Dolphins call each other by name

Dave the dolphin whistles, and his friend Alan whistles back. We can't yet decipher their calls, but some of the time Dave may be calling: "Alan! Alan! Alan! Alan!"

Stephanie King of the University of St Andrews, UK, and colleagues monitored 179 pairs of wild bottlenose dolphins off the Florida coast between 1988 and 2004. Of these, 10 were seen copying each other's signature whistles, which the dolphins make to identify themselves to each other.

The behaviour has never been documented before, and was only seen in pairs composed of a mother and her calf or adults who would normally move around and hunt together.

The copied whistles changed frequency in the same way as real signature whistles, but either started from a higher frequency or didn't last as long, suggesting Dave was not merely imitating Alan.

Copying only happened when a pair had become separated, which leads King to speculate that they were trying to get back together. She believes the dolphins were mimicking another animal's whistle as a way of calling them by name.

King presented her research last week at the summer conference of the Association for the Study of Animal Behaviour in St Andrews.

Justin Gregg of the Dolphin Communication Project in Old Mystic, Connecticut, remains cautious, and points out that the dolphins may copy the signature whistles simply because they hear them a lot. To be sure that they are usingthe whistles to refer to a specific individual, researchers would need to show that dolphins responded when their signature whistle was copied, he says.

There is no other species that is known to combine signature calls and vocal mimicry in this way, says Phyllis Lee of the University of Stirling, UK. "But I bet parrots could do it," she adds. "They have very long lifespans and complex social structures, and they do a lot of mimicry."

Dolphins don't whistle

Signature they may be, but it appears that dolphins' whistles aren't actually whistles. A true whistle relies on pushing air through a chamber, but a similar sound can be produced by a vibrating membrane.

To find out which way dolphins do it, Peter Madsen of Aarhus University in Denmark and colleagues recorded a bottlenose dolphin whistling after breathing helium. The sounds were largely the same whether the dolphin was breathing helium or air. If the dolphin was really whistling, the helium would have changed the frequency of the sound (Biology Letters, DOI: 10.1098/rsbl.2011.0701).

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