1.2. BIOLOGICAL BASIS AND ARTIFICIAL LIFE
Researchers try to examine how collections of animals, such as flocks, herds and
schools, move in a way that appears to be orchestrated. A flock of birds moves like a
well choreographed dance troupe. They veer to the left in unison, and then suddenly
they may all dart to the right and swoop down toward the ground. How can they
coordinate their actions so well? In 1987, Reynolds created a boid model, which is a
distributed behavioral model, to simulate on a computer the motion of a flock of birds
. Each boid is implemented as an independent actor that navigates according to its
own perception of the dynamic environment. A boid must observe the following
rules. First, the “avoidance rule" says that a boid must move away from boids that are
too close, so as to reduce the chance of in-air collisions. Second, the “copy rule" says
a boid must go in the general direction that the flock is moving by averaging the other
boids' velocities and directions. Third, the “center rule" says that a boid should
minimize exposure to the flock's exterior by moving toward the perceived center of
the flock. Flake added a fourth rule, “view," that indicates that a boid should move
laterally away from any boid the blocks its view. This boid model seems reasonable if
we consider it from another point of view, that of it acting according to attraction and
repulsion between neighbours in a flock. The repulsion relationship results in the
avoidance of collisions and attraction makes the flock keep shape, i.e., copying
movements of neighbours can be seen as a kind of attraction. The center rule plays a
role in both attraction and repulsion. The swarm behaviour of the simulated flock is
the result of the dense interaction of the relatively simple behaviours of the individual
boids. To summarize, the flock is more than a set of birds; the sum of the actions
results in coherent behaviour.
One of the swarm-based robotic implementations of cooperative transport is inspired
by cooperative prey retrieval in social insects. A single ant finds a prey item which it
cannot move alone. The ant tells this to its nest mate by direct contact or trail-laying.
Then a group of ants collectively carries the large prey back. Although this scenario
seems to be well understood in biology, the mechanisms underlying cooperative
transport remain unclear. Roboticists have attempted to model this cooperative
transport. For instance, Kube and Zhang introduce a simulation model including