The Evolution of the Giraffe Neck Throughout time, one theory has remained constant in terms of why giraffes developed longer necks. The idea, which was presented by Charles Darwin states quite simply that giraffes selected for longer necks in order to reach the food that was higher off the ground during the dry season. No one has ever challenged that idea until 1996. Initially, Gould argued that "the story-the giraffe evolved its long neck in competition to reach scare foliage-is supported by no evidence" (18). That's when two scientists, Robert Simmons and Lou Scheepers made the claim that necks evolved for a very different reason: sexual selection. Within this paper, information will be presented that argues both for and against the theories made by Darwin and Simmons and Scheepers.
Giraffes are placed in the family, Giraffidae, separate from other animals such as the camel, deer, and cow. Typically, giraffes are about 19 feet tall and can weigh as much as 4000 pounds. The evolution of modern giraffes began about 1 million years ago from a similar species known as Giraffa jumae. Those species were known for their massive skeletons and antler-like structures, not found on giraffes of today (Simmons 772). Today, there are nine widely excepted subspecies of the giraffes which are differentiated by the spots on the trunks and their geographic region.
In the article, "Winning By a Neck: Sexual Selection in the Evolution of Giraffes," Simmons and Scheepers state their purpose as to evaluate the theory proposed by Darwin as well as present their own. The theory by Darwin known as the Interspecific Feeding Competition has many assumptions that must hold up for it to be true. One assumption is that tall trees must have been present on the African savannas back when the selection took place and that competition was initially tough for food. If food was scarce, it makes sense that the giraffes would evolve to keep their species alive. In order for this theory to remain true, there should be length increases in limbs in the same proportion as to the increases in their heads and necks.
On the other hand, the proposed theory deals with the notion that the length of the giraffe neck increased because the neck is used as a weapon during intrasexual combat (Simmons 773). The basic idea of this is that during competition two males stand next to each other and exchange hits by using their necks. The top or back of the skull is actually used to knock the competitor to the ground. It is likely that with a larger neck and head, the giraffe is more destructive. It is also assumed that larger and longer necks would be selected for. The process of necking is unique to giraffes and only male giraffes have ever been viewed doing this. Therefore, it is likely that this action is related to sexual selection. Necking has been observed to be very effective for giraffes to obtain their mates. Often the males are knocked unconscious or even killed during the fighting. The violent nature of these fights is unbelievable. Even when an opponent may be knocked to the ground, that does not stop the fight. They may still be kicked or stepped which can eventually lead to death.
"Sexual selection is a special form of natural selection that is responsible for the evolution of traits that promote success in competition for mates" (Kodric-Brown 309). There are two different forms of sexual selection: it can be involved with the evolution of traits or it can be related to the evolution of characteristics which would make the male look more appealing. Intrasexual competition is involved more with the evolving traits which than can be used as weapons when fighting each other for the right to mate with the female. One general feature of sexually selected traits is that the trait is often more costly for the individual to produce. This holds true for giraffes. Since their necks and heads have disproportionately evolved compared to the rest of their body, the heart has to pump harder in order for blood to reach the brain and through the neck. If the giraffe had evolved consistently throughout its body, it would not have characteristically higher blood pressure than most other animals.
If Simmon's theory is true, there are also some basic assumptions that must take place. For example, males should only use their necks for intrasexual fighting, their necks should be longer and more muscular than females, males with larger necks should be dominant, and lastly, the fossil record should show disproportionate increases in neck size when compared with other body parts (Simmons 774).
Various experiments were run to help to determine which theory has more basis within it. First of all, dissections were performed on a large sample of female and male giraffes that were over one year old. All the heads were cut at the same place and were weighed, and necks were cut at similar areas from the shoulder. When analyzing the results of the sexes, it becomes apparent that females have smaller skulls that contain less protection and their neck length is about 35 centimeters shorter than the average male. If neck length had evolved because of food competition it would not make sense that females would have shorter necks. Both sexes need to survive in order for the species to survive. As stated by Simmons, "for males and females each weighing 800 kilograms, the head masses would be 20.3 kg and 18.0 kg respectively and the neck masses would be 60 kg and 51 kg respectively" (Simmons 777). Another significant difference to note between the sexes is that while female head mass stops increasing at less than 22 kg, the male head mass continued to increase throughout their lives. Also, female neck length leveled off at 60 kg while the males reached weights of over 100 kg. This information also points out problems with Darwin's theory. What would be the point of males continually increasing in size if females were still able to obtain food at their smaller size?
Another factor affecting Darwin's theory is that the giraffes in the Serengeti spend most of the dry season feeding from low bushes and during the wet season, they eat from the tall trees where the new leaves are plentiful (Simmons 775). This information points out that the main idea behind Darwin's theory is false. During the dry season, giraffes are not more likely to eat food from higher trees, the opposite is true. Therefore, there is no direct advantage for the sexes to have their long necks because most of the time they do not feed at a higher level. Females were especially more likely to eat below the shoulder level, but males also feed regularly from below or at that level. If necks had evolved solely for food competition, there would be serious consequences in the event of a drought. Male giraffes are on average two feet taller than females, indicating that the females would be selected against during a rougher climate year (Brownlee 1022).
When fossils were observed, it appears that selection actually "favored heavier bodies and large, heavy necks" (Simmons 776). There was no direct evidence that indicated that elongation had taken place. This information points out another flaw in the original theory about giraffes and the evolution of their necks. Instead of selection occurring throughout the body, it appears to be based specifically on the neck.
One problem with the sexual selection theory presented by Simmons and Scheepers is that it too does not offer a viable explanation as to the reason that females also have long necks. If the necks evolved for sexual selection, there would be no reason for females to also have larger necks: they are not involved in competition and necking does not occur between them. The two scientists offer alternate theories as to why female necks also evolved to be longer. One theory is that past selective pressures may have forced both sexes to increase their neck sizes, but that pressure may no longer be identified. Secondly, the female neck size could have evolved from the genetic components of the female and male mating (Simmons 783).
In conclusion, Simmons and Scheepers may not have backed up their theory completely, but they make it obvious that feeding alone can not be the reason that longer necks evolved. This is shown through the way that the animals feed and the disproportionate increase in neck length compared to other components within the giraffe. The evidence presented in this case offers much more support for the idea that sexual selection is responsible. However, much more investigation must take place in order for the theory to be considered acceptable.
Brownlee, A. Evolution of the Giraffe. Nature. Vol. 200:1963.
Gould, Stephen J. The Tallest Tale. Natural History. May 1996.
Kodrick-Brown, A., and J. H. Brown. Truth in Advertising: The
Kinds of Traits Favored by Sexual Selection. The American
Naturalist. Vol. 124:1984. Pp. 309-323.
Simmons, Robert E. Winning By a Neck: Sexual Selection in the
Evolution of Giraffe. The American Naturalist. Vol.
148:1996. Pp. 771-786.