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Six degrees of separation at sea

October 27, 2005 By Tom Simonite This article courtesy of Nature News.

Even dolphins are linked to Kevin Bacon.

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"It's a small world," we say when a new acquaintance turns out to be linked to several other people we know. It happens surprisingly often. And it seems it happens even more to Scottish dolphins than it does to people.

David Lusseau, from the University of Aberdeen, UK, has spent years researching the social world of dolphins, to find out who knows whom and how often they meet. For the 130-strong community living off the east coast of Scotland, he found, it takes an average of just 3.9 steps to link any two dolphins by the shortest possible route through mutual friends.

Small-world networks, as mathematicians call them, have the property that any 'node', such as an individual person or dolphin, is connected to every other node through a limited number of steps. This is a result of the way that nodes are organised: most people, for example, have a cluster of close friends but also know a few people from different clusters. This means that connecting any two apparently widely separated people is surprising easy.

Researchers have shown it takes about five-and-a-half steps, on average, to get between any two individuals in the world1. This fact is famous for inspiring the play Six Degrees of Separation, by John Guare, and the game 'six degrees of Kevin Bacon', which challenges players to link a given Hollywood actor to the star through a limited number of shared film appearances (see "My friend Flipper").

Small worlds have been found in groups of everything from actors (with an average 3.5 steps between stars) to authors of physics papers (with 6.2 co-author steps between them). Lusseau previously found a small world in a New Zealand population of bottlenose dolphins (Tursiops truncatus), with 3.4 aquatic steps between animals2. Now the Scottish dolphins back up the result3. His work is published in the October edition of the Journal of Animal Ecology.

The life aquatic

Lusseau thinks that the small-world nature of dolphin society may come from evolution favouring efficient information transfer. "It maximizes the opportunity for something to be passed from one individual to another, and minimizes the cost of creating the necessary links," he says.

Such a social network might be good for quickly spreading news about a food source, for example. But Lusseau admits that we cannot know for sure until our understanding of dolphin communication improves. "We have hints that this social structure is explained by information transfer but we don't have any hard evidence."

The thing to do, he says, would be to improve technology so we could tell which dolphin in a group was making sounds, so gossip could be tracked from one group to another.

Beyond that, Lusseau plans to track the social lives of other animals. "Baboons have fluid societies and perhaps we would expect them to be small worlds," he says.

Such work is thought to help us better understand the social fabric of animal societies. "It is a bit of a fad," says Stanley Wasserman, a sociologist at Indiana University, Bloomington. "But these networks can help us describe populations."


  1. Dodds P. S., Muhamad R.& Watts D. J. Science, 301. 827 - 829 (2003).
  2. Lusseau D.Proceedings of the Royal Society Series B published online, doi:10.1098/rsbl.2003.0057 (2003).
  3. Lusseau D., et al. Journal of Animal Ecology, published online, doi:10.1111/j.1365-2656.2005.01013.x (2005).


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