Bottlenose dolphins live in fission–fusion societies and form long-term associations with specific individuals identified by individually unique “signature” whistles. Recognition memory for these whistles lasts for years. Here we investigated long-term recall memory in a dolphin who produced vocal labels to referents. Initially, the adult male dolphin subject, Calvin, learned to label nine objects vocally using arbitrary, human-designed rhythms. Testing occurred in 18-trial sessions in which objects were presented randomly and researchers naïve to an object’s identity identified the vocal label by ear. Performance accuracy on the last 10 sessions of the study averaged 87% (chance = 1/9, 11%). After more than 6 years, Calvin revisited the project in five 18-trial labeling sessions. He immediately resumed the labeling task itself and correctly labeled five of nine objects at least once (56%) in session 1. Performance accuracy ranged from 33% to 78% across the five sessions. Calvin produced eight of nine rhythms without any prompting in session 1 and labeled every object correctly at least once across five sessions. Correct labels for each object across the sessions ranged from 10% to 90%. Overall, these results indicate enduring long-term recall memory for vocal production with variable memory for the associations between label and referent.
Harley et al. (Wed,) studied this question.