Networking in the fish tank: the humble guppy always remembers his friends

Beware of offending your pet fish - it may remember the insult after all. Far from possessing a three-second memory, as is widely believed, scientists have found that fish form social groups and are able to remember their partners.

A team from Leeds and Bath universities has observed guppies living in complex networks, actively choosing - and recalling - social partners.

Darren Croft, a researcher, said these networks were highly structured and appeared to conform to the "small world" theory in humans. This is the idea that everyone in the world can be connected to everyone else through a chain of a few intermediaries.

The phrase "six degrees of separation" was developed to describe this phenomenon in the 1960s, when experiments suggested that in the US most people could be connected to each other by as few as six people.

Dr Croft and his colleagues, who report their findings in the Proceedings of the Royal Society of London journal, found any two fish could be connected through about two others. He said: "Understanding the structure of social networks is of extreme importance. For example, social network analysis has been used to model the spread of diseases such as HIV in human populations, with diseases that are socially transmitted spreading much faster in networks with 'small world' characteristics.

"An understanding of the structure of social networks in wild animal populations may allow us to make predictions about the spread of diseases, which may be particularly important for endangered and threatened species."

Dr Croft and his team found guppies appeared to choose preferred partnerswithin their social network.