Long-term memory for categories and concepts in horses (Equus caballus)

Abstract

Three horses (Equus caballus) with a history of performing cognitive tasks including discrimination learning, categorization, and concept use were tested to evaluate their long-term memory (LTM) in three experiments. In addition, use of LCD multi-displays for stimulus presentation was incorporated into cognition testing protocol for the first time with horses. Experiment 1 tested LTM for discrimination learning that originally occurred 6 years earlier. Five sets of stimuli were used and the two horses tested showed no decrement in performance on four of the sets; however, both horses did score below chance on one set. Experiment 2 examined long-term categorization recall 10 years after horses had demonstrated the ability to make stimulus selections based on shared characteristics within a given category. The horse tested for LTM after the decade-long interval immediately and consistently applied the previously learned categorization rule to not only familiar but also novel sets of stimuli. Experiment 3 tested another horse for LTM for a relative size concept. This horse had originally demonstrated concept rule use in order to select stimuli based on their relative size to one another. More than 7 years later and without further training, this horse reliably applied the previously established size concept to both familiar and novel sets of stimuli. These findings are the first reports of long-term categorical and conceptual memory in horses and are consistent with observations of domestic and wild horses, which indicate that behavioral and ecological events may be remembered for long periods of time. These studies also demonstrate the adaptive nature of horses with regard to their ability to generalize over several different testing conditions.

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Acknowledgments

This research was funded by grants and public donations to the Equine Research Foundation (ERF). Thank you as always to Meinrad and Betty Hanggi for their encouragement and support. Donations and grants from Adobe Systems/Gifts In Kind International, ASPCA, Claire Giannini Fund, Compumentor, Fleet Matching Gift Program, Microsoft and Troxel greatly helped the ERF research, as did the wholehearted involvement of interns and ERF learning and riding vacation participants. We also thank the editor and referees for their valuable suggestions. The article, “Optimizing animal cognition testing protocol using the Equine Research Foundation Training System™ (ERTS)” is available directly through the Equine Research Foundation. Equine care, training, and testing procedures used in these experiments comply with the current laws of the United States of America.

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Correspondence to Evelyn B. Hanggi.

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Hanggi, E.B., Ingersoll, J.F. Long-term memory for categories and concepts in horses (Equus caballus). Anim Cogn 12, 451–462 (2009). https://doi.org/10.1007/s10071-008-0205-9

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Keywords

  • Horse
  • Long-term memory
  • Concept
  • Categorization
  • Discrimination
  • LCD display
  • Equine