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Leadership in Mammalian Societies: Emergence, Distribution, Power, and Payoff

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Leadership is an active area of research in both the biological and social sciences. This review provides a transdisciplinary synthesis of biological and social-science views of leadership from an evolutionary perspective, and examines patterns of leadership in a set of small-scale human and non-human mammalian societies. We review empirical and theoretical work on leadership in four domains: movement, food acquisition, within-group conflict mediation, and between-group interactions. We categorize patterns of variation in leadership in five dimensions: distribution (across individuals), emergence (achieved versus inherited), power, relative payoff to leadership, and generality (across domains). We find that human leadership exhibits commonalities with and differences from the broader mammalian pattern, raising interesting theoretical and empirical issues.
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Review
Leadership in Mammalian
Societies: Emergence,
Distribution, Power, and Payoff
Jennifer E. Smith,
1,
* Sergey Gavrilets,
2
Monique Borgerhoff Mulder,
3
Paul L. Hooper,
4
Claire El Mouden,
5
Daniel Nettle,
6
Christoph Hauert,
7
Kim Hill,
8
Susan Perry,
9
Anne E. Pusey,
10
Mark van Vugt,
11,12
and Eric Alden Smith
13,
*
Leadership is an active area of research in both the biological and social
sciences. This review provides a transdisciplinary synthesis of biological and
social-science views of leadership from an evolutionary perspective, and exam-
ines patterns of leadership in a set of small-scale human and non-human
mammalian societies. We review empirical and theoretical work on leadership
in four domains: movement, food acquisition, within-group conict mediation,
and between-group interactions. We categorize patterns of variation in leader-
ship in ve dimensions: distribution (across individuals), emergence (achieved
versus inherited), power, relative payoff to leadership, and generality (across
domains). We nd that human leadership exhibits commonalities with and
differences from the broader mammalian pattern, raising interesting theoretical
and empirical issues.
Evolution of Leadership Across Mammalian Societies
Leadership (see Glossary) is a pervasive phenomenon in social species, organizing behaviors
ranging from group movement to complex patterns of cooperation and conict. Despite being
an active topic in biological and social sciences, little in the way of theoretical and empirical
synthesis exists. Theory and data indicate that leadership is relevant in a variety of contexts of
current biological interest, including reproductive skew, the evolution of cooperation, intergroup
conict, and evolved personality differences [14]. Biologists have produced a substantial
theoretical and empirical literature on hierarchy and decision-making in non-human societies,
but focus primarily on leadership during movement [59]. By contrast, social scientists have
written extensively about leadership in a wide variety of contexts [10,11], but often lack
evolutionary perspectives and formal models (but see [1215]).
This review offers a transdisciplinary synthesis of biological and social-science models of
leadership from an evolutionary perspective, and analyzes leadership patterns across a sample
of mammalian species. We review evolutionary models of leadership in four domains: group
movement, food acquisition, conict mediation, and between-group interaction (Box 1). We
draw on theoretical work to analyze how leadership varies across these domains for a sample of
human and non-human groups. With evolutionary questions in mind, we classify this variation
into ve dimensions that can be identied across species: distribution across individuals
(skew), leadership emergence, differential power, relative payoff (to leaders versus followers),
Trends
Leadership is an active research area in
both biological and social sciences, but
there has been limited synthesis within
or across these areas; evolutionary the-
ory can assist with such synthesis, but
additional elements are needed for a
robust comparative framework.
Variation in leadership can be mea-
sured in multiple dimensions, including
emergence (how does one become
a leader?), distribution (how widely
shared is leadership?), power (how
much power do leaders wield over fol-
lowers?), relative benet (do leaders
gain more or less than followers?),
and generality (how likely are leaders
in one domain, such as movement or
conict resolution, to lead in other
domains?).
A comparative framework based on
these dimensions can reveal common-
alities and differences among leaders in
mammalian societies, including human
societies.
1
Department of Biology, Mills College,
Oakland, CA 94613, USA
2
Department of Ecology and
Evolutionary Biology, and Department
of Mathematics, University of
Tennessee, and National Institute for
Mathematical and Biological
Synthesis, Knoxville, TN 37996, USA
3
Department of Anthropology, and
Center for Population Biology,
University of California at Davis, Davis,
TREE 2008 No. of Pages 13
Trends in Ecology & Evolution, Month Year, Vol. xx, No. yy http://dx.doi.org/10.1016/j.tree.2015.09.013 1
© 2015 Elsevier Ltd. All rights reserved.
TREE 2008 No. of Pages 13
and generality across domains (Box 1). We advance the study of leadership by synthesizing[30_TD$DIFF] its
theoretical predictions regarding emergence and maintenance, and establish measurable
criteria for comparative study to stimulate empirical and theoretical progress. The hypothesis
that humans have evolved unique leadership attributes not found among other mammals
(e.g., [3,15]) remains largely untested because we currently lack suitable comparative frame-
works for examining leadership across mammalian societies. One aim of this review is to apply
such a framework in a preliminary examination of this hypothesis.
Denitions
The study of collective decision-making suffers from lack of agreement on terminology [16].
Biologists often dene leadership as differential inuence on group activity, however generated.
The usual domain of interest is movement [7], with some additional focus on the duration of
activities [6]. By contrast, few social science uses of this term highlight being the rst to initiate
movement or change activity because this might reect impatience or need rather than
differential inuence [17]. Instead, they commonly point to differences in social power, meaning
the ability of an individual to induce others to act or believe in a particular way [18].Wedene
leadership here as non-random differential inuence on collective behavior. This differential
inuence might vary based on visibility, knowledge, or other factors affecting voluntary decisions
to follow or emulate. Alternatively, it might depend on coercion to control the behavior of
subordinates, for which the term dominanceis often employed. This variation in causal
mechanisms is one issue we address below.
Models of Leadership
Members of social species regularly make decisions to coordinate activities with group-mates,
including where and when to forage, rest, or move. Formal evolutionary models help reveal the
evolutionary causes and consequences of these decisions. Theory predicts that, if accurate
information is to be maximized and the cost of differences in individual preferences is to be
CA 95616, USA
4
Department of Anthropology, Emory
University, Atlanta, GA 30322, USA
5
Department of Zoology, University of
Oxford, Oxford OX1 3PS, UK
6
Centre for Behavior and Evolution,
and Institute of Neuroscience,
Newcastle University, Newcastle NE2
4HH, UK
7
Departments of [28_TD$DIFF]Mathematics and
[29_TD$DIFF]Zoology, University of British
Columbia, Vancouver, BC V6T 1Z4,
Canada
8
Institute for Human Origins, and
School of Human Evolution and Social
Change, Arizona State University,
Tempe, AZ 85281, USA
9
Department of Anthropology,
University of California Los Angeles,
Los Angeles, CA 90095, USA
10
Department of Evolutionary
Anthropology, Duke University,
Durham, NC 27708, USA
11
Department of Experimental and
Applied Psychology, VU University
Amsterdam, the Netherlands
12
Institute for Cognitive and
Evolutionary Anthropology, University
of Oxford, UK
13
Department of Anthropology,
University of Washington, Seattle, WA
98195-3100, USA
*Correspondence: jesmith@mills.edu
(J.E. Smith) and easmith@uw.edu
(E.A. Smith).
Box 1. Leadership Domains and Dimensions
Leadership and alternative forms of decision-making (e.g., via quorum-sensing) can occur in one or more domains or
contexts within social groups. As noted in the main text, collective movement is the domain most com monly treated in the
biological literature (both theoretical and empirical), but social scientists as well as some biologists are concerned with
leadership phenomena in other domains that have signicant tness implications. For our comparative analysis of
mammalian (including human) leadership, we chose to focus on four domains: (i) collective movement, (ii) food
acquisition, (iii) within-group conict resolution, and (iv) between-group interactions (whether peaceful or hostile). The
text discusses examples of leadership in each of these domains for a variety of social groups, as well as related theoretical
models concerning leadership evolution.
Leadership dimensions involve a more abstract categorization. This can be approa ched in the form of a set of questions.
For any given domain, are there leaders? In a purely egalitarian or democratic decision regime, involving either consensus
decision-making or absence of collective behavior, the answer will be no. We recognize that this pureform of
consensus may be rare given the pervasiveness of often subtle forms of leadership and inuence. If the answer is yesfor
any given domain, the following questions can be used to characterize different dimensions along which leadership
varies:
(i) How distributed is leadership? In other words, what proportion of group membership typically makes a decision in a
given domain?
(ii) Emergence. Is leadership achieved (by leader's actions or qualities) or ascribed (by inheritance or some other
process not dependent on the individual's actions or qualities)?
(iii) How powerful is the leader? Our denition of power [23_TD$DIFF](see Glossary[24_TD$DIFF])[25_TD$DIFF]does not necessarily [26_TD$DIFF]entail coercion.
(iv) Relative benet. Do leaders reap tness-correlated benets from their actions equal to, greater than, or less than,
that averaged by other group members?
(v) Generality. To what extent does leadership in one domain predict leadership in other domains?
As discussed further in the text, evolutionary theory as well as empirical evidence suggests that these dimensions are not
independent of each other; for example, greater power should be (and often is) associated with higher relative payoff to
leadership.
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TREE 2008 No. of Pages 13
minimized, [31_TD$DIFF]fully shared decision-making[5_TD$DIFF] should emerge[6_TD$DIFF] (i.e., no consistent leaders) [19,20].
This raises the puzzle of why leaders often do emerge. However, if time is a crucial factor,
consensus decisions can be inefcient, thereby increasing costs for group members. For
example, in the presence of signicant within-group heterogeneity (dened as variation in
preferences, personality traits, and reputation), group decisions by a single leader can be most
efcient [21]. Moreover, recent models suggest that leadership can also emerge under the
inuence of differential information, differential relatedness to group-mates, differential power or
resource control, personality variation, imitation, punishment, and volunteerism, with varying
effects on tness.
Our ultimate goal is to [32_TD$DIFF]derive [33_TD$DIFF]general [34_TD$DIFF]principles [35_TD$DIFF]from [36_TD$DIFF]models [37_TD$DIFF]of leadership[8_TD$DIFF].Whereasthescope
of inference from any single mathematical model is limited, comparison of multiple models
allows for true insights about biological principles. Although some models span multiple
domains, each model depends upon its specic assumptions, which are often domain-
specic. Therefore we rst organize our review of models according to the four domains noted
above before concluding with the overarching conceptual insights to be gained from existing
theoretical models.
Movement
Couzin et al. [7] studied group movement when only a few members have information on the
location of a resource. If members lack a preferred travel direction, and all individuals attempt to
avoid collision while simultaneously maintaining group cohesion, a small proportion (e.g., 5%) of
informed individuals can efciently lead others towards the resource. In a related model [22],
individuals can invest in obtaining personal or social information about the resource location.
With tness tradeoffs, the population evolves to be dimorphic, with a small proportion of
individuals (leaders) actively acquiring information from the environment but ignoring social
information from their group-mates, and the remainder simply follow the leaders. These dynam-
ics are similar to those in other producerscrounger models [23,24]. Stable differences in
propensities to lead dispersal attempts arise when dispersal decreases competition between
kin; the net tness benets of leadership depend upon the costs of dispersal as well as on the
degrees of relatedness and reproductive skew within dispersal groups [25].
Johnstone and Manica [4] modeled an N-player coordination game with conicts of interest, and
analyzed the emergence and maintenance of two personality types: (stubborn) leaders and
(agreeable) followers (see followership). Leaders are maintained by frequency-dependent
selection[38_TD$DIFF].[39_TD$DIFF]They [40_TD$DIFF]benet[41_TD$DIFF]from imposing their preferences on followers, but fail to coordinate
effectively when interacting with other leaders. The fraction of leaders in the population increases
with the degree of conict among group members, with both types being common only at
intermediate levels of conict.
Resource Acquisition
There are several ways leaders improve the efciency of resource acquisition, thereby increasing
the tness of group members.
Leaders as Role Models
In mathematical models of social interactions with a possibility of imitation, leaders (i.e.,
individuals whose strategies are imitated by the rest of the group) emerge naturally. For example,
in a version of the minority game [26,27], individuals chose between two different resources of
equal total value, but being in the minority provides an advantage (e.g., because the resource is
shared among a smaller number of group-mates). Individuals remember group choices over
several rounds and base their decisions on past experiences (individual learning) or choices of
others (social learning). Rapid emergence of inuence-networks can occur when followers copy
Glossary
General Terms
Coercion: control of the behavior of
others through threats or attacks.
Collective action problem: any
situation in which multiple individuals
would all benet from a particular
action, but difculties of coordination
or of ensuring fair contribution to the
costs of the action create obstacles;
also known as a social dilemma.
Coordination problem: situations in
which individual success requires
collective action to achieve a goal.
These range from contexts of pure
coordination, where individuals have
the same preference or tness ranking
across outcomes, to cases where
individuals have different rankings
but still achieve higher payoffs by
coordinating on one choice.
Dominance: ability to win dyadic
agonistic interactions (e.g., contests
or unsolicited appeasements), with
outcomes determining priority of
access to resources or mating.
Egalitarian group: a society in
which the individuals of the same
age-sex category have equal access
to and control over resources and/or
other individuals within the group
(in contrast to a stratied/despotic
society).
Leadership: non-random differential
effect on group behavior of
conspecics through actions evolved
or intended to elicit this effect (a
glossary of leadership types is given
below)
Power: the ability of leaders to
motivate followers to behave in ways
they would otherwise not do, often
but not necessarily through coercion.
Prestige: inuence or deference that
is freely granted (i.e., not generated
by the use of threat or force, in
contrast to dominance).
Shared decision-making: multiple
group members decide on an
outcome (via consensus or quorum-
sensing), in contrast to unshared
decision-making.
Small-scale society: a human
society consisting of one or a few
local communities (several hundred to
a few thousand members in total);
these tend to be egalitarian, but with
some notable exceptions.
Stratied society: one in which
segments have differential access to
and/or control over resources; also
known as a despotic society, or a
society structured by a dominance
hierarchy.
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TREE 2008 No. of Pages 13
the choices of a small number of individuals with the best strategies (leaders) [27,28]. In a version
of the Prisoner Dilemma game with dynamical adjustment of social ties and social learning,
adaptation dynamics favor the emergence of highly connected cooperators with high payoffs
[29]. These leaderssustain the global cooperative state because their social partners imitate
their cooperative strategies. A model of coordination on networks showed that group-mates
often emulate individuals (leaders) with privileged information [30]. In a model of how leadership
inuences group success, players in a multi-player weak-link game become leaders after
publicly choosing to contribute before all other players [31,32]; thus, leadership can increase
efciency but not always guarantee success.
Leaders as Punishers
Punishment has attracted substantial attention as a way to stabilize the production of public
goods [33,34]. However, if punishment is costly, it raises a second-order social dilemma
because mild cooperators who shirk the costs of punishment will restore the original dilemma
to the detriment of all [35]. One solution is to compensate specialized actors (leaders) to administer
punishment [36].Buildingonpatronclient and managerial mutualism models [37], Hooper et al.
[13] showed that a combination of leaders and followers will outcompete acephalous strategies
if followers are taxed to pay leaders to efciently enforce collective action in circumstances
where blindly cooperative strategies, peer punishment, or other alternative mechanisms are
incapable of efciently ensuring cooperation. The amount extracted by leaders in the model
depends on the cost of the leadersefforts, and the efciency of the process of selecting leaders.
This model was extended to the specic public good of success in contests over territory [38]
as well as applied to agent-based simulations of pre-Hispanic Pueblo societies [39,40].
Leaders as Managers
A related model [42_TD$DIFF]assumed that[43_TD$DIFF] the presence of a leader increases a group's productivity in
collective action [41]. Leaders gain more of the reward than the remainder of the group, but they
are tolerated because of increased absolute productivity. Leaders are constrained by the tness
followers would gain from leaving the group; as a result, where leadership is efcient and costs
of dispersal are high, leaders are able to claim unequal shares of production. This class of models
[13,41] is analogous to those of reproductive skew [4244].
Leaders as Volunteers
In many public goods games in which the total group benet is a nonlinear increasing function of
the number of contributors, cooperation can sometimes be maintained without genetic relat-
edness or other forms of assortment [45]. For example, in an N-person volunteer's dilemma [46],
a public good is produced if there are at least kvolunteers (who can be viewed as group leaders);
frequency-dependent selection favors a stable mix of volunteers (leaders) and free-riders
(followers) [47]. If the benet of a collective action declines in time [48], altruistic leaders can
emerge in groups of non-relatives, and larger group sizes and higher costs of volunteering delay
but do not prevent their emergence. In contrast to producerscrounger models where the
producers and scroungers have equal tness at equilibrium, volunteers (that is, producers) have
lower tness than free-riding group-mates.
Within-Group Conict
There is a substantial body of theoretical work on within-group conicts in which dominant
individuals (leaders) participate in or are targets of coalitionary attacks [49,50]. Coalitions can
strongly affect the power of both dominant and subordinate individuals. Within-group competi-
tion and conict can reduce the absolute amount of resources available to the group. [44_TD$DIFF]In [45_TD$DIFF]this
case, individuals can benet from investing in a trait (policing) that simultaneously reduces
the relative amount of resource subject to within-group competition and the costs of this
competition [51]. If group members vary in relative cost of policing, stronger individuals (leaders)
Unshared decision-making: one or
a very few group members make
decisions for the group.
Types of Leadership
Acephalous group: leaderless
group; in the human case, one
without institutionalized leadership.
Achieved status: determined by an
individual's abilities, performance or
effort (in contrast to ascribed status).
Ascribed: assigned at birth or
assumed involuntarily later in life (e.g.,
through maternal rank inheritance
based on maternal interventions or
bequeathal of resources down a
lineage).
Attribute-based leadership:
determined by the specic traits
possessed by a leader (rank, age,
tenure, sex, physiological state) when
that individual assumes role of leader
or follower.
Delegated leadership: when a
leader has tacit or explicit group
consent to act without seeking
consensus on each decisions (thus
intermediate between pure
consensus-based decisions and
despotism).
Despotic leadership: consistent
leadership by a powerful individual or
small set of individuals; also termed
unsharedor personalleadership.
Distributed leadership: different
individuals lead group action on
different occasions, for a given
domain, due to uctuations in
motivation, knowledge, or a turn-
taking convention; also known as
shared leadership.
Domain-specic leadership: when
an individual's leadership is limited to
a specic domain (contrasting with
domain-general leadership, as in
generalized dominance).
Followership: leadership presumed
on the basis of others following (e.g.,
one individual initiates movement and
others follow).
Institutionalized leadership:
consists of durable positions
(regardless of length of tenure of
individuals occupying those
positions), in contrast to situational
leadership.
Situational leadership: leadership
that arises opportunistically and
occurs only in specic situations of
short duration; contrasting with
institutionalized leadership.
Volunteerism: the act of freely
proffering services to a group (e.g.,
agreeing to assume a costly
leadership role).
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TREE 2008 No. of Pages 13
are favored to take on most or all of the policing [52]. For example, game theoretic models used
to analyze eld data on East African pastoralists showed that wealthy households can afford to
enforce conservation of grazing areas, incurring the costs of policing other households from
overgrazing [53].
Between-Group Interactions
Because both conict and peaceful interactions between groups can produce public goods
(e.g., territorial gains), the models reviewed above concerning leaders in collective action can
apply in this context. Leaders can also be modeled as volunteers in between-group conicts.
One model [2] predicts that high-ranking individuals (leaders) who usurp a disproportionate
share of resources from group-mates will act seemingly altruistically in between-group conict,
expending more effort and often having lower reproductive success than low-ranking group-
mates. Similar behavior is expected for individuals with[46_TD$DIFF] the greatest motivation, strength, or
endowments as well as for those paying the lowest costs. Analysis of conict between
egalitarian groups showed that, if the collaborative abilities of individuals are relatively low,
then the population becomes dimorphic, with a small proportion of leaderscontributing to
public goods and the rest free-riding [21].
General Principles
Several general lessons emerge from our theoretical synthesis. First, leaders and followers can
emerge naturally as a result of heterogeneity in preferences, motivation, personality, physical
characteristics, information available,and other features affecting individual performance in tness-
related activities. Under some conditions, this heterogeneity is predicted to be favored by natural
selection. Several models predict relatively low frequencies of leaders, but the results depend on
modeling details. Some models also predict that leaderspreferences and choices will strongly
control those of followers, but theoretical work on this is very limited. To the extent that differentia-
tion of group members into leaders and followers, and subsequent division of labor between them,
is driven by natural selection, leadership tends to be benecial to both social groups and individual
members of these groups. Although sometimes followers can have lower tness than leaders, this
is most likely when the alternatives to accepting the follower's position (e.g., leaving the group
or challenging the leader) are more costly. In some situations, group differentiation into leaders
and followers is predicted to be maintained by negative frequency-dependent selection, such that
both types will have equal tness at equilibrium. Under other conditions, leaders behave as
volunteers and can have lower payoffs than their largely free-riding followers. In sum, there are
many ways in which leadership can evolve, some less intuitive than others.
Empirical Patterns in Leadership
To focus the empirical portion of our review, we compared leadership patterns across mam-
malian societies by examining leadership in a sample of non-human and human societies
(Table 1). We restrict our sample of non-human mammals (NHM) to species for which sufcient
data on social interactions of free-living individuals are available. For comparability, we included
only small-scale societies (SSS) of humans lacking complex political institutions (e.g., more
than two formal administrative levels) [11,54]. Our rationale for comparing human and non-
human leadership is to help to determine the extent to which a unied evolutionary analysis of
mammalian leadership can reveal and explain similarities as well as differences. Although some
may assume that leadership among humans is intrinsically unique, we have no a priori position
on this question, nor on whether we expect greater variation in leadership within or across these
groupings (NHM and SSS).
The domains and dimensions delineated in Box 1 provide a useful framework for describing and
comparing leadership patterns across societies. We coded each dimension for each domain in
our 16 societies according to a ve-point rating system (Table S1 in the [16_TD$DIFF]supplemental [17_TD$DIFF]information
Trends in Ecology & Evolution, Month Year, Vol. xx, No. yy 5
TREE 2008 No. of Pages 13
online), drawing on observational data collected by us or in the published literature (supple-
mented by consultation with the original eld researchers when needed). Each rating is akin to
an item on a Likert scale, a measurement tool utilized extensively by psychologists and
marketing researchers.
This method is clearly preferable to purely qualitative comparisons, but has signicant limitations.
One could raise epistemological concerns: do different raters have different implicit norms of
reference, for example about what might constitute weakversus stronglevels of power? We
attempted to minimize this problem by explicit denitions of the possible ratings for each
dimension (Table 2), scoring each of the examples in dialogue within our group, and consulting
with experts in the eld for those groups not studied directly by one of us. We view the empirical
portion of this review as a foundational contribution towards a unied approach to studying
leadership. Methodologically, it is a survey of current expert opinion on a sample of societies,
with the aim of stimulating further research.
Because leadership dimensions (other than generality) can vary independently across domains
even in the same society, and leadership can be absent in some domains for a given society, we
present the results by domain. For each dimension other than generality, we analyzed ratings to
determine whether these varied signicantly by leadership domain or society type (i.e., SSS
versus NHM) (Table S2).
Leadership Distribution
The distribution of decision-making within a group (i.e., the proportion of group membership that
makes decisions) is of particular interest to biological and social scientists. In the models
reviewed above, this can range from individual autonomy through consensus decision-making
to despotic control.
Table 1. Sixteen Societies Included in the Comparative Analysis
a
Society Units for Collective [11_TD$DIFF]Action ([12_TD$DIFF]Typical [13_TD$DIFF]Number of [14_TD$DIFF]Individuals)
[15_TD$DIFF]Non-humans
African elephant (Loxodonta africana) Multi-family bond group (2550), matrilineal family groups (1020)
Bottlenose dolphin (Tursiops sp.) Pod (1030), alliances (514), other subgroups
Chimpanzee (Pan troglodytes) Community (46), other subgroups
Lion (Panthera leo) Pride (418), other subgroups
Meerkat (Suricata suricatta) Extended family group (clans) (20)
Plains zebra (Equus quagga) Herd (40), harems (4 adults + offspring)
Spotted hyena (Crocuta crocuta) Clans (4590), other subgroups
White-faced capuchin (Cebus capucinus) Multi-male group (539)
Humans
Ache (Paraguay forest) Residential band (35)
Cheyenne (North American plains) Tribe (4000), other subgroups
Inuit (Canadian arctic) Residential band (60)
Kipsigis (Kenyan savannah) Community (200300)
Nootka (Canadian Pacic coast) Winter village (200)
Pimbwe (Tanzanian woodlands) Tribe (25 000), village (dozens to hundreds)
Shoshone (North American Great Basin) Camp (1050), larger temporary aggregations
Tsimane (Bolivian Amazon) Village (dozens to hundreds)
a
See the [16_TD$DIFF]supplemental [17_TD$DIFF]information online for references and details of the societies sampled.
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TREE 2008 No. of Pages 13
We found a similar overall pattern for SSS and NHM societies, with the majority of societies
featuring moderately shared leadership in most domains (Figure 1 and Table S2 for details).
Leadership was rated as signicantly more concentrated (less shared) in the domains of within-
group conict resolution and between-group interactions than in the other domains. There was
no signicant effect of society type (SSS vs NHM). However, leadership in domains other than
food acquisition is less-evenly distributed in NHM than SSS. Food acquisition and consumption
was done individually in most of the NHM societies, but was a cooperative matter within most of
the SSS societies [55,56].
Leadership Emergence
In some cases, leaders emerge through a process of competition or performance. In others, they
are born, not made by inheritance or some other process independent of their own actions or
qualities. Social scientists typically term this contrast achieved versus ascribed leadership
status, labels we adopt here. This ascribedachieved dimension is a continuum which for
convenience we divide into ve steps (Table 2). Even in the same society, leadership in one
domain can be primarily achieved even though in another domain it is ascribed. In common with
many other status differences, leadership is often correlated with age, as one builds up
experience, alliances, and/or competitive ability; we classify this as achieved status, unless
there is an invariant seniority rule (e.g., the oldest female always leads)[47_TD$DIFF]. Mathematical models
reviewed above, however, focus exclusively on achieved leadership status.
Table 2. Rating Codes for Leadership on Mammalian Societies
a
[10_TD$DIFF]
Dimension Rating [18_TD$DIFF]Codes and [19_TD$DIFF]Denitions
Distribution 1 = no leadership (all adults are autonomous in domain X)
2 = semi-autonomy: leadership in domain X is widely shared among a restricted age or sex
category (e.g., females usually lead, older adults usually lead)
3 = moderately unshared (roughly the midpoint between 1 and 5)
4 = very unshared (a small number of adults lead in this domain)
5 = highly unshared (one individual usually leads in this domain)
Emergence 1 = leadership fully achievement-bas ed (includes cases where adults are autonomous)
2 = primarily achievement-based (e.g., adults of one age or sex category usually lead, but
otherwise leadership is achievement-based)
3 = roughly equal mix of achieved and ascribed (e.g., most competent senior males lead)
4 = primarily ascribed (e.g., leadership inherited by a senior member of chief's family)
5 = fully ascribed (e.g., senior female always leads)
Power 1 = weak or non-existent (adult autonomy, highly democratic decision-making, etc.)
2 = delegated leadership (leaders coordinate or execute decisions most or all agree to)
3 = moderate power (roughly the midpoint between codes 1 and 5)
4 = leaders can coerce or persuade many but not all, or often but not consistently
5 = despotic (leaders consistently coerce or persuade others to follow)
Relative benet n.a. = not applicable because no leadership (e.g., full autonomy)
1 = leadership very costly (small payoff relative to that of followers)
2 = leadership somewhat costly, on average
3 = equality (payoff to leadership is roughly the same as the payoff to followers)
4 = leaders gain moderately relative to followers, on average
5 = relative payoff is heavily skewed to leaders
Generality 1 = leadership in one domain rarely predicts leadership in other domains
2 = some correlation[1_TD$DIFF] (e.g., [20_TD$DIFF]individuals [21_TD$DIFF]belonging to one age or sex category usually or always
lead in every domain[22_TD$DIFF], but the precise identity of leaders varies)
3 = moderate correlation (e.g., consistent across two domains)
4 = strong correlation (e.g., consistent across three domains)
5 = leaders are consistently the same across all domains, independently of distribution
(including both cases with consistent autonomy and ones with generalized dominance)
a
See Box 1 for explanations of each dimension.
Trends in Ecology & Evolution, Month Year, Vol. xx, No. yy 7
TREE 2008 No. of Pages 13
Our ratings of the 16 societies indicate that most exhibit achieved leadership in the majority of
domains (Figure 1). There are no systematic differences across domains in this dimension. We
found greater variability within categories (SSS, NHM) than between them, although NHM
societies are somewhat more achievement-based than SSS in domains other than movement.
Spotted hyenas and the Nootka (Northwest Coast Indians) are clear outliers with highly ascribed
(inherited) leadership in all domains.
Differential Power
We dene power as the ability to motivate others to behave in ways they would otherwise not via
mechanisms of coercion, persuasion, or prestige. Leaders in SSS are often particularly skilled at
using rhetoric to express common goals and to persuade and inspire followers [57]. Few
mathematical models explicitly allow for evolving power of leaders (see above), predicting that
leadersactions will have substantial effects on followers unless the latter form coalitions.
The ratings suggest that leaders are generally less powerful in SSS than NHM except in the food-
acquisition domain, where adult NHMs are often autonomous (Figure 1). Anthropological
research indicates that the smallest-scale societies of low-density foragers or horticulturalists
have less-powerful leaders than other human societies, a generalization borne out in our sample
(see ratings for Ache, Inuit, Shoshone, and Tsimane, Table S2). There are also signicant
differences across domains, with leaders being rated more powerful in within- and between-
group interactions, and less powerful for movement and food.
Relative Benet
Do leaders reap tness payoffs from their actions equal to, greater than, or less than those
averaged by other group members? Although intuition might suggest that leaders gain dispro-
portionate payoffs, several models predict otherwise. If payoffs are frequency-dependent (e.g.,
leadership traits are favored when rare, but deleterious when common), tnesses of leaders and
Movement
Within-group conflict resoluon
Distribuon
Distribuon
Emergence
Emergence
Power
Power
Relave benefit
Relave benefit
Rang
Dimension
6
5
4
3
2
1
0
6
5
Human
Key:
Non-human
4
3
2
1
0
Food
Between-group interacon
Figure 1. Ratings for Leadership Dimensions in Non-Human Mammals (white) and Small-Scale Human
Societies (shaded) in Each of the Four Leadership Domains. In each case, the central dark bar represents the
median and the box the inter-quartile range.
8Trends in Ecology & Evolution, Month Year, Vol. xx, No. yy
TREE 2008 No. of Pages 13
followers will be equal at equilibrium. Self-serving leaders can be thwarted by coalitions of the
less powerful, or (particularly in human societies) by rules that remove them from power; one
mechanism to ensure that this is through delegated leadership, by which leaders serve the
larger group and have only limited autonomy to pursue their own interests.
The evidence from our 16 societies shows greater similarity between NHM and SSS in this
dimension than in any other (Figure 1). Notably, across most of our cases, leaders gain tness-
related payoffs equal to or only modestly better than followers. A handful of cases exhibit
negative returns to leadership (i.e., apparent altruism or exploitation by followers), as docu-
mented among lions [58]. Neither domain nor society type has a signicant effect on ratings for
relative benet (Table S2).
Generality
Leadership is domain-specicin some groups, but fairly generalized in others [59].We
therefore asked if individuals who lead in one domain are likely to exercise leadership in other
domains. We expected societies with strong dominance hierarchies or charisma-based leader-
ship would exhibit higher generality than societies with widely distributed leadership. We only
located one modeling study allowing for the effects of leaders over multiple domains (within- and
between-group conicts [2]), and therefore no model-based generalizations are possible yet.
We found that leadership is somewhat more generalized in NHM societies than in SSS; this was
true for both median ratings (median = 4 for NHM, versus 3 for SSS) as well as the range across
societies (three NHM but no SSS societies were rated 5, whereas three SSS but no NHM
societies rated 2; Table S2). As expected, societies with marked dominance hierarchies (e.g.,
capuchins, meerkats, hyenas) exhibit greater leadership generality across domains, reecting
the fact that high-ranking individuals typically occupy most leadership roles.
Cross-Domain Synthesis
Because several of the rating dimensions are intercorrelated, we employed principal compo-
nents analysis to reduce the dimensions of the correlation matrix. The rst two principal
components accounted for 55% of the variation. The rst (39% of the variation) loaded
substantially on all variables, but was specically associated with leaders being powerful,
leadership being concentrated, and the benets of leadership being relatively large across
domains. We thus named this principal component power. The second principal component
(16% of the variation) specically identied variables relating to emergence: societies scoring
highly on this principal component had leaders whose role was more ascribed, whereas those
with low scores had leadership that was more achievement-based. Figure 2 reveals no
systematic divergence between human and non-human mammals; some SSS have low
leadership power scores while others have some of the highest. There is similar variation
amongst SSS for the emergence dimension.
Towards a Unifying View of Leadership
Examination of leadership in a sample of NHM and human SSS in a cross-species framework
revealed some suggestive patterns. We found signicant variation across domains for two
dimensions, distribution and power, with leadership being the most concentrated and powerful
in conict mediation and between-group interactions. Comparing humans to other mammalian
species in our sample, we found both similarities and differences. Despite common assumptions
often made around the notion of human uniqueness, interestingly, we found no clear divide
between human and non-human social mammals with respect to the emergence of leadership in
this exploratory analysis. Instead, both NHM and SSS leadership is based more on individual
achievement than on ascribed (inherited) status, although exceptions to this pattern should
motivate future theoretical and empirical analyses. In addition, both NHM and SSS leaders tend
Trends in Ecology & Evolution, Month Year, Vol. xx, No. yy 9
TREE 2008 No. of Pages 13
to wield more power in the domains of within-group conict-resolution and between-group
interactions, consistent with theoretical analyses of the importance of multilevel selection [60] as
well as within-group dynamics [2]. Furthermore, the paucity of coercive leadership in small-scale
(but not complex) human societies is arguably similar to the way NHM use communicative
or passive mechanisms, but rarely coercion, to recruit followers [9]. These continuities between
humans and other mammals in leadership patterns likely reect shared evolved cognitive mecha-
nisms governing dominancesubordination, alliance formation, and decision-making [3,15].
On the other hand, SSS leaders in our sample generally wield less power than do NHM leaders,
and leadership is marginally more domain-general in the societies of NHM than in SSS. These
differences reect a tendency for humans to develop role specialization and depend less on
dominance as the basis for leadership, which arguably indicates signicant evolutionary diver-
gences between humans and NHM [61,62]. Even in the least complex human societies, the
scale of collective action (for food acquisition and distribution, conict mediation, and between-
group interactions) is greater and presumably more crucial for survival and reproduction
[[48_TD$DIFF]13,54,63].
Pimbwe
Hyena
Elephant Nootka
Meerkat
Cheyenne
Dolphin
More powerful
Principal component 1
Principal component 2
Chimpanzee
Zebra
Capuchin
Kipsigis
Inuit
Tsimane
Ache
Shoshone
Lion
8
4
0
–4
–8
–8 –4 0 4 8
More
ascribed
Figure 2. Position of Each Sample Society in the 2D Space Dened by the First Two Principal Components
Extracted from the Set of Ratings for Four Dimensions (Excluding Generality) in All Four Domains.
10 Trends in Ecology & Evolution, Month Year, Vol. xx, No. yy
TREE 2008 No. of Pages 13
We hypothesize that the factors causing human leadership to potentially diverge from the
prevailing mammalian patterns are those responsible for other aspects of human uniqueness
[64], but additional empirical tests are clearly warranted to investigate this possibility. Biologically
evolved characteristics facilitating syntactic language and high-volume cumulative cultural
transmission underpin the development of norms and institutions shaping all aspects of human
social behavior [63,6567], including leadership [68]. Human socioeconomic systems are
characterized by massive ows of goods and services among large numbers of individuals
who are often non-kin. Such a social system appears to favor the emergence of mechanisms to
solve complex coordination problems [69]. Human cognitive capacities for planning and
communication facilitate solution of more challenging collective action problems that benet
most members of the cooperative social units [21,67]. In such contexts, coercion is not
necessary to motivate group members to follow leaders [13].
Our review reveals notable gaps in leadership studies that differ for SSS and NHM (see
Outstanding Questions[9_TD$DIFF]). Leadership has rarely been quantied in domains beyond group
movement for even the best-studied NHM. Future studies should investigate the degree to
which this lacuna in the literature reects actual patterns of leadership in NHM. In SSS,
leadership is poorly described in the domains of movement and food acquisition, perhaps
because of the rarity of formal leadership roles; here again new studies will be necessary to
examine the dimensions of collective decision-making in these domains. We also have little
systematic understanding of the variable tness costs and benets associated with leadership
status in SSS [68] or NHM societies [9]. Existing theoretical work spans multiple domains of
leadership, but most focuses on only three dimensions of leadership: distribution, emergence,
and payoffs. An important direction for future theoretical work is to provide modeling guidance
on evolutionary processes driving variation in the power of leaders and the generality of
leadership across multiple domains. We hope this review helps to stimulate greater integration
of empirical work and model-building on this emerging research topic.
Acknowledgments
This project was sponsored by the National Institute for Mathematical and Biological Synthesis, supported throug hNational
Science Foundation awards EF-0832858 and DBI-1300426, with additional support from The University of Tennessee,
Knoxville. S.G. was supported by the US Army Research Laboratory and the US Army Research Ofce under grant number
W911NF-14-1-0637. P.H. thanks C. von Rueden, H. Kaplan, and M. Gurven for helpful discussion regarding the Tsimane.
J.E.S. was supported by funds from Faculty Development Funds and from the Provost's Ofce at Mills College. M.v.V.
received funding from the Netherlands Science foundation (NWO). C.H. was supported by the National Science and
Engineering Research Council of Canada (NSERC, RGPIN-2015-05795) and the Foundational Questions in Evolutionary
Biology Fund (RFP-12-10). We are grateful to Richard Connor, Craig Packer, Elizabeth Archie, and Daniel Rubenstein for
advice on rating particular species, and Britt Pimental for bibliographic assistance.
[49_TD$DIFF]Supplemental Information
[49_TD$DIFF]Supplemental [50_TD$DIFF]information associated with this article can be found, in the online version, at http://dx.doi.org/10. 1016/j.tree.
2015.09.013.
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Trends in Ecology & Evolution, Month Year, Vol. xx, No. yy 13
... Here, we use evolutionary game theory to explore the whether the logic of democratic peace can be used to explain variation in intergroup conflict in animal societies, in the absence of uniquely human institutions. A large body of literature now demonstrates that collective movement decisions in animal societies can vary on a continuum from being unshared (i.e., dictated by leaders 28-34 ;) or shared across the group (i.e., democratic [34][35][36][37][38][39][40][41] ). These democratic decisions are observed when the majority can influence group movements more than any one individual or subset of individuals. ...
... Likewise, when d v > 0.5 leaders' share of the benefit V L , is greater than that of the groups' followers, V F . This represents cases where leaders are socially dominant to followers and able to displace them from contested resources such as food or mates that obtained as a reward after victory 33,34,52,53 . We also consider cases where followers pay lower costs, d c < 0.5, or gain more reward, d v < 0.5, following the literature suggesting that there can be greater costs associated with leadership [54][55][56] . ...
... The assumption that the leaders generally benefit more than the followers is well supported in nature, because leadership is often associated with older, dominant, or otherwise privileged individuals who can benefit from priority of access to resources, including those that are gained from fighting 33,34 . Similarly, leaders may be able to lessen their individual costs of fighting relative to followers on account of being larger, stronger, or being able to occupy safer positions during the fight 7,13,20 . ...
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A major goal in evolutionary biology is to elucidate common principles that drive human and other animal societies to adopt either a warlike or peaceful nature. One proposed explanation for the variation in aggression between human societies is the democratic peace hypothesis. According to this theory, autocracies are more warlike than democracies because autocratic leaders can pursue fights for private gain. However, autocratic and democratic decision-making processes are not unique to humans and are widely observed across a diverse range of non-human animal societies. We use evolutionary game theory to evaluate whether the logic of democratic peace may apply across taxa; specifically adapting the classic Hawk-Dove model to consider conflict decisions made by groups rather than individuals. We find support for the democratic peace hypothesis without mechanisms involving complex human institutions and discuss how these findings might be relevant to non-human animal societies. We suggest that the degree to which collective decisions are shared may explain variation in the intensity of intergroup conflict in nature.
... perspectives. The first suggests that decision making is distributed among all group members who adhere to the established rules [13]. Typically, individuals modify their decisions based on interactions with those near them, leading to a collective compromise on routes. ...
... However, it can result in inefficiencies, ambiguous responsibilities, and reduced adaptability. The second perspective posits that decision making in certain advanced animal groups is concentrated among a few leaders with critical information [13]. For instance, a distinct hierarchy is evident in pigeon colony flights [15,16]. ...
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This study investigates coordinated behaviors and the underlying collective intelligence in biological groups, particularly those led by informed leaders. By establishing new convergence condition based on experiments involving real biological groups, this research introduces the concept of a volitional term and heterogeneous networks, constructing a coupled-force Cucker–Smale model with informed leaders. Incorporating informed leaders into the leader-follower group model enables a more accurate representation of biological group behaviors. The paper then extracts the Flock Leadership Hierarchy Network (FLH), a model reflecting real biological interactions. Employing time slicing and rolling time windows, the study methodically analyzes group behavior stages, using volatility and convergence time as metrics to examine the relationship between group consistency and interactions. Comparative experiments show the FLH network’s superior performance. The Kolmogorov-Smirnov test demonstrates that the FLH network conforms to a power-law distribution, a prevalent law in nature. This result further illuminates the crucial role that power-law distribution plays in the evolutionary processes of biological communities. This study offers new perspectives on the evolution of biological groups, contributing to our understanding of the behaviors of both natural and artificial systems, such as animal migration and autonomous drone operations.
... Current insights into group decision-making stem from two primary perspectives. The first suggests that decision-making is distributed among all group members who adhere to established rules [13]. Typically, individuals modify their decisions based on interactions with neighbors, leading to a collective compromise on routes. ...
... However, it can result in 2 of 22 inefficiencies, ambiguous responsibilities, and reduced adaptability. The second perspective posits that decision-making in certain advanced animal groups is concentrated among a few leaders with critical information [13]. For instance, a distinct hierarchy is evident in pigeon colony flights [15,16]. ...
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{This study investigates the coordinated behaviors and the underlying collective intelligence in biological groups, particularly those led by informed leaders. By establishing new convergence criteria based on real biological group experiments, this research introduces the concept of a volitional term and heterogeneous networks, constructing a coupled force Cucker-Smale model with informed leaders. The inclusion of the informed leader aspect distinctively characterizes the dynamics of leaders and followers, enabling a more accurate representation of biological group behavior. The paper then proposes the Flock Leadership Hierarchy Network (FLH), a model reflecting real biological interactions. By employing time slicing and rolling time windows, the study methodically analyses group behavior stages, using volatility and convergence time as metrics to examine the relationship between group consistency and interactions. Comparative experiments show the FLH network's superior performance. The Kolmogorov-Smirnov test indicates that the FLH network follows a power-law distribution, highlighting the evolutionary intelligence of biological groups. This research offers new perspectives on the evolution of biological groups, contributing to our understanding of both natural and artificial systems, such as animal migration and autonomous drone operations.
... But even if future research uncovers new gorilla gestures, the fact that they rely so much on vocalizations is highly relevant. Finally, the success of different leadership models is likely to vary across contexts [93]. We only reported on the mechanisms of group departures after periods of resting. ...
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The ability to coordinate actions is of vital importance for group-living animals, particularly in relation to travel. Groups can only remain cohesive if members possess a cooperative mechanism to overcome differences in individual priorities and social power when coordinating departures. To better understand how hominids achieve spatio-temporally coordinated group movements, we investigated vocally initiated group departures in three habituated groups of western gorillas (Gorilla gorilla) in the Central African Republic. The large sexual dimorphism of gorillas has led to the untested assumption that the silverback males are the sole decision-makers in gorilla groups, although there are also observations that suggest otherwise. To address this, we analysed the direction and timing of group departures and found that high-ranking individuals (silverbacks and high-ranking females) were more successful in indicating the direction of future travel than others, but that the timing of departure was the apparent result of a cumulative vocal voting process among all adult group members. Our findings illustrate that even in species with a large sexual size dimorphism, travel decisions can be taken collectively via a consensus-building process.
... All mammalian groups are hierarchically structured (Smith et al., 2016;Van Vugt & Smith, 2019). Individuals at the top of the social hierarchy possess the most social influence and enjoy privileged access to valued resources (Cowlishaw & Dunbar, 1991;Schjelderup-Ebbe, 1922). ...
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... Leadership can be observed in large-scale hierarchical societies as well as in small-scale relatively egalitarian societies, though in a less institutionalized and more situational form (Garfield et al., 2019(Garfield et al., , 2020Lewis, 1974;von Rueden & Van Vugt, 2015;Service, 1975). To some extent, leadership can even be observed among non-human animals (Couzin et al., 2005;Smith et al., 2016). If the evolution of leadership and followership played a crucial role for sustaining human cooperation and collective coordination, aspects at the heart of group living, then our mind may be endowed with core, reliably and early-developing mechanisms for representing and respecting legitimate leadership as well as dominance, so that human infants need not only live in a Hobbesian 'state of nature' where might makes right. ...
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Research has shown that infants represent legitimate leadership and predict continued obedience to authority, but which cues they use to do so remains unknown. Across eight pre-registered experiments varying the cue provided, we tested if Norwegian 21-month-olds (N = 128) expected three protagonists to obey a character even in her absence. We assessed whether bowing for the character, receiving a tribute from or conferring a benefit to the protagonists, imposing a cost on them (forcefully taking a resource or hitting them), or relative physical size were used as cues to generate the expectation of continued obedience that marks legitimate leadership. Whereas bowing sufficed in generating such an expectation, we found positive Bayesian evidence that all the other cues did not. Norwegian infants unlikely have witnessed bowing in their everyday life. Hence, bowing/prostration as cue for continued obedience may form part of an early-developing capacity to represent leadership built by evolution.
... However, hierarchical structure comes with challenges and costs for social groups [7]. In hierarchical groups, high-ranking individuals may bully subordinates and usurp a disproportionate share of resources, social influence, and reproductive opportunities [8,9], which may amplify intragroup inequality and competitions [10], undermine the authority and legitimacy of group leaders [7]. Small groups overcome these problems through in-group social bonding [11,12], an adaptive means of forming, strengthening, and maintaining interpersonal connections with in-group members [13][14][15]. ...
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Political leadership plays a vital role in shaping the trajectory of political systems, addressing societal issues, fostering inclusive governance, and contributing to sustainable development. It has undergone a major facelift, propelled by the relentless march of technology. Integration of artificial intelligence (AI) into political processes and decision-making occupies a vital space in shaping the future of political leadership and democratic processes.
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This chapter provides a theoretical foundation for the thesis that the social dynamics occurring in the context of economically defensible resources are key generators of large-scale political integration and hierarchy in human societies. This model integrates evolutionary models of economic defensibility, cooperation, punishment, and political hierarchy within a simulation with explicit spatial and population dynamics. It identifies features of the natural, social, and technological environment that favor (or disfavor) the development of an arms race between larger and more hierarchically organized territorial coalitions. The model shows that hierarchy and inequality develop regularly in ecologies that favor the formation of coalitions to defend and contest resources. Hierarchies for competition over resources are particularly likely where egalitarian means of promoting cooperation in coalitions are ineffectual and hierarchies are not unbearably costly. Hierarchies become more costly when group members are restricted in their ability to choose efficient leaders.
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