Supports for this hypothesis:
(A): Why on earth should the animals persist in these kinds of behaviours if they did not get some kind of reward in return?
(B): Stereotypic behaviour goes through a development over time. That suggests that some kind of learning is implicated.
(C): Chronically hungry pigs perform stereotypies.
From a simple energy-perspective it would make more sense if they conserved their energy.
(D): It has been observed that some individuals are willing to work for the possibility to engage in stereotypic activities. A predator that is in the habit of Pacing along a certain wall in its cage will actively push away a member of the group so it can continue its stereotypy. A horse that swallows air (windsucking) will persist in this behaviour even if a belt is tightened around its neck, which does make the swallowing difficult.
Objections for this hypothesis:
(A1): Stereotypies is derived from appetitive behaviour, which is controlled by positive feedback. The most efficient way to end such a behaviour is if the animal passes to the consummatoric phase, which will end due to negative feedback. However, the animal never makes it that far.
(B1): Stereotypies is caused by the before mentioned Sensitization of neuron paths and/or motor programs which make the behaviour more easily/readily performed as time goes on.
(C1): The triggering stimuli are constantly present
(D1): Stereotypies is caused by increased Arousal.
The relevance of this hypothesis, that the main aim of stereotypic behaviours is to be positively self-stimulating, is not evident, and as usual in this business, more research is necessary to answer this question better.
Next topic: Hypothesis: "Stereotypies stems from boredom".