Advances in the Study of Behavior 2003 32,
1-75.
Self-organization and collective behavior in
vertebrates
Iain D. Couzin1,3 & Jens Krause2
1Department of Ecology and Evolutionary Biology, Princeton University,
Princeton, NJ 08544, USA.
2Centre for Biodiversity and Conservation, School of Biology, University
of Leeds, Leeds, LS2 9JT, UK.
3Author to whom correspondence should be addressed.
1.1 Overview
As a ripple of light the fish turn. Like some animate
fluid, the school glides and turns again. The synchrony of motion is
captivating. A similar integration of behavior can be seen in a bird
flock, where the volume and shape of the group changes as it turns and
arcs overhead, and yet the aggregate remains cohesive. Many group-living
vertebrates exhibit complex, and coordinated, spatio-temporal patterns,
from the motion of fish and birds, to migrating herds of social ungulates
and patterns of traffic flow in human crowds.
The common property of these apparently unrelated biological
phenomena, is that of inter-individual interaction, by which individuals
can influence the behavior of other group members. It is on how these
interactions result in the collective behaviors of vertebrate animal
groups that we focus on here. Specifically, we consider systems in which
insights from self-organization theory have been useful in improving
our understanding of the underlying mechanics. Self-organization theory
suggests that much of complex group behavior may be coordinated by relatively
simple interactions among the members of the group. Following this theory,
the form, and therefore often the function, of the collective structure
is encoded in generative behavioral rules. Self-organization has recently
been defined as “a process in which pattern at the global level
of a system emerges solely from numerous interactions among the lower-level
components of a system. Moreover, the rules specifying interactions
among the system’s components are executed using only local information,
without reference to the global pattern” (Camazine et al., 2001).
It should be noted that often in nature, pattern-forming processes may
not strictly conform to this classification: in some instances, such
as animal migration, individuals may modify their local (self-organizing)
interactions with others with reference to global information, such
as a general desire to move in a certain direction. This type of system
therefore self-organizes within the context of global cues.
In recent years there has been an expanding interest
in pattern formation in biological systems (Gerhard & Kirshner,
1997; Maini & Othmer, 2000; Camazine et al., 2001). The field of
pattern formation covers a wide range of areas, including attempting
to explain fetal development (Keynes & Stern, 1998), patterns on
the coats of mammals (Murray, 1981), the structure of social insect
nests (Theraulaz & Bonabeau, 1995), and the collective swarms of
bacteria (Ben-Jacob et al., 1994), army ants (Deneubourg et al., 1989)
and locusts (Collett et al., 1998). In particular there is a growing
interest in the relationship between individual and population-level
properties. A fundamental question is how large-scale patterns are generated
by the actions and interactions of the individual components. Despite
the importance of understanding group dynamics for ecological processes
(Levin, 1999), many collective behaviors are still only qualitatively
understood. Here we will review progress in a newly emerging field,
that of applying self-organization theory to mobile vertebrate groups
composed of many interacting individuals (such as bird flocks, ungulate
herds, fish schools, and human crowds) in an attempt to improve our
understanding of underlying organizational principles.
