Scientists at UZH, the University of Western Australia and the University of MA studied 17 adult bottlenose dolphins in Shark Bay in Western Australia. Additionally, several of these male duos or trios will sometimes form larger, second-level alliances-some of which can last their lifetimes.
They found that males within an alliance use vocal labels that are quite distinct from one another, indicating that they serve a similar objective to names among humans.
The fact that the individual "names" are kept helps males to keep track of their many different relationships and distinguish between friends, friends of friends, and rivals. Dolphins use them to introduce themselves or even copy others as a means of addressing specific individuals.
The discovery paints a picture of the social intelligence of dolphins whereby no other non-human animals have been found to retain an individual "name" when they form long-term cooperative partnerships with one another.
"Convergence onto shared or similar identity signals has been documented in allied male bottlenose dolphins", the study said.
While parrots, bats, elephants and primates are also known to make vocal calls, they are very similar to one another.
"With male bottlenose dolphins, precisely the opposite happens: Each male keeps his own, individual call, and distinguishes himself from his allies, even when they develop an incredibly strong bond", explains Krützen.
Yet, it appears that in the complex network of dolphin alliances in Shark Bay, retaining individual "names" is more important than sharing calls.
Dolphins are intelligent creatures that communicate with high-frequency whistles and are capable of forming strong relationships.
Dr King said while that was already known, researchers were interested in how those male dolphins were able to identify each other and work out who was a friend or foe.
A bottlenose dolphin signature whistle.
To explore the role of these "vocal signals", the team measured the similarity of the signals between and out of alliances of male dolphins.
Signature whistles of two different male dolphins from Shark Bay, Western Australia.
"These alliances with one another retain individual vocal labels, or 'names, ' which allows them to recognize many different friends and rivals in their social network".
She also learned that the dolphins can mimic each other's whistles, seemingly to address or call out to one another. However, they did not know how these males used vocal signals to form and maintain these relationships.
Dr King theorised it could have to do with the very high density of dolphins there, and the higher number of males than females.
"Synchrony has also been linked to oxytocin release in humans, which promotes trust and cooperation", says King.
She said the next step was to investigate the relationships more closely to find whether cooperative relationships within alliances were equal.