MadSci Network: Evolution
Query:

Re: Why do male peacocks have such a large colourful tail?

Date: Tue Jan 21 13:07:16 2003
Posted By: derrick kolling, Grad student, biophysics, university of illinois urbana champaign
Area of science: Evolution
ID: 1040765550.Ev
Message:

Peahens (female bird) usually select a mate based on certain properties of its “tail” (the plumage you are referring to is found in front of the tail and is known as the train). The choice of mate often is based on characteristics (symmetry, size, and distribution) of the eyespots found on the train.

The peacock train is theorized to have evolved by a mechanism known as sexual selection. Sexual selection is distinct from natural selection. Natural selection results in the propagation of traits that enable the organism to survive, whereas sexual selection leads to the ability to produce progeny. It should be noted that this is only one example of sexual selection. There are other types of sexual selection, such as physical confrontation to win favor with the opposite sex.

The sexual selection that results in the peacocks’ ornate train can be easily understood by considering the outcome of the peahens’ choice of a mate. The peahen chooses a mate that has desirable train and therefore the males’ traits are passed on and “survive” in the offspring. A balance is struck however as the size (size is just one possibility, coloration and other factors may also play a role in susceptibility to predation) of the train may shorten the peacocks’ lifespan. If it can’t survive to a reproductive age, then it cannot pass on its traits, including the train, to the next generation. In other words, a balance is struck between the organism’s survival and its ability to reproduce. It is now understood that traits surviving sexual selection do not enhance the survival rate of the individual organism and may actually decrease it. Darwin understood (and explained) this well, as can be seen in the following passage: “[Sexual selection] depends, not on the struggle for existence, but on a struggle between the males for possession of the females; the result is not death to the unsuccessful competitor, but few or no offspring” (The Origin of Species).

Another interesting question is why the large tail appeared at all. A proposal made by Ronald Aylmer Fischer (which I discovered through the second reference) was that train length may have in some way correlated to the fitness of the peacock. If an elongated train was more likely to result in death, a peacock with a long train must have traits that negate this disadvantage in order to survive.

This is, of course, just an overview of sexual selection. I encourage you to use the references below to achieve a better understanding of this theory.

 

The Origin of Species – Charles Darwin

Evolution – Mark Ridley
http://www.necsi.org/projects/evolution/evolution/sexual% 20selection/evolution_sexual.html - Web Site by Shlomiya Bar-Yam at NECSI
http://www.pbs.org/wgbh/evolution/library/01/6/l_016_09.html - Web Page of PBS


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