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The ability to determine the value of dependent variables. Complete controllability means that it is possible to move any state (of a system) to any other state during any finite interval of time by a suitably chosen input (i.e. by manipulation of suitable independent variables). In simple terms, when the value of a dependent variable y is determined uniquely by an independent variable x, the behaviour of y may be said to be controlled by x. When the value of y in no way influences the value of x, the control can be said to be ballistic (or open-loop). Such situations are characteristic of fixed action patterns.

In an open-loop control system, the value of the output is likely to be affected by disturbances (changes in the mechanism due to outside influences). However, in a closed-loop system, such disturbances can be compensated for by feedback from the output, such that the input is appropriately adjusted.

When the value of x is influenced by the value of y, a feedback situation exists and control is of the closed-loop type. Feedback exists between two variables whenever each affects the other. Positive feedback exists when the value of y tends to increase the value of x, which in turn increases the value of y. Positive feedback systems are inherently unstable, but they can have certain advantages in complex systems containing built-in constraints.

Negative feedback systems are characterized by the situation where the value of x is diminished as the value of y increases. The consequence is that the value of y therefore decreases. The value of y is then said to be under negative feedback control. Such negative feedback systems are common in the control of animal behaviour. Examples can be found in motivation, orientation, and thermoregulation.

In ordinary terms, a controls b if, and only if, the relation between a and b is such that a can drive b into whichever of b's normal range of states a wants b to be in. Thus to be a controller, a must have desires (or their equivalent) about the state of b. This means that wherever one animal controls another, that animal must have both the motivation and the ability to exercise control. Similarly, wherever one part of an animal's internal mechanism controls another part, the former must have the ability and the instructions available to exercise control.

Control concepts such as these are important in the study of animal communication, especially manipulation, as well as aspects of motivation, such as hunger and thirst.

Subjects: Zoology and Animal Sciences.

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