Neck of the Giraffe

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Contents 1 The Creationist Argument 2 Quotations 3 Discussion 4 Appendix A: giraffids in the fossil record 5 Appendix B: Darwin and the giraffe 6 Related Articles

[edit] The Creationist Argument

In the case of the giraffe’s neck, the Creationists point to a number of co-adaptations required for the giraffe’s neck to be such a height, including a stronger heart and higher blood pressure, thicker arteries, elastic walls to the arteries, valves in the veins of the neck, pressure sensors in their neck arteries, a shunt between the carotid and vertebral arteries, and the equalization of cerebrospinal pressure with arterial pressure.

Creationists, having listed these features, proudly announce that these features could neither have arisen one at a time (since the intermediate forms would be unviable) nor concurrently (since the coincidence would be too great). Neither of these objections, of course, addresses the evolutionary account of how these features arose.

[edit] Quotations

Some Creationist thoughts on the giraffe’s neck:

Four carefully thought-out design factors nicely solve this problem: (1) The giraffe has in his jugular veins a series of one-way check valves. These immediately close as soon as the head is lowered! But there is still a large amount of blood in the carotid artery; too much. (2) That extra blood Is immediately shunted to a special spongy tissue [note: he means the rete mirabile] located near the brain and filled with small blood vessels, which absorbs it. In addition, (3) the cerebrospinal fluid, which bathes the brain and spinal column itself, produces a counter-pressure to prevent rupture or capillary leakage. Last but not least, (4) the walls of the giraffe's arteries are thicker than those of any other mammal. [1]

When a giraffe is standing in its normal erect posture, the blood pressure in the neck arteries will be highest at the base of the neck and lowest in the head. The blood pressure generated by the heart must be extremely high to pump blood to the head. But when the giraffe bends its head to the ground it encounters a potentially dangerous situation. It must lower its head between its front legs, putting a great strain on the blood vessels of the neck and head. The blood pressure plus the weight of the blood in the neck could produce so much pressure in the head that the blood vessels would burst. Mercifully, however, the giraffe is equipped with an adaptational package, including a coordinated system of blood pressure control. . . . Pressure sensors along the neck’s arteries monitor the blood pressure, and can signal activation of other mechanisms to counter any increase in pressure as the giraffe drinks or grazes. Contraction of the artery walls [which have increased muscle fibre toward the head], a shunting of part of the arterial blood flow to bypass the brain, and a web of small blood vessels (the rete mirabile, or ‘marvelous net’) between the arteries and the brain all serve to control the blood pressure in the giraffe’s head. Notice that adaptations require other adaptations so that a specialized organism such as the giraffe can function optimally. (Davis, Percival, and Dean H. Kenyon : Of Pandas and People)

[edit] Discussion

The Creationist argument combines errors in understanding the theory of evolution, and errors in simple biological facts.

Their theoretical failure involves overlooking the fact that evolution occurs in small stages, each selected for the benefit of the organism.

It is perfectly true that the neck of the giraffe would be fatal if the giraffe had a small heart, or that the blood pressure produced by the heart of the giraffe would be fatal if the giraffe had thin arteries. However, evolutionary theory does not say that evolution does its work first on one feature of an animal, and then on another: it says the very opposite. Obviously random mutations do not form an orderly queue, with those extending the length of the neck going first, those increasing the blood pressure going second, and so forth. Instead, a slightly longer neck gives a selective advantage to a slightly stronger heart, which gives a selective advantage to slightly thicker arteries. So long as there is environmental pressure for slightly longer necks, there is also a selective pressure for these coadaptations, and mutations for them will be favored. It is rather depressing to think that nearly 150 years ago, Darwin gave the same answer to the same grotesque misunderstanding of his theory concerning the same animal, and that it is necessary to repeat it:

With animals such as the giraffe, of which the whole structure is admirably co-ordinated for certain purposes, it has been supposed that all the parts must have been simultaneously modified; and it has been argued that, on the principle of natural selection, this is scarcely possible. But in thus arguing, it has been tacitly assumed that the variations must have been abrupt and great.[2]

With this theoretical error, the Creationists compound a crass factual error: supposing that the particular features named are unique to the giraffe. Not one of them is.

• The giraffe has elasticated arteries? So do you, I trust. Hardening of the arteries is an undesirable medical condition: all arteries are by nature elastic.

• The giraffe’s cerebrospinal fluid produces a counter-pressure to prevent rupture or capillary leakage? This again is true of all mammals, and is a matter not of design nor of evolution, but basic physics.

• The giraffe has valves in the veins of its neck? So do all mammals. You too have a jugular valve.
• Giraffes have pressure sensors in their neck arteries? Again, so do all mammals.
• The giraffe has a rete mirable? So do most of its relatives:

The function of the rete mirabile is to regulate the flow of arterial blood towards the cerebrum as well as the thermal regulation of the brain preventing it from overheating; the arterial system of the cerebrum in ruminants […] is equipped with a safety system. [3]

• The giraffe has a shunt between the carotid and vertebral arteries? So do many of its relatives, including its closest relative, the okapi, which has a short neck.

In summary, the mechanisms which the giraffe uses to control its blood pressure are all either common to mammals in general, or common to the species most closely related to the giraffe.

[edit] Appendix A: giraffids in the fossil record

The following table gives a partial list of giraffes and their relatives as they appear in the fossil record. The table is arranged, like the fossil record, with the most recent fossils at the top.

To summarize: we begin with a deer-like creature, Eumeryx, in the Oligocene. In the Miocene we see creatures such as Samotherium which look much like a modern okapi --- or like a giraffe with a short neck. In the Upper Miocene we see animals such as Honanotherium, which are intermediate in form between short-necked giraffids and the very long neck of the modern giraffe. In the Pliocene we see creatures considered to be species of giraffe. A third group of robust ox-like giraffids known as sivatheres flourished alongside the giraffes and okapis until maybe a few thousand years ago, when their last representatives, the species of Sivatherium, were driven extinct, presumably by human activity.

Epoch	Species
Holocene	Giraffa camelopardalis (the modern giraffe), Okapia johnstoni (the modern okapi)
Pleistocene	Giraffa camelopardalis, Giraffa jumae, Giraffa gracilis, Giraffa sivalensis, Okapia stillei, Sivatherium giganteum, Sivatherium maurusium [4]
Pliocene	Giraffa attica, Giraffa punjabiensis, Giraffa priscilla, Samotherium boissieri, Samotherium sp. , Honanotherium sp. , Samotherium mongoliensis, Helladotherium sp. [5] , Bramatherium perimense [6]
Upper Miocene	Samotherium africanum, Samotherium sp. [7], Palaeotragus germaini, Honanotherium sp. [8], Honanotherium schlosseri, Honanotherium sivalense, Birgerbohlina schaubi [9], Giraffokeryx punjabiensis, Bramatherium megacephalum, Shansitherium fuguensis, Shansitherium tateli [10], Palaeotragus primaevus [11], Progiraffa exigua
Middle Miocene	Palaeotragus primaevus, Giraffokeryx sp., Giraffokeryx tungurensis, Giraffokeryx punjabiensis [12]
Lower Miocene	Prolibytherium magnieri [13], Climacoceras africanus [14], Climacoceras gentryi, Canthumeryx sertensis
Oligocene	Eumeryx sp.

[edit] Appendix B: Darwin and the giraffe

Some quotations from Charles Darwin on the evolution of the giraffe:

With animals such as the giraffe, of which the whole structure is admirably co-ordinated for certain purposes, it has been supposed that all the parts must have been simultaneously modified; and it has been argued that, on the principle of natural selection, this is scarcely possible. But in thus arguing, it has been tacitly assumed that the variations must have been abrupt and great. No doubt, if the neck of a ruminant were suddenly to become greatly elongated, the fore limbs and back would have to be simultaneously strengthened and modified; but it cannot be denied that an animal might have its neck, or head, or tongue, or fore-limbs elongated a very little without any corresponding modification in other parts of the body; and animals thus slightly modified would, during a dearth, have a slight advantage, and be enabled to browse on higher twigs, and thus survive. A few mouthfuls more or less every day would make all the difference between life and death. By the repetition of the same process, and by the occasional intercrossing of the survivors, there would be some progress, slow and fluctuating though it would be, towards the admirably coordinated structure of the giraffe. (Charles Darwin : The Variation of Animals and Plants under Domestication)

The giraffe, by its lofty stature, much elongated neck, fore legs, head and tongue, has its whole frame beautifully adapted for browsing on the higher branches of trees. It can thus obtain food beyond the reach of the other Ungulata or hoofed animals inhabiting the same country; and this must be a great advantage to it during dearths …

So under nature with the nascent giraffe, the individuals which were the highest browsers and were able during dearths to reach even an inch or two above the others, will often have been preserved; for they will have roamed over the whole country in search of food. That the individuals of the same species often differ slightly in the relative lengths of all their parts may be seen in many works of natural history, in which careful measurements are given. These slight proportional differences, due to the laws of growth and variation, are not of the slightest use or importance to most species. But it will have been otherwise with the nascent giraffe, considering its probable habits of life; for those individuals which had some one part or several parts of their bodies rather more elongated than usual, would generally have survived. These will have intercrossed and left offspring, either inheriting the same bodily peculiarities, or with a tendency to vary again in the same manner; while the individuals less favoured in the same respects will have been the most liable to perish.

Assuredly the being able to reach, at each stage of increased size, to a supply of food, left untouched by the other hoofed quadrupeds of the country, would have been of some advantage to the nascent giraffe. Nor must we overlook the fact, that increased bulk would act as a protection against almost all beasts of prey excepting the lion; and against this animal, its tall neck--and the taller the better-- would, as Mr. Chauncey Wright has remarked, serve as a watch-tower. It is from this cause, as Sir S. Baker remarks, that no animal is more difficult to stalk than the giraffe. This animal also uses its long neck as a means of offence or defense, by violently swinging its head armed with stump-like horns. The preservation of each species can rarely be determined by any one advantage, but by the union of all, great and small. (Charles Darwin : On the Origin of Species)

[edit] Related Articles

• Creationist Arguments