THE GIRAFFE: A FAVOURITE TEXTBOOK
ILLUSTRATION OF EVOLUTIONARY THEORIES
High-school biology texts regularly present Darwins theory of evolution in contrast with Lamarcks 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 todays 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:
- 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 giraffes neck, as part of a new section attempting to refute St. George Mivarts 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 giraffes long neck, we find either nothing at all or only the shortest of speculative conjectures. . . . The giraffes neck just wasnt a big issue for the founders of evolutionary theorynot 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).
- 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).
- 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.
- 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 pressuretwice or three times that of a manis 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 necks 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 giraffes 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 giraffes 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 giraffes 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 giraffes 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 theres 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).
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). Africas Gentle Giants. National Geographic Vol. 152, No. 3. pp. 402-417.
Gould, Stephen Jay. 1996 (May). The Tallest Tale. Natural History Vol. 105, No. 5. pp. 18-23, 54-57.
Hofland, Lynn. 1996 (Sep-Nov). Giraffes: animals that stand out in a crowd. Creation Vol. 18, No. 4. pp. 11-13.