The traditionally accepted explanation, proposed by Charles Darwin 130 years ago, is that it is in order to feed on leaves that are out of reach of other browsing animals. But Dr Rob Simmons, a senior conservation officer in Namibia's Ministry of Wildlife, Conservation and Tourism, has a new explanation: male giraffe indulge in 'necking' - fighting with their necks and heads - in order to win females.
Five metres (15ft) in height and weighing more than a ton, giraffes are the world's tallest animals. Because their reach extends more than twice as far as any other browser, it is not surprising that scientists assumed that long necks evolved solely to reduce competition for food. Dr Simmons disputes this as giraffe feed at an average height of two metres (6ft), although they could reach much higher.
Neither do they eat different food plants from other animals. During the winter, when competition for food is most intense, they browse on low-growing raisin bushes that are shared by antelope such as the lesser kudu and the gerenuk. Female giraffes spend more than half their time feeding with their necks below shoulder height and they even feed faster in this position, leading Dr Simmons to suggest that elongated necks may be positively disadvantageous for them.
Fossil evidence does not support Darwin's hypothesis. The largest extinct giraffe, Samotherium, had legs nearly as long as its modern descendant, but its neck was much shorter. If feeding competition were important, evolution would have led to longer legs as well a longer necks. Studies of giraffes reveal that the neck vertebrae have elongated by 130 per cent compared with vertebrae lower down the spine. 'Only in giraffe do we see this rather sudden and disproportionate growth in neck length and the appearance of relatively short, stout horns,' says Dr Simmons.
Instead, he believes that the long neck evolved as a weapon against other males. When giraffes compete for females they stand side by side and violently swing their neck and head at each other. The momentum of the blow is crucial, and the small, blunt horns are a deadly armament as they concentrate the force, sometimes puncturing the neck and spinal column of the adversary. One giraffe can knock another out by 'necking', and the animals are not reserved about kicking an opponent when he is down.
The severity of these attacks has led to the evolution of a more passive dominance display where the nose is pointed skywards, perhaps because this increases the apparent length of the neck. However, full-grown bulls dispense with most of the preliminaries and go straight on to the attack.
The male neck seems to have been adapted for fighting - it is longer and thicker than the female neck by more than a third. Their skulls are also stronger and up to three-and-a-half times as heavy. Females have not been reported to attack one another.
The male pays a price for his longer, thicker neck: handicapped by it, his maximum speed is 56km per hour (34mph), which is unusually slow for such a long-legged animal. This has led to a high mortality rate as the number of males killed by lions is nearly twice that of females.