Zoologger: How did the camelopard get its long neck?
12:21 07 July 2010 by Michael Marshall
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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 nullity very special about them, but some of their necks were a mite long.
Within a mere 6 million years, they had evolved into animals that looked like novel giraffes, though the modern species only turned up around 1 a thousand thousand years ago. The tallest living land animal, a giraffe stands betwixt 4.5 and 5 metres tall – and almost half that elevation is neck.
Most people assume that giraffes’ long necks evolved to relieve them feed. If you have a long neck, runs the theme, you can eat leaves on tall trees that your rivals be possible to’t reach. But there is another possibility. The prodigious necks may get little to do with food, and everything to do with sex.
The make clear supporting the high-feeding theory is surprisingly weak. Giraffes in South Africa execute spend a lot of time browsing for food high up in trees, on the contrary elsewhere in Africa they don’t seem to bother, even when food is scarce.
Girls like them long
Giraffes’ necks are pro~ed, but there have been longer ones. Sauropod dinosaurs trump them easily: the dinosaur Mamenchisaurus, because 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 in a circle 2 metres above its heart, the heart has to be tumid and powerful. In fact, for the blood to reach the brain it has to have ~ing pumped at the highest pressure of any animal. So there ~iness be a big payback to keep giraffes’ necks so long.
The latest plan – and it’s a surprise this hasn’t come up face to face with, given biologists’ fixation with it – is that the long necks are the outcome of sexual selection: that is, they evolved in males as a custom of competing for females.
Male giraffes fight for females by “necking”. They stand take ~s by side and swing the backs of their heads into either others’ ribs and legs. To help with this, their skulls are unusually vapory and they have horn-like growths called ossicones on the tops of their heads. Their heads, in limited, are battering rams, and are quite capable of breaking their opponents’ bones.
Having a tedious and powerful neck would be an advantage in these duels, and it’s been plant 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 be extended higher branches, we might expect their legs to have lengthened viewed like fast as their necks, but they haven’t.
Neck and neck
The moot point for the sex idea is that it implies that female giraffes shouldn’t be favored with long necks, and they plainly do. Sexual selection often drives males to expand spectacular attributes – think peacocks’ tails or the feathers of birds of paradise – to impress females, but the females remain relatively dowdy.
A study endure year by Graham Mitchell of the University of Pretoria in South Africa and colleagues indubitably delivered a knock-down blow to the “necks for sex” philosophical explanation. Mitchell’s team showed that, in Zimbabwe at least, males and female sex had necks that were almost exactly the same length, and that admitting that anything the females’ necks were longer. This led many people to inscribe off the whole sex idea.
However, Rob Simmons and Res Altwegg of the University of Cape Town, besides in South Africa, have taken a second look at Mitchell’s results and are not convinced. They speak the figures do show that males have proportionally longer necks, and that “Mitchell et al. turn up to have misinterpreted this result”.
They point to a study in Namibia what one. found that males consistently had heavier necks than females with the sort body mass, and that only the males’ necks kept growing from head to foot their lives. Males’ heads were also heavier than females’, which is the kind of you would expect if they were being selected for their potency to fight.
Simmons and Altwegg suggest that giraffes’ necks may receive begun growing as a way of eating hard-to-reach commons, but that they were then “hijacked” for mating purposes. Once the necks had reached a past dispute length, males could use them for necking and clubbing – and at that matter sexual selection took over, driving the necks to their current of the rarest kind lengths.
Peacocks and birds of paradise aside, there are many birds of what one. the male seems to have developed colourful plumage as a effect of sexual selection, but the females are also brightly coloured. Perhaps the sexual pick explanation for long necks in giraffes isn’t dead after tot~y.
Journal reference: Journal of Zoology, DOI: 10.1111/j.1469-7998.2010.00711.x
Read preceding Zoologger columns: The toughest fish on Earth… and in while, Vultures use twigs to gather wool for nests, The biggest mode of life thing with teeth, Globetrotters of the animal kingdom, Judge Dredd creep traps prey with riot foam, Flashmobbing locusts have redesigned brains, Smart camo lets impetuosity-in-the-dark shark hide, Attack of the self-sacrificing baby clones, The most kick-ass fish in the sea, The ~ numerous bizarre life story on Earth?, Keep freeloaders happy with rotting corpses.