Tallest of the mammals is the giraffe (Giraffa camelopardalis) whose extremely long neck is a classic example of an evolutionary trait under selection.The long neck is a derived characteristic since giraffes evolved from ancestors that lacked this trait.A few hypotheses were proposed to explain the evolution of the neck which give insight on how the giraffe got its neck.Darwin speculated on the idea that natural selection chooses animals that are best able to feed on the highest treetops, where food is most abundant and competition minimal (Gould, 1996).This interspecific competition could provide a selection pressure that elongated the neck.Lamarck, through his principle of “use and disuse”suggested that the long neck is attributed to the frequent stretching of the neck as giraffes reached for food (Gould, 1996).This hypothesis would infer that long necks were passed to offspring by altered heredity.A novel alternative proposed by Simmons and Scheepers (1996) suggests that the increased neck length has a sexually selected origin.Giraffe males fight for dominance over females by clubbing opponents with their massive heads and necks.This intrasexual combat is called “necking” through which larger-necked males gain the greatest access to estrous females and thus, have a greater contribution to the genetic makeup of the next generation.The most recent explanation is most plausible since it provides better evidence for evolution.
Darwin noted the giraffe in Origin of Species to illustrate the mechanism of natural selection.His suggestion is consistent with the Mendelian nature of heredity, “that giraffes with fortuitously longer necks will tend to leave more surviving offspring that inherittheir 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 top-most leaves” (Gould, 1996).However, present day evidence does not support Darwin’s explanation since during the dry season (when feeding competition should be most intense) giraffes generally feed from low shrubs (Simmons and Scheepers, 1996).This would suggest that long neck did not specifically evolve for feeding at higher levels.
A problem with Darwin’s hypothesis is that if feeding competition was the chief selective pressure for increasing feeding height, anatomically giraffes should have achieved greater height by elongation of all body parts.Other body parts such as the torso, chest and the legs would also undergo stretching when giraffes reached for food, giving rise to equivalent lengthening of all body parts.This morphological allometry is not observed which gives little evidence for selection via feeding competition (Simmons and Scheepers, 1996).
Selection pressures acting today may be different from those that acted in the past.An assumption that selective forces giving rise to morphological traits may not be the same as those maintaining them, give Darwin’s hypothesis some credit.Different forces may have acted in the past to initiate neck elongation of the ancient giraffe, then, once some elongation occurred, necking played a role in further neck lengthening.Other possible uses of the neck that may serve as plausible explanations for evolution of the long neck include the advantage to spot predators far away and shed heat by increased surface area (Gould, 1996).Despite the presence ofthese advantages they should not be considered as primary models for neck elongation because, they are more likely to have arisen as a result of neck elongation.
Lamarck’s principle of “use and disuse” is even less credible.His proposed idea that offspring acquire parental characteristics that were obtained through extraneous stretching of the neck is not valid because offspring can not inherit any changes made by parental striving (Gould, 1996).Lamarck’s ideais not granted any weight.
Giraffes evolved from an ancestor that resembled a 10-foot-tall deer which was much shorter than todays 15-foot-tall species.These creatures roamed what is now Europe and Asia 30-50 million years ago (Stevens, 1993).The modern giraffe radiated on the African savanna about 1 million years ago (Simmons and Scheepers, 1996).Thesparse fossil record which is found in Europe and Asia gives some insight into the relative size of fossil giraffids (Samotherium) which show a leg length 83% that of the modern giraffe.The fossil records fail to show any parallel increase in neck length in relation to other body parts.The ancestor is thought to resemble what is today the okapi, a short four-legged mammal that also displays sexual dominance (Simmons and Scheepers, 1996).Necking must be a recently derived trait since the opaki, giraffe’s closest relative, do not exemplify that behavior.
The novel hypothesis developed by Simmons and Scheepers (1996) argues that the giraffe’s long neck arose from its use as a weapon during intrasexual combat.The sparring between two male giraffes is referred to as “necking”, a behavior used to establish dominance hierarchies in the male herds (Coe, 1967).
The act of necking is initiated by one giraffe, the challenger, who approaches another giraffe, and receives a faster response if the opponent is large in size.The challenger acquires a proud posture and stares at his opponent propagating the fight.The swinging of the necks begins as the challenger swings his neck and head into the neck of the opponent.The neck is the organ that provides the main force of the blow, and the head and horns are used at the termination of the hit (Coe, 1967).The force with which the hits are delivered is capable of knocking a competitor off balance.There are incidents recorded in Nairobi National Park and Kruger National Park, Africa where male giraffes were knocked to the ground unconscious and killed; others suffered from splintered vertebra which pierced the spinal cord, and broken bones.Most of the time necking does not end up deadly, but male bulls will fight hard for their position in the herd (Coe, 1967).
Necking only occurs in all male herds, which may consist of two or more males of varying sizes and large bulls.It is motivated by the presence of estrus females near by andthemost vigorous animal will take on a female group himself and mate if females are ready (Simmons and Scheepers, 1996).The bull determines a females time of estrus by taking a sample of urine on his tongue which signals if she is ready to mate (Stevens, 1993).Also, females allow themselves to be urine tested more frequently by larger males.Therefore, males with the largest neck size will mate most often.Since giraffes don’t have a definite breeding season, necking and sexual activity takes place at any time of the year, and the larger males will genetically contribute more to the next generation.
From the above observations, the following predictions can be made.Since males only use their necks (and heads) for intrasexual combat they should exhibit morphological adaptations such as longer and more powerful necks across generations.Sexually selected traits arise because males with larger and longer necks are dominant and gain reproductive advantages over smaller males.This would result in offspring inheriting the genes for a longer neck and eventually eliminating the genes that call for shorter necks since these males were constrained from mating.Because, each generation inherits genes for a longer neck, “female necks arose a neutral by-product of genetic correlation between sexes” (Simmons and Scheepers, 1996).Due to directional selection acting on the neck an increase in the mean length of necks among giraffes is be observed.
“Fossil and modern skeletal evidence should reveal a disproportionate increase in the neck length over and above expected isometric increases associated with lengthening of other body proportions” (Simmons and Scheepers, 1996).When modern and extinct giraffids are compared this is just the case.The few fossil records suggest that fossil giraffids were stout in structure with shorter necks.
When fossil records are incomplete, embryology is used to acquire morphological information about the ancestor.The ancestral character state is likely to be the character state that appears early in development (Ontogeny recapitulates phylogeny).Young giraffes exhibit a leg-to-neck ratio of 1: 0.62, which reflects their ancestor who resembles the okapi with a ratio of 1: 0.44.For an adult giraffe the leg-to-neck ratio is measured to be 1: 0.93 which suggests specific selection on neck length over other body proportions.The okapi which are thought to resemble their common ancestor has a neck about 50% smaller than that of the giraffe.In further support of disproportionate elongation of the neck, skeletal studies show that a 29% disproportionate elongation takes place over the thoracic and lumbar vertebrae.Necks of the females are on average 35cm shorter than those of males because selection only acts on the males (Simmons and Scheepers, 1996).The above pieces of evidence suggest that selection has been mainly on the neck.
Throughout this discussion it has been illustrated that that feeding competition alone as an evolutionary explanation oflong necks, proposed by Darwin and in part by Lamarck, is unlikely.First, this is because giraffes exhibit no direct advantage from their long neck since they feed more often with their heads unextended.This suggests that feeding advantages poorly explain maintenance of the trait.Second, a disproportionate increase in neck length has been observed implying selection primarily on the neck.If in fact feeding competition was the reason for the increased height it would not occur just on the neck.There seem to be an unnecessary input of energy to evolve only a long neck for the purpose of reaching food.These extra costs include high blood pressure, enlargement of the heart that is required to deliver blood to the brain, and a larger over all body which restricts giraffe maneuverability in running, drinking, and getting up (Gould, 1996).Therefore, it is easy to see that Darwin’s natural selection via feeding competition theory holds weak ground.
Sexual selection
is a more valuable explanation for the evolution of the long neck.Behavioral
analysis, courtship rituals, fossil data, and anatomical scaling all provide
support for this type of evolutionary model.First,
giraffe males use their necks in combat which determines hierarchy among
male herds.The bull male with the
longest neck gains access to estrous females and, thus, passes his “long-neck-genes”
to the next generation.With geological
time the giraffe species would tend to show increased neck length.Second,
since mainly necks are used in necking this would give rise to asymmetric
increase of the neck over other body parts which in fact is observed.Third,
males have larger necks than females of the same body mass, hence there
is sufficient evidence that necks are maintained by sexual selection.It
can be concluded that neck elongation via sexual selection provides the
strongest evidence for the evolution of the giraffe’s neck.
Bibliography:
1. Brownlee, A. (1963). Evolution Of The Giraffe. Nature, 200, 1022.
2. Coe, M. (1967). “Necking” Behaviour In The Giraffe. Journal of Zoology, 151, 313-321.
3. Gould, S. (1996). The Tallest Tale. Natural History, 105, 18-27.
4. Simmons, R. & Scheepers, L. (1996). Winning By A Neck: Sexual Selection In The Evolution Of Giraffe. The American Naturalist, 148, 772-786.
5. Stevens, J. (1993). Familiar Strangers. International Wildlife, 23, 6-10.
6. du Toit,
J. (1992). Winning By A Neck. Natural History, 101, 29-32.
