May 19 2009
Giraffe Necks
The story of the giraffe’s neck is a classic of high school biology textbooks. For this reason everyone “knows” that giraffes evolved longer necks in order to reach the leaves at the top of trees. However, this has never been clearly established and the real story is much more complex. There is, in fact, an enduring controversy over exactly what factors led to the elongation of the giraffe neck, highlighted by a recent study examining one hypothesis – sexual selection.
But before we get to that study, some background on giraffe necks.
The most obvious feature of the giraffe is its long neck. For some reason the evolution of the giraffe neck became the standard example in textbooks. Stephen J. Gould did a survey of biology textbooks and found that 100% used giraffe evolution as the example to distinguish Lamarckian evolution from Darwinian evolution (which itself is based upon a misconception of Lamarck’s career, but that’s another story). Meanwhile, Darwin did not use the giraffe’s neck as an example of natural selection, and regarded it as a speculative “just so” story.
Since Darwin there has been speculation and controversy over the evolution of the giraffe’s neck, but never any consensus. There is therefore a stark contrast between the scientific reality and the textbook fiction regarding giraffe evolution – but what’s new.
The standard (textbook, that is) story is that ancestral giraffes were selected for longer necks because that enabled them to reach leaves higher up in trees than other animals. Therefore in times of scarcity they would have access to food that other animals could not access, and this conferred a survival advantage. There is nothing wrong with this story, and it likely holds a kernel of truth. There is simply no evidence to support this particular selective pressure, and there are plausible competing hypotheses. Unfortunately the giraffe fossil record is sparse, so we cannot turn to fossil evidence to settle the question.
It is true that the giraffe’s reach does give them access to leaves in tall acacia trees. It also gives them access to leaves deep within trees. However, giraffes also feed off low bushes by bending their neck, and they do not show any preference for high or deep leaves during the dry season when food is scarce. So at least at present the long neck does not seem to be a significant hedge against starvation.
An alternate hypothesis is that the long neck evolved in response to the evolution of long legs, which themselves evolved for some other reason, such as speed in evading prey. Giraffes cannot support themselves with their knees bent. In order to drink water on the groun they must splay their forelegs (with knees straight) and then use their long necks to reach the ground. Therefore it is possible that necked elongate simply to keep up with the growing limbs.
Other hypotheses include the increase in skin surface areas for cooling, and increased head height to keep a better eye on predators.
Male giraffes also use their long necks in mating competition. They slam their necks against together to settle male rivalries. This could explains why males have slightly longer necks than females. This hypothesis was the focus of this latest study.
Giraffe researcher, Professor Graham Mitchell of the University of Wyoming, and colleagues addressed the sexual selection hypothesis by seeing if the giraffe’s neck meets the criteria for sexual selection: they should be exaggerated in the male, the neck should be out of proportion to other body parts and they should confer no other advantage, and may even be a detriment.
To be clear, it is possible for a feature to be selected for sexually but still have some other advantage. The criteria above are used only to demonstrate that sexual selective pressures are present. If other selective pressure are also present, that makes it more difficult to isolate sexual selection as a factor.
What Mitchell found was that the giraffe’s neck does not meet any of the above criteria. Male necks are longer than female necks, but only in the same proportion to overall body size. Males are bigger than females, but their necks are not propotionally longer.
Giraffes do invest more energy into developing their necks than other parts of their body, but there is no difference beetween the males and females. If males were being sexually selected for long necks, then males should invest more proportionally than females.
There also does not appear to be any detriment to the long necks for males – they are no more vulnerable to prey, for example. It is not possible to say that the long necks do not convey any other advantage – that is the very heart of the controversy.
It occurs to me, however, that all three criteria in the end derive from the same fact – male and female giraffe necks are proportionately similar, therefore males do not invest more energy in their neck nor do they suffer any more disadvantage from them. So one might argue that the first criteria being negative (no difference between males and females) automatically negates the other two. However, if the first criteria were positive (relatively longer necks in males) then the second and third would still be required, so they are not entirely redundant.
Conclusion
So after this study we are still left without any definitive answer to the question of what selective pressures lead to the giraffe’s long neck. This question may not be resolved unless and until we find some fossil evidence to examine. And even then, the fossils may not settle the question.
It is possible that the question cannot be answered because there is no single answer. The more we learn about evolution the more it is undestood that selective pressures are complex and inter-dependent, and there are forces at work in evolution other than natural selection – such as genetic drift and pre-adaptation.
Therefore perhaps all of the hypotheses regarding the evolution of the giraffe’s neck are true. Longer legged giraffes would have been able to run faster and see farther, but needed longer necks to then drink ground water. Meanwhile the longer necks extended access to food. Males later started banging their long necks together are part of their mating competition.
The whole giraffe system had to work together every step of the way, with each change opening up new possibilities and also creating its own selective pressures back on the whole system.
Why did giraffe ancestors head down the tall-necked path in the first place, while other African mammals did not? I chalk that up to the quirkiness and contingent nature of evolutionary history. The giraffe ancestor had a morphology which made the eventually evolution to its current long-necked form possible, perhaps even probable. Some set up circumstances, including the behavior of the ancestors themselves, then created a positive feedback loop of selective pressure sending them down the evolutionary path to tall necks.
Reverse engineering these circumstances may be as complex as watching pingpong balls bouncing around in one of those the air-chambers used to select numbers for the lotto, and then trying to explain why a particular number came up based upon all of the collisions that took place. In other words, there is a certain amount of chaos to the particular details of evolutionary history that therefore may forever defy detailed explanation.
But what we can define are the broad brushstrokes – the dominant factors at work. We can also expand our understanding of the many possible ways in which evolutionary changes occur, even though we cannot always determine which factors were present in any particular case.
Further, while Mitchell’s study is far from conclusive by itself, he demonstrates that we can apply scientific methods to questions of evolutionary history and can use data to infer which selective factors were more likely to be dominant. That may not lead to a simple and clean evolutionary tale (suitable for a high school textbook), but nature is notoriously insensitive to our desires and sensibilites.
43 Responses to “Giraffe Necks”
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Fascinating. Thanks for this. It reminds me of the Eskimo words for snow legend, which is still repeated frequently in newspapers, magazines, textbooks, and the like. I’ll make sure I remember these lessons the next time I am digging up examples of natural selection to tell my students.
There needs to be conservation plans for threatened giraffe species. Some subspecies only have one hundred or so of their African group living. To preserve the tears of the giraffe, that’s precious.
even ‘the obvious’ needs testing, and the acknowledgment
that conclusions are not always possible. this is an easy-
to-understand example of such.
a fun & popular example of productive empiricism is-
the America’s Test Kitchen show on PBS.
Skeptical cooking. Hey, its a start-
[...] May 19, 2009 — Skepdude READ THE FULL ENTRY AT “NEUROLOGICA” The story of the giraffe’s neck is a classic of high school biology textbooks. For this reason [...]
You wrote: “Why did giraffe ancestors head down the tall-necked path in the first place, while other African mammals did not?”
This could be a systems issue. There is a cool growing body of speculative research on why bird calls are the frequencies they are. This tries to see the forest soundscape as a competetive system. So while a robin and a cardinal aren’t necessarily competing for food, their songs are competing to be heard above the din of the rest of the animal communication. The theory suggests that a a cardinal evolves the unique frequency of its song not because that particular frequency is advantageous, but because any unique, non-interfering frequency is an advantage.
So over time, species of birds which find a unique, non-interfering frequency are better able to find each other for mating, find their children or parents and so are more successful as a species. The forest soundscape selects for unique frequencies.
So now I’m stepping out on my own speculative limb: perhaps giraffes found long necks advantageous while the other animals did not, not because long necks are better than short necks inherently, but because the African “neckscape” favors a variety of non-competing neck-lengths: long necked herbivores get the deep or tall leaves, shorter necked herbivores get the surface or low leaves.
Of course the big advantage of the long necks is that they can get both if they want, but the concept may be valid: that the African flora can only support one species at that particular neck length and giraffes just occupied that space first.
We could partially test this theory by killing off the rest of the giraffe population and waiting to see if something else fills this niche with their own long necks…
Very interesting post, and a good reminder that evolutionary changes do not occur in order to accomplish a specific goal, but rather they occur because of the impact of selective pressures.
I definitely remember the lessons in high school re: giraffe necks & Lamarck, and explicitly or not the lessons supported an ends-based view of evolution.
One other thing I would be interested in reading/hearing about is at what point in earth’s history were the most “difficult” or “implausible” developments occurring. What I mean by that is that it seems once there existed a fish (or fish-like) ancestor the development of all manner of sea & land vertebrates makes total sense. You have a good set of raw materials (body plan, organs, central nervous system, etc.) The same could be said for insects & other invertebrates, getting the first prototype makes the development of extant species much more likely
As much as evolution deniers harp on whales (even after the discovery of fossil explaining cetacean evolution), for me the initial development of multi-cellular life has always been much more amazing. And I feel pretty lucky to be living at a time when we have some pretty good answers (or at least plausible hypotheses) to explain this wondrous thing we call life. It is amazing what will happen given 3-4 billion years. You can even get a long-necked giraffe
The first thing a baby giraffe asks its mama is why do all the other kids have necks that are too short.
Great post, Steve.
The other additional point I wanted to add to this discussion is one of constraint, which is an important consideration when trying to understand natural selection and adaptation. The question that I have always found a bit more interesting relating to the long neck of the giraffe is not, “what is it for,” but “how the heck do they pump blood all the way up there?” Granted, the mechanics of pumping blood to a brain that is quite far away from the heart was mostly solved in other long necked hoofers like horses and camels, but to do so on the scale of a giraffe neck is really extreme. The point being that there are serious physical constraints on the vascular system of the giraffe that needed to be overcome in order for the neck to elongate. Often, when a trait or feature has an obvious downside, we figure that the benefits of the trait (or a trait that co-evolved with the trait of interest) must out weigh the cons. E.g., flashy colors in birds attract both mates and predators. There are always trade-offs, you can never seem to have your cake and eat it too.
Incidentally, it never ceases to amaze me how little science knows about the charismatic megafauna of the world. They are so big, but SO difficult to study in the wild, and the opportunities to do so are disappearing fast.
Actually Lamarck was correct in hypothesizing that evolution was purposeful, but as a human, he was thinking of purpose as a long term proposition, while evolution “thinks” of purpose in extremely short term increments.
Fascinating, as always, I always enjoy learning new thing from you, Dr. Novella
This is totally of topic, and maybe the wrong way to bring this to your attention, but I just read something interesting that might benefit from your skeptical eye. A blog posted about the human corpses showing a slowing rate of decomposition (http://www.blog.sethroberts.net/2009/05/16/how-fast-do-we-rot/) This is just the kind of story that tickles me; strange enough to be fascinating, and weird enough to hang just on the limit of the plausible. I’m in the middle of exams and don’t have the time to research myself, and anyways your mind has proven to be sharper than mine on several occasions. (I always guess wrong at science or fiction:)
I have a problem understanding purposeful evolution. I think it’s more like the kid who got a bulls eye every time he shot an arrow at a barn by painting the targets around the arrows.
Why did the kid shoot the arrow to begin with? Was not the purpose to hit the barn at least once? The kid painting the target after the fact is more like a creationist trying to deny it wasn’t in any sense an accident.
Trial and error is purposeful. Deal with it.
Let me make it clearer. He could just as well shoot them in the air and draw a target around where it fell and say he got a bulls eye too. Some will hit the barn, some will hit the tree, etc.
Then any adaptation, if that was his purpose, will be consequentially unintended.
The analogy breaks down here. No purpose needed.
Have you ever heard of a bow and arrow going off by accident? But then of course it was your analogy.
In case you still don’t get it, even if all genetic changes were caused by accidental mutations, there would be no evolutionary adaptation until trial and error sorted out the potentially successful results.
Trial and error? You mean the selection pressure of the given environment.
Evolution ‘thinks’? Evolution has purpose? Any sane definition of ‘purpose’ would have to include the concept of conscious direction towards a goal. If I throw a bucket of water on a hillside, the water will take the path of least resistance and trickle downwards as far as it can. But the water doesn’t employ ‘trial and error’, it doesn’t experiment or think about it and plan a route, it just passively follows the usual natural laws. Doing anything else would be ‘super’ natural.
weing, yes I mean the selection pressure of the given environment. The organism acts to achieve success in the smallest of increments against that pressure, with no plans, goals or expectations other than what those pressures have taught it over time. And all organisms do learn from that experience. And that learning contributes to their evolution.
As to what the other poster says, water is not life (unless to a homeopath). All life forms calculate or compute. Water doesn’t. The water flowing downhill analogy is extremely limited. It was meant to demonstrate how evolution works in the absence of the type of goals that Lamarck and his brethren assumed it had. The analogy has nothing to do with the trial and error strategies employed by all life forms. Water doesn’t learn. Life forms learn.
as an explanation of
Why not, the organism exposed to the environment and expressing its traits survives to reproduce and the traits are passed on to the next generation?
No matter how you slice and dice it, the organism must react to its environment, even if with the simplest of strategies. It tries because it has to. That’s its purpose.
Life forms are molecular structures that have acquired the ability to seek out and compete for energy. That seeking out is their purpose. The competition determines their strategy. The relative effectiveness of their strategies is determinant of their evolution.
One could just as easily say that water is seeking its purpose, to flow down hill. The water is competing with other matter for gravitational energy. Objects with a higher density out compete water, and sink, objects with a lower density are not as adept at competing with water and float.
daedalus:
I know asking you this question is an exercise in futility, a purpose devoid of tangible expectation, but here goes anyway:
Does water evolve?
I will grant you that living things strive to survive and reproduce and they use the traits that they have inherited to do so. If the environment changes, and the mix of traits they have inherited are no longer giving them an advantage or even a disadvantage, they are less likely to survive and pass them on. Their brothers and sisters who may have had a slightly different mix of traits may now have an advantage and pass on these traits. It’s the relative effectiveness of their strategies that is selected by the chaotic environment.
But you have granted that the purpose is in the striving. The incremental actions developed the strategies, without which there would have been nothing effective to select. The purpose of evolutionary trial and error is to find and present at least one viable option for selection.
I have no problem with organisms striving and having a purpose. I have a problem with evolution being purposeful. I’m not sure I would even call it trial and error. There is no need to present at least one viable option for selection, that’s why species go extinct.
Why species go instinct is that when push comes to the wrong shove, they can’t come up with that one viable option.
But the point is that without some sort of purposeful actions taken by organisms, they would not evolve. Of course a species may eventually cease to evolve – and some have already without as yet becoming extinct. And eventually life itself will be extinguished, on earth at least. So if the purpose is or was to ensure survival for the long term, good luck with that. But if it is an incremental process, then so far, so good.
Ok. I can agree to the purposeful actions being on the part of the organism.
artfulD-”Of course a species may eventually cease to evolve – and some have already without as yet becoming extinct. ”
How could you tell for sure that a species has stopped evolving if it has yet to go extinct?
OK, good on ya for that.
tmac,
You mean can I be sure that after a few million years they won’t start up again? Yeah, I think biologists have ways of being pretty sure when that has minimal chances of happening. But nothing is certain. Is that the disclaimer I failed to include?
“there is a certain amount of chaos to the particular details of evolutionary history that therefore may forever defy detailed explanation.”
No, you may not ever understand every single detail about evolution. Especially since you have no evidence whatsoever for why or how anything evolved. You make it sound like most traits of most species are already figured out, based on scientific evidence, except for a few odd ones like the giraffe. But NONE have been figured out.
So is this not just a bit deceptive?
And please refrain from calling me an idiotic creationist, because I believe in evolution. I just don’t pretend to understand how or why it happened.
Disclaimer: Any proximity between my comments and pec’s has no significance except to demonstrate the vagary of coincidence.
artfulD-”Yeah, I think biologists have ways of being pretty sure when that has minimal chances of happening. But nothing is certain. Is that the disclaimer I failed to include?”
I just never have heard that before. That’s all.
Creationists have heard of this and as a matter of fact use examples of this to “prove” that evolution doesn’t occur at all.
http://www.ambersdenydarwin.com/amber_05.html
Broccoli doesn’t learn. It evolved.
You’re all a bunch of animalists.
Broccoli didn’t get that bitter taste by accident.
Seems like a species that has not changed in many millions of years could be said to have stopped evolving, but the term ‘stasis’ might be more accurate since it connotes stability as opposed to a more rigid state. But I don’t want to be pedantic. Oops too late.
tmac57
You’re the one that used the phrase “stopped evolving.” My initial phrasing that you seem determined to find fault with was “cease to evolve.”
You don’t want to be pedantic? But isn’t that where you have long found your purpose in life?
artfulD-Yeah, you got me on that one (damn!). I caught it after I sent it. Think stasis. Don’t give up on the little flies and aphids, I just know they can do better!
You mean they may have ceased but not necessarily desisted?
Zoologist Darren Naish did an excellent article on giraffe necks back in early 2007.
If you ever want to know fascinating and unusual things about four-footed animals, his blog is the best there is.