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How giraffe got long neck not just tall story

 
 
Reply Fri 29 Dec, 2006 01:36 am
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How giraffe got long neck not just tall story
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Walter Hinteler
 
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Reply Fri 29 Dec, 2006 01:37 am
The American Naturalist:
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Elissa Z. Cameron (University of Pretoria) and Johan T. du Toit (University of Pretoria and Utah State University), "Winning by a neck: tall giraffes avoid competing with shorter browsers"
Giraffes are well known for their unusual height, and they generally feed high in the tree canopy, above the height that other herbivores can reach. Text-books use the giraffe's elongated neck as an example of evolution by natural selection caused by competition between different species for the same leaves. Research by Elissa Cameron and Johan du Toit at the Mammal Research Institute in the Department of Zoology and Entomology at the University of Pretoria tested whether foraging competition could explain why giraffes feed mostly on leaves high in trees despite being able to feed at lower levels as well. Giraffes receive more leaves per bite by foraging high in the tree, which could be because small browsers eat some of the leaves at lower heights, or because more leaves grow at higher levels. Fences were built around trees in the greater Kruger National Park ecosystem to stop these smaller browsers from eating the leaves. After a complete growing season there was the same amount of leaves throughout the fenced trees, demonstrating that leaves were depleted by small browsers. Therefore, competition appears to drive the giraffes to forage high in the trees. "This provides the first real experimental evidence that the long neck of the giraffe might have evolved as a consequence of competition," states Elissa Cameron, "which provides support for the previously untested text-book example of natural selection."

http://i10.tinypic.com/2iitnd3.jpg
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aidan
 
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Reply Fri 29 Dec, 2006 05:30 am
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Giraffes are the tallest mammals on Earth, with some males standing 19 feet high. Fossils show they evolved from a deer-like ancestor with a shorter neck.


I wonder what triggered and/or made the evolutionary change from shorter neck to the longer neck necessary.
eg. Did the acacia trees that provided their diet of leaves begin growing taller due to climate change (increased rainfall for instance) over a period of generations or did they originally feed on different vegetation which grew lower to the ground?
I guess there's no way to gather that information from fossils, but it'd be interesting to trace the chronology of the change with what is known about available plant life during the same period or era in which the evolutionary change was taking place.
This is fascinating Walter.
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farmerman
 
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Reply Fri 29 Dec, 2006 06:28 am
That has got to be one of the lamest and dummess assed ezperiment Ive ever read about. Selectively fence out smaller browsers and then claim that your experiment was a success. Thats total sludge. They should canopy fence the tree tops also..
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aidan
 
  1  
Reply Fri 29 Dec, 2006 06:34 am
But what do you think about my question farmerman? Aren't you some kind of plant or animal evolutionist-leaning scientist of some sort? Do you have any ideas about why a giraffe's neck has lengthened as it's evolved?
*Sorry to be so vague, but I've just picked up little bits and pieces from your posts in the whole ID/Evolutionist debate thread.
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farmerman
 
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Reply Fri 29 Dec, 2006 06:49 am
genetics of the giraffe clan includes a number of related species including theokapi. The fossil record of the features of this clan are also pretty good. There were about three breakoffs with very similar skeletal structures and tyhe fused cannon bone (which, as much as they look for more specialty features, the feet in artiodactyles are always the first common evolutionary structure). The fossil record coincides with a lot of climate changes about 15 to 20 my ago. This coincides with the rise of savannahs and the kinds of border forests where such features as a long neck would have conceivably been selected for.

My problem with the experiment is tjhat it takes some basic givens and doesnt really test anything of value. A giraffe has a heck of a problem drinking but it does get into that weird squat to sip water. So theres really no Evidence that , becuase the giraffe eats at higher leaf levels, this is proof of anything of evolutionary significance.
The basic giraffe design is accomplished. Now look at the okapi.
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aidan
 
  1  
Reply Fri 29 Dec, 2006 07:10 am
Yeah - Thanks - I was thinking that it had to have something to do with the availablity and placement of dietary staples.

I also found this which addresses what you said about the whole drinking issue as well as some other interesting points: http://natureinstitute.org/pub/ic/ic10/giraffe.htm

I thought the points about total surface area of the animal in terms of distributing and withstanding the effects of what can be the extreme heat of their habitat and perspective when looking at different parts of the animal separately and with relation to purpose were interesting points as well.
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Walter Hinteler
 
  1  
Reply Fri 29 Dec, 2006 07:39 am
farmerman wrote:
That has got to be one of the lamest and dummess assed ezperiment Ive ever read about. Selectively fence out smaller browsers and then claim that your experiment was a success. Thats total sludge. They should canopy fence the tree tops also..


That might be, I didn't read the full report but only the abstract. (Via same link as given above.)

Quote:
ABSTRACT: With their vertically elongated body form, giraffes generally feed above the level of other browsers within the savanna browsing guild, despite having access to foliage at lower levels. They ingest more leaf mass per bite when foraging high in the tree, perhaps because smaller, more selective browsers deplete shoots at lower levels or because trees differentially allocate resources to promote shoot growth in the upper canopy. We erected exclosures around individual Acacia nigrescens trees in the greater Kruger ecosystem, South Africa. After a complete growing season, we found no differences in leaf biomass per shoot across height zones in excluded trees but significant differences in control trees. We conclude that giraffes preferentially browse at high levels in the canopy to avoid competition with smaller browsers. Our findings are analogous with those from studies of grazing guilds and demonstrate that resource partitioning can be driven by competition when smaller foragers displace larger foragers from shared resources. This provides the first experimental support for the classic evolutionary hypothesis that vertical elongation of the giraffe body is an outcome of competition within the browsing ungulate guild.
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