The Giraffe: A Favourite Textbook Illustration of Evolutionary Theories

With a special Appendix: Harvard paleontologist Stephen Jay Gould on the inappropriate textbook use of the giraffe to introduce students to evolutionary theories! (following the conclusion of this article)

by Richard Peachey

High-school biology texts regularly present Darwin’s theory of evolution in contrast with Lamarck’s earlier explanation, and the organism most often used to illustrate the difference between the two views is the giraffe (e.g., Creager et al., pp. 233-240).

Lamarck, it is said, told a story of giraffe necks becoming longer as the animals tried to stretch their necks to reach food (Law of Use and Disuse). The longer necks acquired in this way would then be passed on to their offspring (Law of Inheritance of Acquired Characteristics). Continued stretching over the generations led to today’s long-necked giraffes.

Darwin, on the other hand (it is said), proposed that early giraffes had necks of different lengths, some longer and some shorter (Variation). Limited food supplies meant that not all giraffes could obtain enough food to survive (Competition). Giraffes with longer necks could survive better and reproduce, passing their long-necked trait to their offspring, while those with shorter necks more often died off before being able to reproduce (Natural Selection). Over the generations the average giraffe neck became longer due to this process.

But a number of things are wrong with this story:

1. Historically, there is no evidence that either Lamarck or Darwin used the giraffe as a significant part of their presentation of evolution. In his sixth edition of Origin of Species (though not in the first five editions), Darwin (201-203) did discuss the giraffe’s neck, as part of a new section attempting to refute St. George Mivart’s objections to the theory of natural selection (Gould 54). But then, Darwin explicitly accommodated Lamarckian thinking in his explanation; he accounted for the giraffe using natural selection “combined no doubt in a most important manner with the inherited effects of the increased use of parts” (202, cf. 133-139). Harvard evolutionist Stephen Jay Gould states: “When we look to presumed sources of origin for competing evolutionary explanations of the giraffe’s long neck, we find either nothing at all or only the shortest of speculative conjectures. . . . The giraffe’s neck just wasn’t a big issue for the founders of evolutionary theory—not as a case study for arguing about alternative mechanisms, not for anything much at all. No data from giraffes then existed to support one theory of causes over another, and none exist now [emphasis added]” (21).

2. Female giraffe necks, on average, are two feet shorter than male necks! “If a longer neck were needed to reach above the existing forage line, then the females would have soon starved to death and the giraffe would have become extinct” (Davis and Kenyon 71).

3. Many researchers now suggest that the primary function of giraffe neck length is not for reaching leaves on tall trees, but for male combat (“necking”), or for spotting predators, or for shedding heat through increased skin surface area. All of these functions “have been viewed by prominent scientists as a chief reason for the evolution of the long necks” (Gould 56f.). Darwin himself (202) alludes to some of these as alternate possibilities.

4. There is no fossil record showing a gradual increase in giraffe neck length. “All giraffes belong to a single species, quite separate from any other ruminant mammal, and [allegedly] closely related only to the okapi (a rare, short-necked, forest-dwelling species of central Africa). Giraffes have a sparse fossil record in Europe and Asia, but [alleged] ancestral species are relatively short necked, and the spotty evidence gives no insight into how the long-necked modern species arose [emphasis added]” (Gould 56).

The giraffe neck is not simply a longer version of an okapi neck; it is a well-designed “adaptational package” — a combination of unique features that work together to help the giraffe survive in its environment:

• “To drive blood eight feet up to the head, the heart is exceptionally large and thick-muscled, and the blood pressure—twice or three times that of a man—is probably the highest in any animal” (Foster 409). “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 and Kenyon 71). The giraffe also has special “control valves in the jugular veins” (Foster 409); these “heavily valved veins control return of blood to the heart” (Davis and Kenyon 70).

• “The lungs are oversize to compensate for the volume of dead air in the long trachea. Without this extra air-pumping capacity a giraffe would breathe the same used air over and over” (Foster 409). “The giraffe’s lungs are eight times the size of those of humans, and its respiratory rate is about one-third that of humans. Breathing more slowly is necessary in order to exchange the required large volume of air without causing windburn to the giraffe’s rippled 3.6 metres (12 feet) of trachea. When the animal takes in a fresh breath, the oxygen-depleted previous breath cannot be totally expelled. For the giraffe this problem is compounded by the long trachea that will retain more dead air than man can inhale in one breath. There must be enough lung volume to make this ‘bad air’ a small percentage of the total” (Hofland 12).

• “Equally marvellous is the fact the blood does not pool in the legs, and a giraffe does not bleed profusely if cut on the leg. The secret lies in an extremely tough skin and an inner fascia [fibrous connective tissue] that prevents blood pooling. This skin combination has been studied extensively by NASA scientists in their development of gravity-suits for astronauts. Equally helpful to prevent profuse bleeding is that all arteries and veins in the giraffe’s legs are very internal. The capillaries that reach the surface are extremely small, and the red blood cells are about one-third the size of their human counterparts, making capillary passage possible. It quickly becomes apparent that these unique facets of the giraffe are all interactive and interdependent with its long neck. But there’s more. The smaller red blood cells allow for more surface area and a higher and faster absorption of oxygen into the blood. This helps to retain adequate oxygen to all extremities, including the head” (Hofland 12).

 

Giraffe's Neck (adaptational package)

 


For further discussion on evolutionists' inappropriate use of the giraffe, see Jerry Bergman, "The giraffe's neck: another icon of evolution falls." <http://creation.com/images/pdfs/tj/j16_1/j16_1_120-127.pdf>


References

Creager, Joan, Paul G. Jantzen, and James L. Mariner. 1985. Biology. New York: Macmillan.

Darwin, Charles. 1958. Origin of Species. (reprint of 6th edition). New York: Mentor.

Davis, Percival, and Dean H. Kenyon. 1993. Of Pandas and People. 2nd edition. Dallas: Haughton Publishing. See especially pp. 12-13, 69-71.

Foster, Bristol. 1977 (Sep). "Africa’s Gentle Giants." National Geographic 152(3):402-417.

Gould, Stephen Jay. 1996 (May). "The Tallest Tale." Natural History 105(5):18-23, 54-57.

Hofland, Lynn. 1996 (Sep-Nov). "Giraffes: animals that stand out in a crowd." Creation 18(4):11-13.


APPENDIX: Extract from Harvard paleontologist Stephen Jay Gould's article "The Tallest Tale" (subtitle: "Is the textbook version of giraffe evolution a bit of a stretch?"). Natural History 105(5):18-23, 54-57. Bold print indicates emphasis added.

[page 18] ". . . I made a survey of all major high-school textbooks in biology. Every single one—no exceptions—began its chapter on evolution by first discussing Lamarck's theory of the inheritance of acquired characters, and then presenting Darwin's theory of natural selection as a

[page 19] preferable alternative. All texts then use the same example to illustrate Darwinian superiority—the giraffe's neck.

"Giraffes, we are told, got long necks in order to browse the leaves at the tops of acacia trees, thereby winning access to a steady source of food available to no other mammal. Lamarck, the texts continue, explained the evolution of long necks by ar-

[page 20] guing that giraffes stretched and stretched during life, elongated their necks in the process, and then passed these benefits along to their offspring by altered heredity.

"This lovely idea may embody the cardinal virtue of effort rewarded, but heredity, alas, does not operate in such a manner. A neck stretched during life cannot alter the genes that influence neck length—and offspring cannot reap any genetic reward from parental striving. We therefore prefer the Darwinian alternative, consistent with the Mendelian nature of heredity, that giraffes with fortuitously longer necks (in a varying population with a large range of neck lengths among individuals) will tend to leave more surviving offspring that inherit their genetic propensity for greater height. This slow process, continued for countless generations, can lead to a steady increase in neck length, so long as local environments continue to favor animals with greater reach for those succulent topmost leaves. . . .

"A story so often repeated should rise from firm foundations and bear both strong and graceful support throughout the length of construction. In short, this most familiar of examples should, like the subject's own head, stand tall above everything else, buttressed by a device as supple and and as well designed as the neck of the giraffe. . . .

"When we look to presumed sources of origin for competing evolutionary explanations of the giraffe's long neck, we find either nothing at all or only the shortest of speculative conjectures. . . .

"The giraffe's neck just wasn't a big issue for the founders of evolutionary theory—not as a case study for arguing about alternative mechanisms, not for anything much at all. No data from giraffes then existed to support one theory of causes over another, and none exist now. . . .

[page 54] "Darwin does speculate about the adaptive advantage of giraffes' necks, but he cites both natural selection and Lamarckism as probable causes of elongation. Thus, obviously, Darwin never regarded giraffes' necks as an illustration for the superiority of natural selection over other valid mechanisms. He writes in two passages of the 1872 edition [the 6th and final edition of his Origin of Species), marrying Lamarck with natural selection:

By this process [natural selection] long continued . . . combined no doubt in a most important manner with the inherited effects of the increased use of parts, it seems to me almost certain that an ordinary hoofed quadruped might be converted into a giraffe. [square brackets and ellipsis are Gould's]

In every district some one kind of animal will almost certainly be able to browse higher than the others; and it is almost equally certain that this one kind alone could have it neck elongated for this purpose, through natural selection and the effects of increased use. . . .

[page 56] "If we choose a weak and foolish speculation as a primary textbook illustration (falsely assuming that the tale possesses a weight of history and a sanction in evidence), then we are in for trouble—as critics properly nail the particular weakness and then assume that the whole theory must be in danger if supporters choose such a fatuous case as a primary illustration. . . .

"As a closing point, we might excuse this thoughtless repetition of an old legend without presumed historical sanction if later research had established the truth of the tale nonetheless. But when we turn to giraffes themselves, we encounter the final irony of this long story. Giraffes provide no established evidence whatsoever for the mode of evolution of their undeniably useful necks.

"All giraffes belong to a single species, quite separate from any other ruminant mammal, and [allegedly] closely related only to the okapi (a rare, short-necked, forest-dwelling species of central Africa). Giraffes have a sparse fossil record in Europe and Asia, but [putative] ancestral species are relatively short necked, and the spotty evidence gives no insight into how the long-necked modern species arose. . . .

[page 57] "In short, we have no basis for any firm assertion about the most famous inquiry among Darwinian just-so stories: how did the giraffe get its long neck? . . .

"In the realm of ideas, current use of the giraffe's neck as the classic case of Darwinian evolution does not grow from firm and continuous historical roots. The standard story, in fact, is both fatuous and unsupported. In the realm of giraffes, current use of maximal mammalian height for browsing leaves does not prove that the neck evolved for such a function. Several reasonable alternative scenarios exist [from an evolutionary viewpoint], and we have no evidence for preferring any plausible version over another. Caveat lector.

"Why then have we been bamboozled into accepting the usual tale without questioning? I suspect two primary reasons: we love a sensible and satisfying story, and we are disinclined to challenge apparent authority (such as textbooks). . . . Darwinian evolution may be both true and powerful, but if we continue to illustrate our conviction with an indefensible, unsupported, entirely speculative, and basically rather silly story, then we are clothing a thing of beauty in rags—and we should be ashamed, "for the apparel oft proclaims the man." [quoting Shakespeare's Hamlet]