What is it about March Of The Penguins that has made it so successful? The film opened quietly in the States on four screens in late June 2005. Four months later it had become the second most successful documentary of all time, after Fahrenheit 9/11, with the emperors waddling their way to a staggering $100 million worldwide. Industry insiders have been gobsmacked. "It's a nature film! A movie about birds... and they don't even fly!" you hear them say.

But the life cycle of these famous inhabitants of the frozen South has all the ingredients of great drama - survival, tragedy, new life, rites of passage and death. In his voice-over narrative, Morgan Freeman calls it "a love story". That's stretching it. although it would take a hard heart not to soften at the tender courtship ritual and the inexplicable bond that develops between male and female emperors, but the "mythic" narrative does tend to over project human traits onto the penguins. It's more a story of life and survival. The march of the title refers to the arduous 70 mile trek that the emperors make each April from their feeding area by the sea to their traditional inland breeding ground. Nature clearly did not design penguins for walking, with their clumsy webbed feet and heavy set bodies, and it is a funny thing to see the long columns of birds waddling across an empty, white landscape. In their slick tuxedoed coats, they could almost be a column of wedding guests stranded in the snow, until they drop down and start belly surfing across the icy wasteland.

As well as humour, the darker survival theme is balanced out with many shots of the penguins being cute, particularly when males and females pair up. The emperors have many interesting quirks of nature. They are monogamous for a year, have a baby chicken and then part their ways. When the female lays an egg, it is the male who will store it in a furry pouch until it is born. The delicate and awkward act of exchanging this egg is well documented. Facing each other, the female rolls the egg from a furry fold in her abdomen to the male who balances it on his toes and slips his coat over it to keep it warm. During this dangerous procedure some eggs roll away, leaving the forlorn parents to watch as the shell cracks and freezes over.

The females, now exhausted and hungry, make the long, dangerous march back to the sea, while the males huddle in a scrum against the blizzards. There they stand in the wintry perma night, taking it in turns to enjoy the warmth in the middle of the huddle. Amazingly, all this time, the penguins balance their egg on their feet. As daylight returns, the chicks start hatching, and the mothers, now fattened on fish, make the trip back to take over from the starving males. This marching to and from breeding ground to feeding ground continues as the days grow longer, until the ice draws back to within close range of the adolescent chicks, who now must take their first plunge into the sea.

It is an amazing story of Darwinian survival and vividly captured by French director Luc Jacquet and his team in exquisite photography. The penguins themselves are surprisingly social creatures and not used to humans, so the cameramen are able to get up close and intimate, even with the downey chicks.

Perhaps, in the interests of keeping the film family friendly, it doesn't seem as bloodthirsty as some of our own fine wildlife programmes, such as David Attenborough's series. Nonetheless, you do get a strong sense of the environment's hostility, as you watch distraught parents arched over the frozen corpses of dead chicks and older penguins left to die in the ice-fields.

The narrative, which Freeman delivers in his best fireside manner, may serve up too much cheese at times, but it does ensure that the connection with the penguins is maintained at an emotional level. The narrator also readily admits to not having all the answers. And thankfully, the producers decided against going the way of the French version, where penguins were given human voices.

There's also a noticeable absence of information about how their numbers are threatened by global warming and man's impact on its habitat. However, as you marvel at nature's strange design, you will understand the precariousness of these fascinating creatures' existence.