Costs of resources can be imposed on animals by an operant task ( e.g. lever - pressing ) or a homeostatic challenge ( e.g. increased body temperature ) .
An operant task ( e.g. lever - pressing ) is a natural aversion ( e.g. crossing water ) .
Costs of resources can be imposed on animals by an operant task ( e.g. lever - pressing ) , a natural aversion ( e.g. crossing water ) , or a homeostatic challenge ( e.g. increased body temperature ) .