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Categorization learning was investigated in 2 horses (Equus caballus). Both horses learned to select a 2-dimensional black stimulus with an open center instead of a filled stimulus in a 2-choice discrimination task. After a criterion of 10 out of 10 correct responses in a random series for 2 consecutive sessions was reached, 15 additional pairs of open-center versus filled stimuli were tested. Each was run to criterion and then incorporated into sessions of randomly mixed problems. Both horses solved the 1st problem by simple pattern discrimination and showed evidence of categorical processing for subsequent problems. New pairs were learned with few or no errors, and correct responses on novel trials were significantly above chance. These results suggest that the horses were making their selections on the basis of shared characteristics with the training stimuli and were using categorization skills in problem solving.This study was funded in part by Earthwatch and its Research Corps and by public donations to the Equine Research Foundation.I thank Meinrad and Betty Hanggi for their continual support and encouragement, Hansens Feed of Aptos, California, and all others who have made donations. I also thank Jerry Ingersoll for his invaluable assistance throughout the project. Likewise, I thank Julie Yi and the many student assistants and Earthwatch volunteers for their enthusiastic participation and George H. Waring for valuable suggestions on an earlier version of this article.

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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