In this task, the elephants were presented with three different odor combinations for two sessions each. The S+ (amyl acetate) was kept constant during the entire experiment while the S- (ethyl acetate, propyl acetate and butyl acetate) was altered. The graph shows the performance of the animals during the first session with each odor combination. All three individuals performed above criterion level and were successful in discriminating between all of the three odor combinations (p < 0.05). The results show that elephants can discriminate between structurally related odorants belonging to the same chemical class, even when these are structurally very similar and only differ by one carbon chain length.

There was a significant negative correlation between discrimination performance and structural similarity in terms of differences in carbon chain length (r_s=0.905, p < 0.05) with a decrease in performance with increased structural similarity. These results are in accordance with the results from other studies with squirrel monkeys and humans (Laska and Freyer, 1997; Laska and Hübener, 2001). Esters constitute a major part of the odors of a variety of fruits (Sun Pan and Kuo, 1994) and even though the Asian elephant is a mixed feeder that mainly feeds off browse and grass (Steinheim et al., 2005) it also selectively feeds on seeds and fruits when available (Kitamura et al., 2007). The ability of the elephants in the present study to discriminate between the structurally related acetic esters therefore makes sense from an ecological point of view.

Esters have also been identified in urine from African elephants, where the urine from young non-musth males contained a higher concentration of esters and acids than that from males in musth (Rasmussen and Wittemyer, 2002) suggesting a role for this class of odorants in assessing the reproductive status of an individual. For a social species as the Asian elephants which strongly rely on the sense of smell for individual recognition and social status assessment through chemical signals (Schulte and Rasmussen, 1999), the ability of the elephants in the present study to discriminate between structurally related acetic esters was therefore not unexpected.