After a given period of recess in testing, the elephants were presented with a previously learned odor combination to assess the long-term odor memory of the animals. After a two, four, eight and 16 weeks of recess, all three individuals were successful in remembering the reward value of both the S+ and the S- (p < 0.01). This finding of an excellent long-term odor memory is in agreement with the results from other studies assessing the same capability in other species ( Hübener and Laska, 1998; Hübener and Laska, 2001; Laska and Hudson, 1993; Laska et al., 2003; Laska et al., 2008). The results from the present study are also in accordance with the only previous study examining the odor memory of elephants. Male Asian elephants have been shown to possess an extraordinary long-term chemical memory in the sense that they can recognize the scent of maternal urine even when they have been physically separated from their mother up to 27 years (Rasmussen, 1995).

The memory tests for the two-, four- and eight-weeks of recess in testing in this study, were implemented in between other tasks, while the 16-weeks memory test was conducted after a total recess in testing for eleven weeks. During this period of time, the elephants were not subjected to any olfactory training or testing. Still, after this longer recess, there was no change in either the behavioral response or the discrimination performance in any of the individuals. This demonstrates that the animals did not only show excellent retention of an odor combination, but that they also remembered how to execute the task they had learned to perform even after a total recess of almost three months. Another memory-test performed on a 20 year old captive Asian elephant which was retested in a visual discrimination task eight years after she had first been trained to perform the task also showed excellent retention. Even after this extended period of time, the elephant showed no problem in remembering the task she had learned years ago (Markowitz et al., 1975). The results of the present study strengthen the evidence for an outstanding long-term memory in elephants.