Object permanence in marine mammals using the violation of expectation procedure

Object permanence refers to the ability to process information about objects even when they are not visible. One stage of object permanence, called visible displacement, involves being able to find an object that has been fully hidden from view. Visible displacement has been demonstrated in many animal species, yet very little is known about object permanence in marine mammals. In addition, the methodology for testing visible displacement has sometimes been called into question because alternative explanations could account for subjects' success. The current study investigated visible displacement in Atlantic bottlenose dolphins and California sea lions using a methodology called violation of expectation, in which the animal's fish bucket was placed on a table surrounded on three sides by curtains. A solid screen placed in front of the bucket was then rotated in an arc from front to back. The screen was rotated either 120° (possible event) or 180° (surprising event), appearing as if the bucket disappeared. Both dolphins and sea lions looked significantly longer during the 180°, unexpected, trials than the expected event trials. Results suggest that both dolphins and sea lions pass visible displacement tests without the use of perceptual cues. This article is part of a Special Issue entitled: Tribute to Tom Zentall.