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The Behavior System 129

the behavior of animals, who must behave effectively in a complex dynamic environment
in order to satisfy their needs and maintain their well-being. This entails having the animal
apply its limited resources (finite number of sensors, muscles and limbs, energy, etc.) to
perform numerous tasks. Given a specific task, the animal exhibits a reasonable amount of
persistence. It works to accomplish a goal, but not at the risk of ignoring other important
tasks if the current task is taking too long.
For ethologists, the animal’s observable behavior attempts to satisfy its competing phys-
iological needs in an uncertain environment. Animals have multiple needs that must be
tended to, but typically only one need can be satisfied at a time (hunger, thirst, rest, etc.).
Ethologists strive to understand how animals organize their behaviors and arbitrate between
them to satisfy these competing goals, how animals decide what to do for how long, and
how they decide which opportunities to exploit (Gallistel, 1980).
By observing animals in their natural environment, ethologists have made significant
contributions to understanding animal behavior and providing descriptive models to ex-
plain its organization and characteristics. In this section, I present several key ideas from
ethology that have strongly influenced the design of the behavior system. These theories and
concepts specifically address the issues of relevance, coherence, and concurrency, which are
critical for animal behavior as well as for the robot’s behavior. The behavior system I have
constructed is similar in spirit to that of Blumberg (1996), who has also drawn significant
insights from animal behavior.
Behaviors

Ethologists such as Lorenz (1973) and Tinbergen (1951) viewed behaviors as being com-
plex, temporally extended patterns of activity that address a specific biological need. In
general, the animal can only pursue one behavior at a time such as feeding, defending
territory, or sleeping. As such, each behavior is viewed as a self-interested goal-directed
entity that competes against other behaviors for control of the creature. They compete for
expression based on a measure of relevance to the current internal and external situation.
Each behavior determines its own degree of relevance by taking into account the creature’s
internal motivational state and its perceived environment.

Perceptual Contributions
For the perceptual contribution to behavioral relevance, Tinbergen and Lorenz posited the
existence of innate and highly schematic perceptual filters called releasers. Each releaser
is an abstraction for the minimal collection of perceptual features that reliably identify a
particular object or event of biological significance in the animal’s natural environment.
Each releaser serves as the perceptual elicitor to either a group of behaviors or to a single
behavior. The function of each releaser is to determine if all perceptual conditions are right