Bottlenose dolphins have a knack for language. They can understand both the meaning and the order of words conveyed through human hand gestures—correctly putting an item on the right side of their tank into a basket on the left, for example. Now humans, too, are beginning to understand dolphin language as more than just a cacophony of clicks, pulses, and whistles. A new study shows that dolphins use their own unique calls, known as signature whistles, to introduce themselves to others when meeting at sea.
Until recently, researchers could study signature whistles only in captive animals—raising the question of whether the whistle developed in response to capture, isolation, or stress. A 2004 study showed that a group of free-swimming bottlenose dolphins in Florida did indeed use signature whistles. But information about how they used these sounds was scant.
Marine biologists Vincent Janik and Nicola Quick of the Sea Mammal Research Unit at the University of St. Andrews in the United Kingdom were focusing on signature whistles as a way of understanding how dolphins communicate in the natural world. "Dolphins are comparable to great apes in their cognitive skills, but all we know is what they do in a lab," Janik explains. "We wanted to understand how dolphins use their intelligence outside of the tasks that humans set for them."
To learn more, Quick and Janik followed a group of bottlenose dolphins that swim long distances around the eastern coast of Scotland. The researchers used a small boat to tow an array of underwater microphones, called hydrophones, about 2 meters below the surface and recorded the sounds of individual dolphins identified by their dorsal fins. The animals were matched with their calls by their surfacing locations and swim speed, as well as by digital cameras while they were near the surface.
The dolphins used signature whistles when meeting up with another group, Quick and Janik report online today in the Proceedings of the Royal Society B. What's more, they gave the distinctive whistle only if they actually mingled with the other dolphins. Of 11 "conversations," only two did not result in the groups' joining together, and only once did the groups join up without first exchanging whistles.
Even more intriguing was that only one member of each group gave the signature whistle. According to Janik, there could be several explanations. The group could have a leader doing the "talking;" the dolphins may have identified each other using echolocation (the clicks the dolphins send out that echo back from nearby objects), and the whistle was more of a ritual; or the groups may have been together previously and already known each other.
Both the methods and the findings are the first of their kind, according to Laela Sayigh, a cetacean biologist at the Woods Hole Oceanographic Institution in Massachusetts. (Sayigh, who co-authored the 2004 study of Florida dolphins, has collaborated with Janik in the past but was not involved in the current research.) Understanding how dolphins use signature whistles opens up many exciting research possibilities, according to Sayigh. "We know dolphins learn to copy each other's signature whistles. What if they use a signature whistle to refer to a 'third party' or a dolphin that isn't there?"
The study also sheds some light on dolphin society, says Heidi Harley, a comparative cognitive psychologist at the New College of Florida in Sarasota. "Dolphins live in groups that come together and break up, often ending up composed of different individuals," she says. "Exchanging signature whistles may be one way they manage these interactions."