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  #1  
Old 01-12-2008, 03:18 AM
Indian Indian is offline
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giraffes and long necks..

If the long necks of giraffes evolved since they needed to graze leaves at higher levels, why didn't other animals' necks also evolve ??

Surely, the basic reason ( probably taller trees / no grass) would have been the same for all animals at that time?? Or is there some other reason ??

Wikipedia article is not much helpful ..
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  #2  
Old 01-12-2008, 04:02 AM
Sage Rat Sage Rat is online now
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1) Evolution isn't intelligent. A cockroch can't look at me and go, "Opposable thumbs! Why didn't I think of that? " and then proceed to evolve to have opposable thumbs. As humans with an understanding of evolution, we could theoretically set up laws to breed the human race towards a certain goal, but this is beyond the capabilities of all other animals on the planet.

2) There is no "right" answer. Longer necks allowed giraffes to get those leaves, but climbing ability let monkeys get those leaves too.

3) Market Forces. Giraffes may just have gotten there first, at which point any other animal wouldn't have the ability to enter the race since as the guy in the back, they can't compete with the giraffe. They still have to eat, so trying to eat the stuff that's already been et off the trees by the herd of giraffes over yonder is just impossible.

Overall, when there's a niche of food available, some species will drift in (evolve in) until the supply of food equalises with the rate at which the species are eating it. Any more species won't be able to move in since it would cause famine among all the species in that niche, including the new guy.

Famine is the great balancing factor in over-population.
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  #3  
Old 01-12-2008, 04:24 AM
Pullet Pullet is offline
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Quote:
Originally Posted by indian
If the long necks of giraffes evolved since they needed to graze leaves at higher levels, why didn't other animals' necks also evolve ??
Because the giraffes were already taking up that ecological niche.

Quote:
Originally Posted by indian
Surely, the basic reason ( probably taller trees / no grass) would have been the same for all animals at that time?? Or is there some other reason ??

Wikipedia article is not much helpful ..
I think you're falling into a trap a lot of people find when they first start studying evolution. Namely, creatures don't change their genetics to meet new environments; the ones who don't have the genetics to survive well in the new environment die and the ones who already have genetics that let them survive do and propagate.

Let me explain using your example:

Proto-giraffes had much shorter necks than modern ones, but within the population there was a range of neck lengths. Some had necks 2'3 inches long, some had 2'4", some were crazy long at 2'9" (Obviously, these are fake numbers, but play along). If you were to graph out the neck length of each proto-giraffe, it would make a bell curve, with most of the proto-giraffes having a neck of 2'4", but a couple odd balls having really short necks and a couple having really long 2'9" necks.

Now, as it happens, something in this environment shifted such that the food sources close to the ground were harder to get than ones high up. You have a couple of theories, but what is more likely is that there were already other animals competing for leaves and grass lower down.

The couple of proto-giraffes with crazy long 2'9" necks are going to town because they get to feast on leaves whenever they want. Since they didn't have to fight with the others over food, they have more energy for finding and winning mates, so they make more babies. If you made your graph of neck lengths now, it would still make a bell curve, but the exact numbers will have shifted such that average length is 2'7", there's one guy with a 2'4" neck and a couple guys with crazy long 3' necks.

Bingo, evolution.

ETA: Sage Rat beat me to it
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  #4  
Old 01-12-2008, 05:11 AM
Gymnopithys Gymnopithys is offline
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If the long necks of giraffes evolved since they needed to graze leaves at higher levels...

Or did giraffes graze at high levels because they had long necks ?
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  #5  
Old 01-12-2008, 06:29 AM
Barrington Barrington is offline
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I'm not sufficiently savvy to have an opinion of my own on this, but this article by British evolutionary biologist Darren Naish discusses the issue, and makes for an interesting read.
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  #6  
Old 01-12-2008, 11:31 AM
Zebra Zebra is offline
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Long necks worked for Giraffes, other things worked for other species. There is no single way to 'adapt' to a situation. Some animals probably moved to locations better suited to what they were, some just ate the giraffes.
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  #7  
Old 01-12-2008, 11:40 AM
Giraffe Giraffe is online now
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Quote:
Originally Posted by indian
Surely, the basic reason ( probably taller trees / no grass) would have been the same for all animals at that time?? Or is there some other reason ??
We spit on all the leaves we don't eat, so no other animals would want them.
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  #8  
Old 01-12-2008, 03:43 PM
Polycarp Polycarp is offline
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Take note that while the animals called "giraffe" in everyday parlance are the long-legged, long-necked treetop leafeaters, there were (and in one case still is) a variety of animals in the Giraffidae, not all of which were adapted for upper-storey dining.

The okapi lives in dense forest, largely in the Congo basin. It is a giraffid, but built rather short and stocky, somewhat reminiscent of "What would you get if you crossed..." with the two progenitors being giraffe and tapir.


The Sivatherium was another giraffid, evidently adapted to fill the moose niche in the Old World during the Pleistocene. It resembled a somewhat larger okapi with horns (not antlers) looking vaguely like what you might find on Bullwinkle T. Moose (as opposed to the tined antlers of real moose). There is some evidence that the Sivatheres survived until the beginnings of civilization; statues looking like slightly-stylized reprsentations of a sivathere have been found in Sumeria.
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  #9  
Old 01-13-2008, 12:21 AM
t-bonham@scc.net t-bonham@scc.net is offline
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Quote:
Originally Posted by Gymnopithys
If the long necks of giraffes evolved since they needed to graze leaves at higher levels...

Or did giraffes graze at high levels because they had long necks ?
Neither is correct.

Regarding the second, the fossil record clearly shows that giraffes started out with shorter necks, and ended up with longer ones.

Regarding the first statement, the wording of the statement tends to be misleading. A more accurate way to state it would be:
Quote:
Giraffes with long necks were able to graze leaves at higher levels, thus they survived & reproduced during hard times.
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  #10  
Old 01-13-2008, 08:31 AM
Zebra Zebra is offline
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Wouldn't the long neck help spot predators? Kind of like a look out tower.
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  #11  
Old 01-13-2008, 09:05 AM
Moirai Moirai is offline
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Yes, which would be a selection factor. IIRC, the steps in natural selection are these-

1. More individuals will be born than can survive.

2. Individuals with traits that are better suited to their environment are more likely to survive than those without those traits.

3. Individuals that survive to sexual maturity will pass on their traits.

4. Over time, the whole population will end up with these traits.

"Survival of the fittest" is a terribly misunderstood term. Try and think of it as "best suited to their particular environment." Then it will be easier to understand how smaller, slower animals can still triumph in their environment.
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Last edited by Moirai; 01-13-2008 at 09:06 AM..
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  #12  
Old 01-13-2008, 01:30 PM
Darwin's Finch Darwin's Finch is offline
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Quote:
Originally Posted by EJsGirl
Yes, which would be a selection factor. IIRC, the steps in natural selection are these-

1. More individuals will be born than can survive.

2. Individuals with traits that are better suited to their environment are more likely to survive than those without those traits.

3. Individuals that survive to sexual maturity will pass on their traits.

4. Over time, the whole population will end up with these traits.
Natural selection really isn't a series of steps. It's a logical deduction, based on key points: 1) more individuals are produced than can survive (as you noted); 2) all orginsms vary, such that each individual will typically be distinct in some way from its brethren; 3) at least some of this variation is hereditary.

Now, assuming those facts to be true, then natural selection can be formulated as follows: if only some offspring can survive, then those which vary in such a way as to be better suited for the particular environment in which they find themselves will thus be more likely, on average, to survive. Those individuals will thus tend to leave more offspring, and the average composition of the population will then trend toward more individuals possessing those more favorable, hereditary traits.

It is key to understand that natural selection is a probability, not an inevitablity; the race is not always to the swiftest. The bestest and the stongest can still be squashed by a rockslide, and weaklings can still get lucky.


As for the OP, as has been noted, the key to survival is not always to outcompete other critters. Sometimes, simply finding a new food source that isn't already heavily harvested can work to one's advantage. This is the path taken by giraffes; rather than become more eficient at mowing grass or low browsing, they became better adapted at high-browsing -- going after the tasty leaves higher up, the ones the rest of the herds couldn't reach. Having a slightly longer neck was an advantage is such a situation (especially when the food lower down was sparse), and those who could, indeed, reach those tasty treats were the ones more likely to pass on their long-necked genes. Being long-necked comes with other advantages, too, such as being better able to spot predators.

Other animals may not have evolved long necks for a variety of reasons (though some others did, notably camels and sauropods). Some could have been more constrained in terms of the degree to which neck lengths could vary, some had put energy and efort into becoming more efficient at what they were already doing, others may have just moved on to a different location. The beauty of evolution is that it isn't "one size fits all"; there can be multiple solutions for any given problem.

Note also that long, prehensile tongues evolved in giraffes as well, the better to get at acacia tree leaves while avoiding the thorny spikes that the trees had evolved to prevent those leaves from being eaten in the first place!
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  #13  
Old 01-13-2008, 05:45 PM
t-bonham@scc.net t-bonham@scc.net is offline
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Quote:
Originally Posted by Zebra
Wouldn't the long neck help spot predators? Kind of like a look out tower.
Giraffes don't have many natural predators. They are too big for the small canines (hyenas, jackels) to be much of a threat and even the big cats like the lions tend to hesitate, since a giraffes' legs can kick a long ways, and can deliver a bone-breaking, disabling injury.

Generally, only young calves and sick or old giraffes are in danger from predators.

But the long necks do assist in looking for other trees to eat.
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  #14  
Old 01-14-2008, 09:55 AM
Flander Flander is offline
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Why is a giraffe's neck so long?


SPOILER:
Because its head is so far away from its body!


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  #15  
Old 01-14-2008, 12:31 PM
Sal Ammoniac Sal Ammoniac is offline
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Quote:
Originally Posted by Sage Rat
Evolution isn't intelligent. A cockroch can't look at me and go, "Opposable thumbs! Why didn't I think of that? " and then proceed to evolve to have opposable thumbs.
File under "thank heaven for small mercies."

Quote:
Originally Posted by Darwin's Finch
Note also that long, prehensile tongues evolved in giraffes as well, the better to get at acacia tree leaves while avoiding the thorny spikes that the trees had evolved to prevent those leaves from being eaten in the first place!
Interestingly, it now appears that the acacia trees need to be regularly grazed by giraffes and elephants. Article here.
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