In this study, six adult dolphins were trained to approach an underwater speaker when either their own signature whistle or a biologically irrelevant “trivial” sound was played, and their ability to discriminate between their own and others’ signature whistles and their assigned and other "trivial" sounds, was tested.

Two out of three dolphins were successfully trained to be called over, using their assigned "trivial" sound, but neither of them was able to discriminate between their assigned, and other "trivial" sounds.

All three dolphins trained to be called over, using their own signature whistle, were successful in this task, and two of them were, at least to a degree, able to discriminate between their own signature whistle and the signature whistles of other dolphins in the group.

There was a large variation in how fast each dolphin learned their new call-over signal. Although the differences were not significant, signature whistles seemed to be easier for the dolphins to learn to be called over with than "trivial" sounds.

However, both signature whistles and "trivial" sounds were successfully used as call-over signals, indicating that the variation in learning speed may not only have been caused by what sound was used, but also by differences in each individual's motivation. 
 

Upon beginning to grasp the concept of swimming towards the Whistle caller speaker when his sinature whistle was heard, the only male in the study, David, started to whistle back whenever his sound had been played.

David, described by the trainers as a relatively quiet individual, all of a sudden started using his signature whistle throughout the training sessions with the Whistle caller, and after a while also during other training sessions.

This increased use of his signature whistle was a surprise, and the same behaviour was not seen in the two female dolphins (Ariel and Pärla) that were also trained with their signature whistles. 

As David was the only dolphin displaying this increased whistleling, and he was also the only male dolphin in the study, I suggest that future studies investigate the possible male-female differences in the use and recognition of signature whistles. 

Out of the five dolphins taking part in the discrimination sessions, only two were able to discriminate between call-over signals. Both of these dolphins (David and Ariel) were trained with a playback of their own signature whistle. The dolphin with the highest percentage of correct responses was David. 

As mentioned, David was the only male dolphin in this study. As his results from the discrimination sessions were better than the females' results, one may ask the question if there are male-female differences in how well dolphins discriminate between different signature whistles.

In the literature on the subject of dolphin signature whistle use, much points to male and female dolphins both using their signature whistles in order to keep in contact with their social group, but that they also benefit from this individual identity broadcasting in different ways - females use their signature whistle in order to keep vocal contact with their calf, whereas males have been observed to use it to form alliances and affiliative social bonds between themselves and other males. Males have also been seen to use vocal learning to match other non-signature whistles, to converge on a shared whistle within their alliance. 

Taking all this into consideration, there might be an innate evolutionary reason for why David performed better during his five discrimination sessions than any of the other dolphins in this study. Perhaps male dolphins have a greater need for recognizing and responding to their own whistle than females do, and perhaps females instead tend to be more responsive to the signature whistles of others (such as their calves).

This theory needs to be tested more in depth in future studies. However, male-female differences have already been observed with regards to both environmental enrichment and during other cognitive tasks.