杏吧原创

Species:

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

Most people assume that giraffes鈥� 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鈥檛 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 , but elsewhere in Africa , even when food is scarce.

Girls like them long

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

Long necks come at a cost. Because a giraffe鈥檚 brain is around 2聽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 . So there must be a big payback to keep giraffes鈥� necks so long.

The latest theory 鈥� and it鈥檚 a surprise this hasn鈥檛 come up before, given biologists鈥� fixation with it 鈥� is that the long necks are the result of sexual selection: that is, they evolved in males as a way of .

Male giraffes . They stand side by side and . To help with this, their skulls are unusually thick and they have horn-like growths called on the tops of their heads. Their heads, in short, are battering rams, and are quite capable of breaking their opponents鈥� bones.

Having a long and powerful neck would be an advantage in these duels, and it鈥檚 been found that .

The 鈥渘ecks for sex鈥� 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鈥檛.

Neck and neck

The problem for the sex idea is that it implies that female giraffes shouldn鈥檛 have long necks, and they plainly do. Sexual selection often drives males to develop spectacular attributes 鈥� think peacocks鈥� tails or the feathers of 鈥� to impress females, but the females remain relatively dowdy.

A by Graham Mitchell of the in South Africa and colleagues apparently delivered a knock-down blow to the 鈥渘ecks for sex鈥� theory. Mitchell鈥檚 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鈥� necks were longer. This led many people to .

However, and of the University of Cape Town, also in South Africa, have taken a second look at Mitchell鈥檚 results and are not convinced. They say the figures do show that males have proportionally longer necks, and that 鈥淢itchell et al. appear to have misinterpreted this result鈥�.

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

Simmons and Altwegg suggest that giraffes鈥� necks may have begun growing as a way of eating hard-to-reach food, but that they were then 鈥渉ijacked鈥� for mating purposes. Once the necks had reached a certain length, males could use them for necking and clubbing 鈥� 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鈥檛 dead after all.

Journal reference:

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