Dolphins address each other using "secret handshake" whistles, a finding that suggests they may have embarked on an evolutionary path towards real language.
A biologist made underwater recordings of a group of bottlenose dolphins off Scotland in the Kessock Channel of the Moray Firth. He found they learned specific whistles and used them in "matching interactions" to address certain individuals.
Dr Vincent Janik couldn't tell whether the exchanges were meant to express aggression or kinship. Such matching recognition signals were thought to be an important step in the evolution of human language, he writes in the journal, Science. Recognisable vocal interactions using learned signals have been documented only in humans and birds.
Dr Janik, formerly of St Andrew's and Aberdeen Universities and now working in the US, took recordings of 1,719 whistles made during seven summer days in 1994 and 1995 by groups of up to 14 dolphins. He used six hydrophones which could locate the position of individual dolphins by measuring the time taken for the sounds to reach each unit.
Of those dolphins for which locations could be determined, a total of 176 whistle interactions were identified. Among these, 39 were classified as matching. Some exchanges took place over distances of up to 579 metres but most were much closer, with an average distance of 179 metres.
There might have been many more matching exchanges but they could not be measured reliably, Dr Janik pointed out. If the chatting pairs got closer than 26 metres, the recording equipment could not separate their calls and prove them to be two dolphins rather than a single loquacious individual.
"These results show that bottlenose dolphins use their learned whistles in matching interactions, most likely to address each other," Dr Janik said..
"The character of such addressing might be either aggressive or affiliative. However, matching could also signal alliance membership to third parties, or be used to prevent deception of third parties by a whistle-copying dolphin."
Recognising and repeating vocal signals was thought to be an important step in the development of human language, he said. "The results presented here show that reaching that step can be achieved in very different environments."
