Dolphins are bilingual during sleep

A group of five Bottlenose dolphins at the Planète Sauvage aquarium in Port-Saint-Père, France, was found to mimic whale sounds at night. These dolphins were exposed to whale songs through the soundtrack from their daily shows, which included bird cries and other marine sounds.

Martine Hausberger and her colleagues of the University of Rennes discovered the dolphins' mimicry when trying to learn more about the sounds that dolphins make at night. The dolphins had never been heard mimicking whale song during or after shows, but when researchers hung underwater microphones in the dolphins' tank overnight, they heard 25 instances of whale-like sounds that had never been heard from these dolphins before.

To confirm their observations, the researchers played regular-speed and slowed-down versions of the calls to 20 volunteers. In addition, volunteers were exposed to the audio of regular dolphin whistles, slowed-down dolphin whistles and authentic whale songs. The volunteers correctly differentiated the dolphin whistles from the whale song 88 to 99 percent of the time. However, they also mistook the dolphin's whale-like whistles for real whale song 72 percent of the time.

These whale-like whistles, which constituted approximately one percent of the dolphin noises recorded, only occurred when the dolphins were resting, which was mostly between midnight and three a.m. This suggests that the whale sounds are dolphins' version of sleep-talking.

Bottlenose dolphins produce vocalizations using six air sacs near their blow hole. They use whistles to identify other dolphins, coordinate hunting strategies, and determine which individuals have priority access to prey items. Despite the versatility of their communication strategies, researchers have not yet discovered a dolphin language.

Dolphins are known to mimic the vocalizations of other animals. But unlike birds, their mimicry was thought to be limited to the time right after hearing a foreign sound. The dolphins in this study are the first example of dolphins retaining a sound to practice later. The researchers suggest that the dolphins were rehearsing their daily shows at night, which is possible, as the dolphins were more attentive during the time of their shows.

Hausberger hopes to monitor the dolphins' brain activity to determine if they are making the whale calls while sleeping. If confirmed, these findings could give insights to dolphin learning processes and mental representations.