Evolution of Giraffe (Giraffa camelopardalis) Necks: By Sexual Selection or Competing Browsers?


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At a height of 18 feet, the giraffe is extremely unique and the world’s tallest mammal. All nine species of giraffes are found in Africa, which limits the amount of research that has been done with them (“Giraffe”). Some analysis on this mammal has been performed, and one immense question surrounding the giraffe is the origin of the evolution of its long neck. From many of the studies done over the years, two hypotheses have arisen. Some scientists believe the cause of the evolution of the giraffe’s increasing neck length is due to sexual selection, while others argue it is due to competing browsers. The sexual selection hypothesis argues that the larger males with longer necks are dominant against smaller males in fights for mating access with female giraffes. The competing browsers hypothesis argues that giraffes feed at a higher level to avoid competition with other organisms for food. Scientists agree evolutionary changes have occurred in giraffe’s necks, but the origin of this evolution is unclear. In this paper, I will describe the two hypotheses in detail and give supporting evidence for each. I will also show contradicting evidence and refute each argument. The evolution of the neck length in giraffes has been disputed among scientists for many years, and will continue to be in future years.
Evolution by Sexual Selection
When two male giraffes fight using their heads and necks as clubs to bash each other it is commonly referred to as “necking” (Simmons and Scheepers 1996). These fights occur because there is competition for priority in mating with estrous females. Female giraffes prefer the stronger, dominant males to mate with so they will have successful offspring. Through fossil evidence from the last...

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... (Giraffa camelopardalis) cervical vertebral column: a heuristic example in understanding evolutionary processes? Zoological Journal of the Linnean Society 155:736-757.
Cameron E. Z. E., du Toit, Johan T JT. 2007. Winning by a neck: tall giraffes avoid competing with shorter browsers. The American naturalist 169:130-135.
"Giraffe." National Geographic. Web. 26 Feb. 2012. .
Mitchell G., S. J. van Sittert, and J. D. Skinner. 2009. Sexual selection is not the origin of long necks in giraffes. Journal of Zoology 278:281-286.
Simmons R. E., R. Altwegg. 2010. Necks-for-sex or competing browsers? A critique of ideas on the evolution of giraffe. Journal of Zoology 282:6-12.
Simmons R. E., L. Scheepers. 1996. Winning by a neck: Sexual selection in the evolution of giraffe. American Naturalist 148:771-786.

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