Zoologger: How did the giraffe get its long neck?

By Michael Marshall

Species: Giraffa camelopardalis

Habitat: grasslands and open woodlands of sub-Saharan Africa

Around 15 million years ago, antelope-like animals were roaming the dry grasslands of Africa. There was nothing very special about them, but some of their necks were a bit long.

Within a mere 6 million years, they had evolved into animals that looked like modern giraffes, though the modern species only turned up around 1 million years ago. The tallest living land animal, a giraffe stands between 4.5 and 5 metres tall – and almost half that height is neck.

Most people assume that giraffes’ long necks evolved to help them feed. If you have a long neck, runs the argument, you can eat leaves on tall trees that your rivals can’t reach. But there is another possibility. The prodigious necks may have little to do with food, and everything to do with sex.

The evidence supporting the high-feeding theory is surprisingly weak. Giraffes in South Africa do spend a lot of time browsing for food high up in trees, but elsewhere in Africa they don’t seem to bother, even when food is scarce.

Girls like them long

Giraffes’ necks are long, but there have been longer ones. Sauropod dinosaurs trump them easily: the dinosaur Mamenchisaurus, for instance, had a neck over 9 metres long, four times the longest of giraffe necks.

Long necks come at a cost. Because a giraffe’s brain is around 2 metres above its heart, the heart has to be big and powerful. In fact, for the blood to reach the brain it has to be pumped at the highest pressure of any animal. So there must be a big payback to keep giraffes’ necks so long.

The latest theory – and it’s a surprise this hasn’t come up before, given biologists’ fixation with it – is that the long necks are the result of sexual selection: that is, they evolved in males as a way of competing for females.

Male giraffes fight for females by “necking”. They stand side by side and swing the backs of their heads into each others’ ribs and legs. To help with this, their skulls are unusually thick and they have horn-like growths called ossicones on the tops of their heads. Their heads, in short, are battering rams, and are quite capable of breaking their opponents’ bones.

Having a long and powerful neck would be an advantage in these duels, and it’s been found that males with long necks tend to win, and also that females prefer them.

The “necks for sex” idea also helps explain why giraffes have extended their necks so much more than their legs. If giraffes evolved to reach higher branches, we might expect their legs to have lengthened as fast as their necks, but they haven’t.

Neck and neck

The problem for the sex idea is that it implies that female giraffes shouldn’t have long necks, and they plainly do. Sexual selection often drives males to develop spectacular attributes – think peacocks’ tails or the feathers of birds of paradise – to impress females, but the females remain relatively dowdy.

A study last year by Graham Mitchell of the University of Pretoria in South Africa and colleagues apparently delivered a knock-down blow to the “necks for sex” theory. Mitchell’s team showed that, in Zimbabwe at least, males and females had necks that were almost exactly the same length, and that if anything the females’ necks were longer. This led many people to write off the whole sex idea.

However, Rob Simmons and Res Altwegg of the University of Cape Town, also in South Africa, have taken a second look at Mitchell’s results and are not convinced. They say the figures do show that males have proportionally longer necks, and that “Mitchell et al. appear to have misinterpreted this result”.

They point to a study in Namibia which found that males consistently had heavier necks than females with the same body mass, and that only the males’ necks kept growing throughout their lives. Males’ heads were also heavier than females’, which is what you would expect if they were being selected for their ability to fight.

Simmons and Altwegg suggest that giraffes’ necks may have begun growing as a way of eating hard-to-reach food, but that they were then “hijacked” for mating purposes. Once the necks had reached a certain length, males could use them for necking and clubbing – and at that point sexual selection took over, driving the necks to their current extreme lengths.

Peacocks and birds of paradise aside, there are many birds of which the male seems to have developed colourful plumage as a result of sexual selection, but the females are also brightly coloured. Perhaps the sexual selection explanation for long necks in giraffes isn’t dead after all.

Journal reference: Journal of Zoology, DOI: 10.1111/j.1469-7998.2010.00711.x

Read previous Zoologger columns: The toughest fish on Earth… and in space, Vultures use twigs to gather wool for nests, The biggest living thing with teeth, Globetrotters of the animal kingdom, Judge Dredd worm traps prey with riot foam, Flashmobbing locusts have redesigned brains, Smart camo lets glow-in-the-dark shark hide, Attack of the self-sacrificing child clones, The most kick-ass fish in the sea, The most bizarre life story on Earth?, Keep freeloaders happy with rotting corpses.