Why are these men going topless in winter?

It's all to do with machismo.

They look as though they could have invented a new soccer chant... "Come and have a go if you think it's cold enough". These bare-chested lads pictured at a football match played on one of the coldest nights of the year are exponents of the latest "cool" trend on the terraces: passive machismo.

Dozens of young men are demonstrating their toughness by standing stripped to the waist in the bitterest weather at away matches. The club with the fans out to give the opposition the cold shoulder is Leeds United.

Rogan Taylor, Director of the Football Research Unit at the University of Liverpool, has his own theories about what drives these fans to make these teeth- chattering gestures of defiance.

"On the face of it this is just another example of primate posturing, the difference being that this is harmless, and I for one would much rather see them doing this than punching seven bells out of each other," said Mr Taylor.

He added: "This is quite fascinating. There is more than just posturing going on here, and it is quite possible this trend will spread. It is a rare form of passive machismo." He believes that the fans are using their bodies to articulate a serious point about the modern game.

"In a way they are a throwback to an earlier age when watching football was an altogether less comfortable experience. It recalls the days of the 50s and 60s when lads used to queue on a drizzling Tuesday night and it was considered positively effeminate to wear anything more than trousers and a T-shirt.

"These fans have just taken that culture to its logical conclusion. It is also significant that this is an `away' fans phenomenon as away supporters are traditionally the most committed of the dedicated club supporters."

Leeds fans were first spotted stripping in numbers on a bitterly cold Saturday in December at Middlesborough, soon after the Italian ace Fabrizio Ravanelli scored a goal and celebrated by pulling his shirt up over his head and running a lap of honour with arms outstretched.

"That is significant because it revealed that Ravanelli was wearing a vest," Mr Taylor said. "A lot of continental players wear a vest or undershirt. These fans are saying: `We don't wear vests. You foreigners may have fancy footwork but we've still got our bulldog spirit.'

"Leeds have always had a reputation for ferocious support. Visiting teams are often intimidated for the first few minutes when they step out on the turf at Elland Road. The fans there know they have a role to play. Together they can be as good as a twelfth player."

Nigel Pleasance, the Leeds United club secretary, said: "It looks very uncomfortable but they do not do any harm. I think it gives a lot of people something to smile about."

Dr Alan Swingewood, a senior lecturer in sociology at the London School of Economics, believes that the protest may be directed at the Premier League clubs. "This sort of primeval behaviour may be a commentary of the way that soccer has become so stratified of late, with hospitality boxes and ever increasing admission charges. They may want to get back to a more basic form of the game."

But Dr Simon Eassom, senior lecturer in the philosophy of sport at De Montfort University, Leicester, believes that the fans themselves may not be sure why they do it.

"There could be a number of reasons for this activity but it is clearly highly visual and as such serves to create a group identity," said Dr Eassom.

"Just as players now have their own unique way of celebrating after scoring a goal, recently I noticed a new routine among Arsenal players, who put their hands over their mouths as if stifling a giggle after scoring. The fans are the same, constantly searching for ways of marking themselves out from the opposition."

BBC Radio Five Live's Alan Green was commentating at freezing Selhurst Park last Tuesday when Leeds played Crystal Palace. "Personally, I regretted not wearing my thermals. It was a terribly cold night," he said.

"I applaud them because it's all done in the best humour. Five or six years ago it would have been a different story."