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What do dolphins (Tursiops truncatus) understand about hidden objects?

Abstract

Object permanence, the ability to mentally represent and reason about objects that have disappeared from view, is a fundamental cognitive skill that has been extensively studied in human infants and terrestrial animals, but not in marine animals. A series of four experiments examined this ability in bottlenose dolphins (Tursiops truncatus). After being trained on a “find the object” game, dolphins were tested on visible and invisible displacement tasks, and transpositions. In Experiments 1 and 2, dolphins succeeded at visible displacements, but not at invisible displacements or transpositions. Experiment 3 showed that they were able to pass an invisible displacement task in which a person’s hand rather than a container was used as the displacement device. However, follow-up controls suggested they did so by learning local rules rather than via a true representation of the movement of hidden objects. Experiment 4 demonstrated that the dolphins did not rely on such local rules to pass visible displacement tasks. Thus, like many terrestrial animals, dolphins are able to succeed on visible displacement tasks, but seem unable to succeed on tasks requiring the tracking of hidden objects.

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Fig. 1
Fig. 2

Notes

  1. For one dolphin, the target object initially consisted of three interlocking rings (each 16 cm in diameter), made from flexible plastic tubing (1 cm in diameter) and covered with colored electrical tape. When these rings seemed to be contributing to problematic behavior, the object was switched to the plush alligator.

  2. For the first dolphin tested, the protocol differed slightly. Any test trial in which she touched early was aborted, and then repeated at the end of the session. If the aborted trial was the last trial of the session, a training trial (i.e., with a partially hidden object) was added before the repeated test trial. However, because the first dolphin continually touched early, this protocol led to an unfortunate cycle of repeatedly adding trials. Therefore, after her first condition, we switched to the current protocol.

  3. For the first dolphin tested, the design also included three training (i.e., partially hidden) trials in addition to the errorless and testing trials in each session. However, when this dolphin encountered difficulties during her first two testing sessions (see footnote 2), these training trials were dropped for all remaining test sessions.

  4. Note that during the invisible displacement condition, one of the dolphins (Tanner) repeatedly chose the displacement cylinder itself, except for two trials in which he touched the bucket closest to the displacement cylinder. For purposes of analysis, we scored his performance in this condition at zero. Alternately coding his responses as chance did not change any of the statistical outcomes.

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Acknowledgments

We wish to thank Denise Cabrisas, Jane Hecksher, Julie Lockard, Courtney Smith, Cheryl Sullivan, and numerous research interns at DRC for help with data collection, as well as Josep Call, James Ha, Francys Subiaul, and three anonymous reviewers for helpful comments on earlier drafts of this paper. Finally, a special thanks to the staff and dolphins of DRC for their cooperation and patience during this project. All of the experiments complied with the current laws of the United States of America.

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Correspondence to Kelly Jaakkola.

Appendix

Appendix

Training trials before each experiment consisted of an incremental sequence of gradually introducing (or reintroducing) new components, along with any additional sessions between experiments for the purpose of maintaining the “find the object” game in the animals’ repertoires. In no case did between-experiment sessions include the component which was to be tested in upcoming experimental sessions. The total number of training/maintenance trials between experiments is detailed for each animal in Table 6, divided into three trial types: non-choice trials, which were either errorless or no-container trials in which the object was placed directly on the dock; choice trials in which the object was visible; and (after Experiment 1) choice trials in which the object was hidden.

Table 6 The total number of training/maintenance trials between experiments

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Jaakkola, K., Guarino, E., Rodriguez, M. et al. What do dolphins (Tursiops truncatus) understand about hidden objects?. Anim Cogn 13, 103 (2010). https://doi.org/10.1007/s10071-009-0250-z

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Keywords

  • Dolphins
  • Object permanence
  • Visible displacement
  • Invisible displacement
  • Secondary representation