Thursday, 14 May 2009
Did you know?
For centuries experts have argued over how the giraffe got its long neck.
Some said it helped the giraffe feed on leaves other animals cannot, while some suggested it evolved as a consequence of giraffes evolving long legs.
But evidence for such ideas remains flimsy. Now one of the more recent hypotheses has also bitten the dust.
Giraffes did not grow elaborately long necks as a sexual signal, scientists have shown, leaving its origins a mystery.
In the journal Zoology, Professor Graham Mitchell of the University of Wyoming, in Laramie, US, and colleagues Professor John Skinner and Dr S J van Sittert of the University of Pretoria in South Africa report that there is still no consensus on the origin of the giraffe's neck.
The theory which most support, they say, is that the neck confers a feeding advantage, allowing the giraffe to reach leaves beyond more numerous smaller browsers such as gazelles or antelope.
Research has shown that having a long neck does confer an advantage when the lower leaves of trees have already been eaten, while it also helps giraffes reach more inaccessible leaves in the centre of low trees.
But giraffes tend to prefer particular types of leaves rather than leaves at particular heights, suggesting that competition for different leaves may not have naturally selected strongly for longer necks.
Another hypothesis is that giraffes evolved longer legs to run away from predators and needed an equally long neck to reach the ground to drink.
Sexually attractive?
More recently, another popular idea emerged: that sexual selection, rather than natural selection, drove the evolution of the giraffe's neck.
The idea is that down the generations, males evolved ever longer necks to dominate rivals for the affections of female giraffes. The fact the male giraffes uniquely wrestle for dominance by "necking" and "head clubbing" one another, with males with the longest necks and heaviest heads tending to win, has been forwarded as evidence to support the hypothesis.
So Professor Mitchell and his colleagues decided to put the sexual selection hypothesis to the test by examining 17 male and 21 female giraffes.
If long necks were a sexually selected trait, they expected to find a number of things:
Long necks should be more exaggerated in males than females
They should evolve to be bigger in size more than other parts of a giraffe's body
They should confer no immediate benefit to survival, and may come at a cost
Their results didn't support any of these propositions.
They could find no significant differences in the relative size of male or female necks.
What's more, although giraffes do invest more in growing their necks than other body parts as they grow, both sexes do so equally.
And there is little evidence that a large neck is costly to male giraffes, as generally male giraffes are no more vulnerable to predators than females.
It will never be possible to conclusively prove what advantages selected for longer necked giraffes, say the researchers.
But they can say that any advantages that were gained don't appear to have been sexual in nature. "Better explanations for neck elongation must be sought elsewhere," they write.
http://news.bbc.co.uk/earth/hi/earth_news/newsid_8050000/8050298.stm
Did you know?
For centuries experts have argued over how the giraffe got its long neck.
Some said it helped the giraffe feed on leaves other animals cannot, while some suggested it evolved as a consequence of giraffes evolving long legs.
But evidence for such ideas remains flimsy. Now one of the more recent hypotheses has also bitten the dust.
Giraffes did not grow elaborately long necks as a sexual signal, scientists have shown, leaving its origins a mystery.
In the journal Zoology, Professor Graham Mitchell of the University of Wyoming, in Laramie, US, and colleagues Professor John Skinner and Dr S J van Sittert of the University of Pretoria in South Africa report that there is still no consensus on the origin of the giraffe's neck.
The theory which most support, they say, is that the neck confers a feeding advantage, allowing the giraffe to reach leaves beyond more numerous smaller browsers such as gazelles or antelope.
Research has shown that having a long neck does confer an advantage when the lower leaves of trees have already been eaten, while it also helps giraffes reach more inaccessible leaves in the centre of low trees.
But giraffes tend to prefer particular types of leaves rather than leaves at particular heights, suggesting that competition for different leaves may not have naturally selected strongly for longer necks.
Another hypothesis is that giraffes evolved longer legs to run away from predators and needed an equally long neck to reach the ground to drink.
Sexually attractive?
More recently, another popular idea emerged: that sexual selection, rather than natural selection, drove the evolution of the giraffe's neck.
The idea is that down the generations, males evolved ever longer necks to dominate rivals for the affections of female giraffes. The fact the male giraffes uniquely wrestle for dominance by "necking" and "head clubbing" one another, with males with the longest necks and heaviest heads tending to win, has been forwarded as evidence to support the hypothesis.
So Professor Mitchell and his colleagues decided to put the sexual selection hypothesis to the test by examining 17 male and 21 female giraffes.
If long necks were a sexually selected trait, they expected to find a number of things:
Long necks should be more exaggerated in males than females
They should evolve to be bigger in size more than other parts of a giraffe's body
They should confer no immediate benefit to survival, and may come at a cost
Their results didn't support any of these propositions.
They could find no significant differences in the relative size of male or female necks.
What's more, although giraffes do invest more in growing their necks than other body parts as they grow, both sexes do so equally.
And there is little evidence that a large neck is costly to male giraffes, as generally male giraffes are no more vulnerable to predators than females.
It will never be possible to conclusively prove what advantages selected for longer necked giraffes, say the researchers.
But they can say that any advantages that were gained don't appear to have been sexual in nature. "Better explanations for neck elongation must be sought elsewhere," they write.
http://news.bbc.co.uk/earth/hi/earth_news/newsid_8050000/8050298.stm
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