Where next? Coordination and decision-making in primate groups” at
the XXIIIth Congress of the International Primatological Society (IPS) at Kyoto,
Japan, in September 2010. As a result of the strong positive feedback, we decided to
initiate a public discussion about these terms via an interactive online platform
joined by participants of the symposium and associated researchers. The glossary
contained 20 central terms regarding coordination and decision-making research,
most of which were previously compiled in a comprehensive review by Conradt
and Roper (2005). Each participant had the opportunity to contribute additional
terms, definitions, references, and commentaries. Table I provides the final
glossary based on the summary of all entries. In addition, we identified a number
of conceptual and methodological issues that have been discussed rarely or
controversially in the literature so far. We here discuss the most debated terms of
the glossary and a number of crucial conceptual issues in more detail.
Terminology
Combined Decisions
Conradt and Roper (2005) defined a “combined decision” as the sum of individual
group members` decisions that —unintentionally— affects the group as a whole
(Table I). However, this definition is ambiguous and cannot be separated clearly
from the often used and well established term “quorum decisions,” in which group
members also choose individually between different options until a certain threshold
is reached that affects the entire group (Seeley et al. 2006; Ward et al. 2008; Table I).
In fact, the mechanisms of reaching a group decision employed in a large colony of
ants (Leptothorax albipennis) choosing between several new nest sites (Franks et al.
2002) are principally the same as those employed by Tonkean macaques (Macaca
tonkeana) in a predeparture quorum (Sueur et al. 2010) or meerkats (Suricata
suricatta) increasing travel speed via a vocal voting mechanism (Bousquet et al.
2010). In these species, each group member decides individually where and when to
move, even if this decision is influenced by the behaviors of conspecifics. Further,
L.W. Pyritz et al.

Table I Central terms of coordination and decision-making research and their respective definitions,
selected references, and commentaries compiled by participants of the XXIIIth Congress of the
International Primatological Society (IPS) in Kyoto, Japan
Term Definition References (selection) Comments
Combined
decision
Members of a group choose
individually (but not
necessarily independently)
between ≥2 actions. They
do not aim for consensus,
but the combined result of
their decisions usually
affects the group as a whole.
Conradt and Roper 2005 The distinction between a
quorum threshold, e.g., in
ants or Tonkean macaques,
and a combined decision
seems inconclusive. See
text for detailed discussion.
Consensus
decision
Group members choose
between ≥2 mutually
exclusive actions to
reach a consensus on
the group level.
Conradt and Roper 2005;
King et al. 2008; Sueur
and Petit 2008a,b
Consistent
leadership
The same individual always
leads group actions.
Conradt and Roper 2005 Opposite of distributed/
variable leadership.
Democratic and
despotic
A consensus may be reached
by the averaging of
preferences (democracy),
or by following the choices
of specific leaders
(despotism).
Democratic=shared decision-
making; despotic=unshared
decision-making.
King et al. 2008; King
and Cowlishaw 2009
In primatology, we usually
use “despotic” and
“egalitarian” (or “tolerant”)
to characterize the social
system of a species and
“shared” or “unshared” to
describe consensus
decisions. See text for
detailed discussion.
Follower An individual that follows/
joins the initiator/leader
for a certain activity.
Jacobs et al. 2008; Pyritz
et al. 2010; Ramseyer
et al. 2009a, b, c; Sueur
and Petit 2008a,b
The definition of a follower
(of a group movement)
should be operational and
taxon-specific. See text for
discussion.
Hidden
leadership
The same individual initiates
and terminates a group
activity (movement),
although
it is not guiding the
movement in front of
the group.
Kummer 1968;
Pyritz et al. 2010
The new definition of a
leader (see below) includes
hidden leadership. Therefore,
we no longer need this term.
Initiator The group member that
initiates a group activity.
Bourjade and Sueur 2010;
Jacobs et al. 2008; Pyritz
et al. 2010; Ramseyer et al.
2009a, b, c; Sueur and
Petit 2008a,b;
The definition of an initiator
(of a group movement)
should be operational and
taxon-specific. See text for
discussion.
Leader Individual eliciting follower
behavior/exerting social
influence on others, by its
rank into the progression,
its behavior, or its social
status.
Harcourt et al. 2009;
King 2010; Petit and
Bon 2010; Pillot et al.
2010; Sueur and Petit
2008a,b, 2010
The definition of a leader
should not be restricted
by the spatial position
during a group movement
(Kummer 1968) but
comprise the whole process
of a collective action. It is
also a defining characteristic
of a leader that he exerts
social influence on
conspecifics. See text for
detailed discussion.
Mimetism The probability that an
individual performs a
behavior depends on
Camazine et al. 2001;
Deneubourg and Goss
1989; Meunier et al. 2006;
Terminology in Decision-Making Research

Table I (continued)
Term Definition References (selection) Comments
the number of individuals
already performing this
behavior (anonymous
mimetism, allelomimetism).
It can also depend on the
social relationships the
individual has with group
members already displaying
the behavior (selective
mimetism).
Petit et al. 2009;
Sueur et al. 2009;
Sumpter 2006
Overtaking Followers overtake the
individual
at the forefront of the group
without diverging >45°
from the initial trajectory.
Boinski 1991; Erhart
and Overdorff 1999;
Pyritz et al. 2010;
Trillmich et al. 2004
Predeparture
behavior
Behavior performed before
the departure of the initiator,
making the timing of the
departure predictable and
potentially indicating the
direction in which
individuals want to move.
Bourjade and Sueur
2010; Sueur and
Petit 2008a,b
Other names: preliminary
behavior, notifying
behavior, voting
behavior, intention
movement, priming
behavior.
Predeparture
period
Period preceding the departure
of the initiator and delineated
by the presence of
predeparture behaviors.
Bourjade and Sueur
2010; Sueur and
Petit 2008a,b
Other names:
preliminary period.
Quorum Minimum number, i.e.,
threshold, of group members
that need to take or favor a
particular action for the
whole group to adopt
this action.
Bousquet et al. 2010;
Franks et al. 2002;
Seeley et al. 2006;
Sueur et al. 2010;
Ward et al. 2008;
In principle, the quorum
could be a majority,
submajority (less than a
majority), or supermajority
(more than a majority) of
members. In practice,
animals are likely to
determine whether a
quorum has been reached
by estimating the relative
numerousness of members
contributing to the quorum,
often by relying on
indirect cues.
Recruitment
behavior
Behavior that increases the
probability that other
group members will join a
certain activity. It results
in a larger number of joiners
or in quicker joining of the
collective action than
when not performed.
Bourjade and
Sueur 2010
Self-organizing
system
Individual group members
follow local behavioral
rules, resulting in
organized behavior by the
whole group without the
need for global control.
Camazine et al. 2001;
Conradt and Roper 2005;
Couzin et al. 2002;
Sueur et al. 2009
Emergent properties due to the
interactions between
individuals in self-organized
systems are more complex
than the emergent properties
that should be observed by
the sum of individual
behaviors.
Shared consensus
decision
All members contribute equally
(and independently of
individual identity) to the
Conradt and Roper
2005; Sueur and
Petit 2008a,b
Opposite of unshared
consensus decision.
L.W. Pyritz et al.

there is no easy way to test whether individuals aim to reach a group consensus,
which was the criterion Conradt and Roper (2005) used to distinguish consensus
decisions from combined decisions. We therefore recommend not using the term
“combined decision” in future research.
Democratic and Despotic Decisions
Researchers have used the terms “democratic” and “despotic” interchangeably with
“shared” and “unshared” in a number of recent studies on decision-making (Conradt
and Roper 2003; King et al. 2008; King and Cowlishaw 2009). “Democracy”
describes a consensus reached by the majority principle, whereas “despotism”
characterizes a consensus reached by following the choices of specific leaders in
these studies. However, in primatology the term “despotic” is also used to
characterize the social structure of a species, opposed to “egalitarian” or “tolerant”
societies (Matsumura 1999), and the social structure of a species undoubtedly
influences the process and outcome of consensus decision-making. For instance,
Sueur and colleagues studied the decisions of 2 macaque species, with
contrasting social structures, beginning a group movement after a resting period
(Sueur and Petit 2008a,b). They found that egalitarian Tonkean macaques
displayed an equally shared consensus whereas more despotic rhesus macaques
(Macaca mulatta) displayed a partially shared consensus. These differences were
attributable to the fact that in the despotic rhesus macaques, dominant individuals
Table I (continued)
Term Definition References (selection) Comments
decision outcome. The
consensus is usually
determined by a quorum
or by averaging over
all votes.
Terminator Individual that stops and
seems to suggest the
termination of a group
movement.
Pyritz et al. 2010 Group movements can
feasibly comprise 2 linked
decisions: 1) initiation:
when and in which
direction to move and 2)
termination: when and
where to stop See text
for detailed discussion.
Unshared
consensus
decision
One particular group member,
e.g., the dominant, makes
the decision on behalf of all
group members. All other
members abide by this
decision.
Conradt and Roper 2005;
Sueur and Petit 2008a,b
Opposite of shared
consensus decision.
Variable/
distributed
leadership
Different group members
lead group actions on
different occasions.
Conradt and Roper 2005;
Jacobs et al. 2008;
Petit and Bon 2010
Opposite of consistent
leadership.
Voting An individual communicates
its individual preference
with regard to the
decision outcome.
Bousquet et al. 2010;
Prins 1996; Sellers
et al. 2007; Sueur
et al. 2010
Terminology in Decision-Making Research

had a disproportionate influence on the movement decisions of group-mates: they
were followed more often than the lower-ranked individuals. In contrast, all
individuals in the more egalitarian Tonkean macaques had a similar influence on
one another’s decisions to move and the decision was equally shared. However, the
social structure of a primate group can more broadly be viewed as both the cause
and consequence, reinforced via a feedback loop, of social interactions such as
intensity of aggression, grooming, or reconciliatory patterns. These social
interactions may be independent of group level decision-making processes.
Thus, so as to not confound characteristics of the social system and collective
decision-making in a group or causal relationships between these 2 realms, we
suggest that future decision-making research should use only the terms “shared”
and “unshared,” and not “democratic” and “despotic.”
Leadership
The definition of a leader should not be restricted to its spatial position during a
group movement because individuals may also lead from behind, i.e., initiate and
terminate a movement without being at the forefront of the group (Kummer 1968;
called “hidden leadership” in Pyritz et al. 2010). Instead, a leader should be defined
as an individual that elicits follower behavior (from a majority of or all group
members) and exerts social influence on group members either by its rank,
experience, social status/connectedness, or specific behavior (King 2010; Petit and
Bon 2010; Sueur and Petit 2008b). Further, an individual that leads may not do so
intentionally, i.e., leadership can be a passive process (Fischer and Zinner 2011;
King et al. 2009; King and Sueur 2011). For instance, in sheep (Ovis aries),
individuals triggered follower behavior by merely moving away from their group
following a sound they had been trained to, thus eliciting group movements as
incidental leaders (Pillot et al. 2010).
Concepts and Methods
Taxon-Specific Definitions
It remains a major practical challenge for human observers to identify reliably and
define operationally a group movement in the field. The following is the most
detailed definition employed in recent studies on the coordination of group
movements in different species: An individual moves a certain distance toward the
edge of the group either in a defined time period, e.g., 10 m within 40 s (Leca et al.
2003; Stueckle and Zinner 2008; Sueur and Petit 2008a,b), or without stopping and
feeding (Bourjade et al. 2009; Ramseyer et al. 2009a, b, c), and is followed by a
certain number of group members. However, even in this fairly detailed definition,
most parameters were not assessed empirically. In fact, a definition of group
movements must account for a number of taxon-specific characteristics such as
different traveling types (directed movements vs. “feed-as-you-go,” ameboid-like
movements that do not necessarily require an initiator or coordination among group
members, e.g., bonobos [Pan paniscus]: Wrangham 2000); mean travel distances;
L.W. Pyritz et al.

ecological conditions (resource abundance, predation risk); as well as the size,
composition, and cohesion of the group. Therefore, we encourage the use of
operational group movement definitions for different taxa that are built upon
empirical data collected before the study period used for the analyses of group
movements. In the long term, these empirical details will generate a comprehensive
database for multiple species and different contexts. Researchers will then be able to
compare and contrast the results to see if general patterns are shared among different
species. For practical details of how to operationalize definitions see Trillmich et al.
(2004) and Pyritz et al. (2010).
Individual-Level Process and Group-Level Outcome
Studies of group coordination and decision-making examine 2 different levels
that should be clearly distinguished. First, studies can focus on the group level,
i.e., the question of whether there is consistent or variable leadership among
several group movements or other collective actions (Erhart and Overdorff
1999; Jacobs et al. 2008; King et al. 2008; Pyritz et al. 2011). Second, studies can
focus on the level of single movements and study the process, i.e., the question of
whether decisions are shared or unshared and if they are mediated by mimetism,
affiliative/genetic network relations, quorum thresholds, or self-organized process-
es (King and Sueur 2011; Petit and Bon 2010; Sueur and Petit 2008a,b, 2010;
Sueur et al. 2009). It is important to separate these levels and clearly highlight the
level at which the research is focused. King and Sueur (2011) and Sueur and
Deneubourg (2011) explain how a consistent leadership or a consistent order of
individuals could be observed even though the decision process is shared. For
example, imagine n % of group members have to follow the first-moving
individual, i.e., initiator, within n minutes for a group movement to occur;
otherwise the initiator stops moving and the whole group remains stationary. A
study observing movement patterns during departure would conclude that the
process is shared because multiple individuals contribute to the decision of the
group to move. Researchers who observed only the order of departure when the
group chose to move would observe group movements that were always led by the
same individuals, and the decision appears unshared. Because the outcome at the
group level, i.e., departure order, is easier to observe, it is no surprise that most
early studies in the area of coordination focused on who leads (King 2010).
However, identifying who leads may not tell the whole story. For instance, an
individual can always, or in most cases, be the leader of a group because his
energetic reserves are always the first to be depleted, and he has to move to
feed, but might have the same, or at least no stronger, influence on the joining
process as other group members, e.g., in the case of an anonymous mimetism.
In fact, a number of recent findings, e.g., macaques (Sueur and Petit 2008a,b,
2010; Sueur et al. 2009), and horses (Bourjade et al. 2009) suggest shared
consensus between group members at the movement level, even though a single
leader or a few leaders, i.e., an unshared consensus with consistent leadership, has
previously been reported, e.g., macaques (Reinhardt et al. 1987) and horses (Feist
and Mc Cullough 1976). For a further discussion of this topic see Bourjade and
Sueur (2010).
Terminology in Decision-Making Research

Initiation and Termination of Collective Actions
During the course of a group movement, animals have to make 2 basic decisions, 1)
initiation: when and in which direction to move and 2) termination: when and where
to stop. Theoretically, the decision where to stop may be a second consensus
decision independent from the decision during initiation, and the 2 decisions can be
initiated by the same or different leaders (Boinski 1991; Erhart and Overdorff 1999;
Pyritz et al. 2010). Although this issue has been addressed rarely, it yields important
implications for the decision type, i.e., there could be unshared decision-making at
departure that is mitigated by individuals overtaking the initial leader and
terminating the movement, resulting in a shared decision-making with regard to
the whole movement process. Accordingly, we suggest making a distinction between
these 2 events and exploring the consistency of leadership during travel. For
instance, one might classify leadership as “stable,” i.e., the initiator leads the
movement until termination, or “unstable,” i.e., the terminator is different from the
initiator (Pyritz et al. 2011). In general, it is important to study the entire process of
group movements from initiation to termination to gain a realistic image of
coordination, leadership, and decision-making in a given species (Fichtel et al. 2011;
Pyritz et al. 2010; Trillmich et al. 2004). Indeed, such detail will be crucial if we are
to determine the temporal scale over which decisions are made, and outcomes occur.
Take an example where the same individual, X, both initiates and terminates the
movement. This could imply that 1) X made the decision to move and the decision
to stop, which was followed by group-mates, or 2) X made the decision to move in a
particular direction of travel (Noser and Byrne 2007; Sueur et al. 2010) in a
“goal-directed fashion” toward a particular destination, and was first to arrive, but
did not make the decision to terminate the movement, because all group-mates that
followed were aware of the target destination at the initiation (Pyritz et al. in
prep.). In other words, the decision of where to terminate had been made at the
initiation.
Successful Group Movements
Early studies set a threshold of 50% of group members having to follow an initiator,
within a certain time frame, to consider a movement as successful (Erhart and
Overdorff 1999). More recent studies considered an initiation attempt as failed only
if no individual followed (Jacobs et al. 2008; Sueur and Petit 2008a; Trillmich et al.
2004). However, the mean number of followers that determines a successful group
movement is presumably taxon-specific and may change with ecological season or
resource abundance (King et al. 2008; Pyritz et al. 2011). Further, fission into
subgroups —and an accordingly lower number of followers for single initiators—
may represent the most beneficial group decision under certain conditions (Jacobs
2010; Kerth 2010). Hence, it is difficult to provide a general definition for successful
or unsuccessful movements. We suggest giving a taxon- (and season-) specific
threshold of success regarding followers below which the initiator does not stop and
show recruitment behavior (Table I), or “give-up” on the movement entirely, e.g., 5
followers in chacma baboons (Papio hamadryas: Stueckle and Zinner 2008); 3
followers in Verreaux’s sifakas (Propithecus verreauxis: Trillmich et al. 2004),
L.W. Pyritz et al.

white-faced capuchins (Cebus capucinus: Petit et al. 2009), meerkats (Suricata
suricatta: Bousquet et al. 2010), and Tonkean macaques (Sueur and Petit 2010).
Combining this with operational group movement definitions for different taxa, it
may be possible to uncover a common threshold —a universal— to primate group
movements. In fact, examination of the aforementioned studies indicates that a
threshold of approximately 3 followers seems to be sufficient to elicit a group
movement, whatever the species. This number may provide sufficient protection
against predators or enough collective knowledge to orientate within the home range
and detect resources, for example.
Conclusion
After a public discussion about terms used in coordination and decision-making
research via an interactive online platform, we have come to a number of
conclusions that will aid future research on this topic. First, we highlighted a
number of ambiguous terms (“combined decision”; “despotic/democratic decision”)
and suggested that these not be used in future coordination research. We also suggest
maintaining the terms “leader” and “leadership” because although they can
sometimes be ambiguous, as long as an operational definition is provided these
terms are useful to broader scientific communication and integration (King 2010;
Pyritz et al. 2010). Further, we discussed a number of conceptual issues: group vs.
movement level; operationalized successful group movements; collective action
process including initiation and termination. Ultimately, the implementation of our
suggestions should provide comparable data on coordination and decision-making in
different species and facilitate the identification of general patterns that are shared
among different taxa.
Acknowledgments We thank the participants of the symposium “Where next? Coordination and
decision making in primate groups” at the XXIIIth Congress of the International Primatological Society
(IPS) for valuable discussions and comments on the glossary and conceptual issues. Henning Lahmann
kindly implemented the online glossary. We also thank Joanna Setchell and the International Journal of
Primatology for providing financial support and making this special issue possible. L. W. Pyritz is funded
by the German Research Foundation (DFG: KA 1082/16-1), A. J. King by a NERC Postdoctoral
Fellowship (NE/H016600/2), and C. Sueur by the Japan Society for the Promotion of Science. Joanna
Setchell and 2 anonymous reviewers provided valuable comments on the manuscript.
Open Access This article is distributed under the terms of the Creative Commons Attribution
Noncommercial License which permits any noncommercial use, distribution, and reproduction in any
medium, provided the original author(s) and source are credited.
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Terminology in Decision-Making Research