The reason why domestic cats still hunt when they are not hungry and are well fed is because the trigger to hunt is completely separate from the trigger to eat. Sensory cues such as prey movement and visuals triggers the hunting sequence. This behaviour is based upon the need to survive because the success rate of hunting is quite low and therefore if they waited until they were hungry they might be weakened and the success rate would be further driven down to the point where they would become vulnerable
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