Introduction
The giraffe’s long neck is a phenomenon that has perplexed researchers for a long time. It is also classified as the tallest mammal. The unique feature that distinguishes this animal from other animals is the long neck. People are not only perplexed by the way the giraffe is able to handle the long neck but also how the long neck came to be. Modern giraffes have long necks relative to their ancestral counterparts. Various explanations have been provided towards giraffe’s long neck. Charles Darwin gave the initial explanation that the long neck is as a result of natural selection. Darwin explained that giraffe selected for longer necks for them to be able to reach for food in tall trees during scarcity (Butzer 2457). This is a form of adaption which enables it to survive and fit within the ecosystem. Over a long time, Darwin’s theory has been the dominant theory explaining long neck in giraffes until recently. Lou Scheepers and Robert Simmons’s work have changed perception over evolution of giraffe. These scholars provide a different explanation by suggesting that the long necks in giraffes result from sexual selection. Necking is a common trait in giraffes. Necking refers ways male giraffe compete and fight for females by use of their necks and heads. Giraffes with longer and stronger necks often win in necking. This can support that the long necks in giraffes is a result of sexual selection.
Sexual Selection
Theory of sexual selection suggests that evolution can be explained through intraspecific competition. This theory was introduced by Charles Darwin who explained sexual selection as competition between individuals of same sex, especially male, for the other sex. Sexual selection is manifested in three ways: intrasexual selection, sexual conflict and intersexual selection (Darwin 79). Intrasexual selection refers to a phenomenon where males fight for female. Intersexual selection on the hand refers to a case where female choose their mates depending on some traits. With regard to intra-sexual selection, the males mainly depend on certain secondary sexual characteristics; these include the antlers and horns which also act as weapons. For mate choice, the trait chosen by females are referred to as ornament.
Sexual selection can be viewed as a form of natural selection. It occurs where some individuals reproduce more than others. The individuals that out-reproduce others demonstrate different genetic characteristic as the fewer individuals. In a certain type of sexual selection, individuals of given sex form a reproductive differential through competition for mating opportunities (Krumbiegel 173). The individuals that win in completion out-produce the others. In this case natural section takes place when the characteristics that enable some members to win are inherited. The other type of sexual selection occurs when a members of a particular sex result into formation of reproductive differential in another sex. This occurs through selection of mating partners with particular traits. Natural selection then take place when the individuals that are chosen as makes are genetically different from the ones rejected.
Natural selection and evolution
Giraffes are classified in the family Giraffidae separate. Their height is about nineteen feet while their weight can be as high as 4000 pound. Giraffes originated from a common species referred to as Giraffa jumae. It is estimated that evolution that has occurred amongst the giraffes to their current state began approximately 1,000,000 years ago. Giraffa jumae had some features which include antler-like structure and enormous skeleton. The antler-like structure is no longer present in modern giraffes. From this species evolved various subspecies. Today, about nine subspecies of Giraffes are present. The species are differentiated some different characteristics that include differences in spots. The subspecies are also differentiated by geographical regions that they occupy.
Modern giraffes are significantly different from their ancestors. The ancestor to giraffes was close to a 10 foot deer. It was even shorter than a 15 foot tall species of giraffe that we have today. According to Stevens, the creature lived in what is today Europe more than thirty million years ago (Gould 79). The African Savannah is the core origin of Modern giraffes. The relative size of the ancestors a given by Thesparse fossil record is 83% of modern giraffes. Records however do not show proportionality in increase of the size of the neck as other body parts. A close remembrance of giraffe ancestor is the today Okapi. Sexual selection in giraffes is thought to be a new train as Okapi does not show any sigh of necking.
Darwin’s Natural Selection
Darwin explains evolution in giraffes as a result of natural selection. In Origin of Species, he explains that the long show mechanism of nature selection. This opinion is in agreement with Mendelian nature of hereditary which suggest that giraffes with longer necks are more likely to leave more offspring that those that have shorter necks. According to Charles, giraffes with long neck are more adaptable to harsh condition (Darwin 79). In case of scarcity of food, the giraffes with long neck are able to reach food on treetops. As these giraffes are able to access more food than those with shorter necks then they grow strong and are able to survive making them more prevalent. According to Gould, the process of natural selection in giraffes continued for generations (Gould 107). He explains that as the process continues, there is likelihood for the length of neck gradually increasing with generation. The giraffes with long necks thus are able to leave more offspring that inherit their genetic characteristic for long necks. The process consequently continues to favour giraffes with longer necks leading to evolutional increases in the length of neck (Pincher 31). Darwin’s natural selection explanation has been the most common but it is inconsistent with what is observed in modern giraffes. For example, contrary to Darwin’s assumption, modern giraffes tend to feed from low shrubs during dry seasons rather than feed from treetops. Since during dry season competition for food is expected to be high, giraffes would be expected to compete to reach tall trees thus giving the giraffes with longer necks an added advantage. This however is not the cases as all giraffes tend to go for low shrubs in dry seasons. This observation weakens Darwin’s explanation long neck is an evolution to enable the giraffes to feed on high levels.
Darwin’s hypothesis that long neck result from competition in feeding and feeding height increases is weakened by various aspects. According to Simmons and Sheepers, if increasing feeding height was the cause long necks, then elongation of all other body parts was also expected. They explain that other body parts such as chests, torso and legs would have stretched leading to proportional increase in length of all body parts (Coe 343). They insist that this expected phenomenon does not happen, thus giving little evidence of natural selection motivated by competition for food. Despite of this criticism, Darwin’s theory still remains relevant when it is considered that conditions may have changed over time. This implies that selection pressures that affect giraffes currently be completely different as in the past.
Lamarckian Theory
Besides Charles Darwin’s natural selection theory, Lamarckian theory is another popular theory explaining the long necks in giraffes. Lamarck explains the long necks in giraffes as resulting from continuous stretching. He argues that continuous stretching led to gradual elongation of giraffe’s neck. This theory suggests that the benefits of elongated neck were passed down to offspring leading to modern long necks as compared those of ancestral giraffes (Young and Lynne 81). This explanation to long necks in giraffes is however challenged under several grounds. The first criticism is that the stretching is a mechanical process that cannot be passed down to offspring. Even if stretching could lead to long neck on a giraffe that stretches, the feature cannot be inherited (Ginnett and Montague 163). Another criticism to the theory comes when giraffe is compared to Okapi- a mammal almost similar to giraffe. Although Okapi also stretch to reach for food, there is no difference in length of neck between modern Okapi and ancestral Okapi. If stretching could lead to elongation of neck in giraffe, then it would be expected that the same would happen in Okapi. According to Whitfield, this demonstrates that the long neck in giraffes did not result from mere pattern of use and disuse. Whitfield also argues against natural selection by saying if natural selections was responsible for long necks is giraffes then it would also have led to elongation of Okapi’s neck (Whitfield 57). He explains that Okapi would also benefit from longer necks by being able to access more food but there is no evidence of this in Okapi.
Apart from Lamarckian and Darwinian theories, long necks in giraffes are sometime attributed to adaptation to weather condition. This theory argues that ancestral giraffes lived in cold areas. As the weather started to be warmer, the neck lengthen and an adaptation trait. Long neck increase surface area and thus it facilitates cooling. Although this is relevant, it fails to account why other animals don’t do the same.
Sexual Selection and Long Necks in Giraffes
Sexual selection theory suggested by Simmons and Sheepers propose that long necks in giraffes result from competition for mates. It explains long necks as resulting from use of the neck as weapon during intersexual competition. Simmons and Sheepers explain that during competition for a mate, make giraffes stand close to each other and use their neck as weapons (Simmons and Scheepers 776). During necking male giraffe exchange hits by use of their necks. This implies that a giraffe with longer neck would have advantage over the other and will be more likely to win. The longer the neck would mean the more destructive a giraffe would be. This phenomenon would imply that the giraffes with longer would be selected over those with short necks. Since necking is unique to male giraffes, sexual selection would be a better explanation to the long necks. Necking is known to be effective in giraffes where make giraffes that are able to neck effectively being able to win mates. Necking is often very violent with some make giraffes being knocked to unconsciousness or even killed on the process. The fights for mate often prolong. Even after a giraffe brought to ground, it may be stepped on or kicked leading even leading to death. A male giraffes that has a long neck is often able to win competition for a mate. Giraffes with short necks are not only unable to win for mates but may also die during necking.
Sexual selection can be used to explain evolution of traits or characters that enable and individual to be more successful in completion for mates. Sexual selection takes two forms: evolution of traits that promote winning in competition for a mate and evolution of characteristics that make a make more appealing to a female. Intersexual competing mainly involves evolution of traits. The evolved traits are used as weapons and assist an individual to be able to win mates (Brownlee 1071). Sexual selection traits are mainly acquired at the expense of other parts of the body. This is actually true with giraffes where the neck and head have evolved more than other parts of the body. Due to their long neck; the giraffe’s heart is forced to beat at a higher rate. This is vital so as to ensure that the brain is supplied with sufficient oxygen. This explains the reason as to why giraffes have relatively higher blood pressure than other mammals. Figure below how two male giraffes necking.
Limitation of Sexual Selection Theory
Although sexual selection theory seems to give a better explanation to long neck in Giraffes, it does have some limitation. The theory fails to offer an explanation to female giraffes having longer necks than male. Since females are not involved in competition nor do they neck, sexual selection cannot account for their long necks. Simmons and Scheppers attempt to offer explanation to this phenomenon. One explanation given is that the longer necks in female could result from other past selective pressures that do not exist today. They also attribute the longer necks in female giraffes to genetic components that result from mating.
Conclusion
Sexual selection offers a better explanation to evolution giraffes’ long necks. There are a number of issues that support the theory. These includes:
- Fossils data
- Behavioural analysis
- Courtship rituals
Giraffe males use their necks to compete for a makes, a feature that determine hierarchy of a male giraffe in a heard. The male giraffe with long neck is able to win in competitions and able to pass down its long neck genes. As time goes, giraffes with longer necks become dominant. Use of neck in necking explains the disproportionate increase of length of the neck to other body parts. Darwin’s explanation that the long neck result from natural selection resulting from completion for food fail to hold since it fail to account for failure of other body parts to increase proportionately. Lamarckian theory on the other hand fails to explain how long necks resulting from stretching passed down. Sexual selection gives a convincing explanation to evolution of giraffes’ long neck. In summation, sexual selection gives a satisfactory explanation behind giraffe’s elongation of their necks.
Works Cited
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