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11/13/14 Final Paper

The Evolution of the Peacock’s Train

The peacock’s train has been a prime example of sexual selection for years. Peafowl exhibit intersexual selection, in which the female peahens are choosey. They have a preference for a certain trait in males, the peacock’s train in this example. The male Indian peafowl has iridescent blue and green feathers with eye spots at the end. The peacock uses his train in courtship displays in order to attract females. It is widely believed that peahens choose their mate based on the size, color, and quality of the male’s train (Loyau 2008). This idea has become highly debated over the years. There are now multiple theories which seek to explain this evolutionary process and what exactly females are choosing for (Loyau, Jalme, and Sorci 2005). This paper deeper explores these theories of how the peacock’s train evolved. It is important to gain more knowledge on this subject in order to better understand sexual selection. Sexual selection is the ability of male and female organisms to exert selective forces on each other with regard to mating activity (Herbers 2014). The strongest driver of sexual selection is gamete size. In general, eggs are bigger than sperm and females produce fewer gametes than males. This leads to eggs being a bigger investment, and therefore to females being choosey about the traits that will be passed on to her offspring by males (Herbers 2014). The peahen’s reproductive success and the likelihood of survival of her chicks is partly dependent on the genotype of the mate. Females generally have more to lose when mating with an inferior male due to her gametes being more costly than the male’s. The peacock’s train is a prime example of a secondary sexual characteristic. It distinguishes male and female peafowl, but it is not directly part of the reproductive system. How did female choice evolve? There are multiple hypotheses that explain the evolution of female choice. These hypotheses include direct benefits to the female (Herbers 2014). In some instances, females are provided with benefits such as protection, shelter, or nuptial gifts that sway the female’s choice of mate. Another possible hypothesis is that females choose mates with good genes (Herbers 2014). Males with more exaggerated secondary sexual characteristics, such as bigger, brighter peacock trains, tend to be more fit and therefore have better genes in the peahen’s eyes. These better genes will directly benefit her offspring as well as her fitness and reproductive success. Runaway selection also seeks to clarify the evolution of the peacock’s train. In the case of runaway sexual selection, linked genes in males and females code for sexually dimorphic traits in males and preference for that trait in females. The linkage disequilibrium of the genes causes a positive feedback loop that exaggerates the trait in males and the preference in females (Herbers 2014). Another hypothesis for the evolution of female choice is sensory bias, in which females have a preference for a trait in a non-mating context that becomes transferred to mating (Herbers 2014). It is important to remember that there may be multiple causations for the evolution of female choice. We will now delve deeper into these hypotheses and apply them more directly to the evolution of the peacocks train. For years it has been thought that peahens make their choice based on the number of eyespots, train length, and symmetry of the train of the peacock (Loyau 2008). It’s thought that the peahen is attracted to peacocks with more eyespots, and longer, more symmetrical trains. The good genes hypothesis explains this thought well. The female peahen is thought to be attracted to peacocks with better, more elaborate trains because the male’s good traits will then be passed on to her offspring. This will not only increase the chicks’ likelihood of survival, but also increase her reproductive success. Studies even suggest that peahens will lay more eggs and preferentially have more males when they mate with peacocks that have larger trains. Petrie and Williams investigated whether peahens laid more eggs when they mated with peacocks with larger trains. They predicted that females invest more in reproduction when mating with males with larger trains (Petrie and Williams 1993). This is because the potential benefits are greater for the female when she mates with a male of higher ornamentation. In their research, Petrie and Williams randomly mated peahens to peacocks that varied in ornamentation. They found that in these random matings, females that reproduced with males with more elaborate trains produced more eggs (Petrie and Williams 1993).

A study by Pike and Petrie also found interesting results related to peacock train elaboration. Birds have the ability to manipulate sex ratios of their offspring before they lay the eggs. This would allow the peahen to choose which sex to have depending on the quality of her mate. It is thought that the female would produce more males when mating with peacocks with more elaborate trains, which correspond to better genes. She would produce more males so that the sons benefit from the good genes the father possesses. Vice versa, the female would produce more females if the male was inferior, therefore not passing those inferior genes on to as many males. In Pike and Petrie’s study, they were interested in the effect of paternal attractiveness on the resulting sex ratio of offspring produced (Pike and Petrie 2005). In the experiment, they mated females to males with a varying degree of attractiveness, which was mainly based on the number of eyespots in the train. Certain male peacock trains were physically manipulated by removing eyespots in order to make them less attractive to the female. The study found that peahens that were mated to peacocks that were less attractive, therefore having fewer eyespots, produced significantly more daughters than when mated to more attractive males (Pike and Petrie 2005).

Another explanation as to why the peahen prefers longer, more elaborate trains is because the train length could be an indicator of age (Manning 2002). An older male with more eyespots would indicate a more fit male. This is due to the fact that he has survived longer than other males, even though he has a larger, more costly train. The male has therefore demonstrated his genetic quality by surviving the high selective pressures acting on him (Manning 2002). This leads the female to believe that he must have better genes and those genes will benefit her offspring. It is not known for certain whether number of eyespots actually correlates with age however. Data collected by Manning indicates that age is more associated with the total number of feathers per train, rather than any one feather type (Manning 2002). Manning’s data also shows that the eyespot’s color intensity, mean size, and feathers per train is more correlated with age than any one variable by itself. Manning concludes that the peacock’s train seems to be a good advertisement of age (Manning 2002).

There are a number of studies recently emerging within the last decade that provide evidence that peahens do not prefer peacocks with more elaborate trains. Mariko Takahashi investigated the role of the male’s train in mate choice in a population of Indian peacocks for over 7 years. Attractiveness of male peacocks was based on number of eyespots, more symmetrical trains, and longer feathers (Takahashi et al 2008). In their experiment, they found no evidence that peahens preferred peacocks with more elaborate trains (Takahashi et al 2008). Their results found that the train is not the target of peahens when choosing their mates. They also found that the train shows small variance among males across populations, and that the train may not even reliably reflect male condition (Takahashi et al 2008). They believe that peahens exercise choice based on other cues, possibly behavioral, aside from the peacock’s train. Takahashi and his partners had a few problems with the idea that the train is an indicator of good genes and that female choice is based on the train. The first was that since studies have evidence both for and against the effect of elaborate trains on female choice, therefore the train must not be the universal cue of choice (Takahashi et al 2008). Another problem they had is that there is no consensus on which traits of the train characterize males as more attractive (Takahashi et al 2008). While it is believed that length, symmetry, and eyespots play a role, there is no consistency between experiments that prove this. The peahen’s ability to assess these traits has also been highly questioned (Takahashi et al 2008). It is even believed that female preference for traits can be reduced, lost, or even reversed (Wiens 2001). In certain species, it appears that male ornamentation has been lost, even though the ornamentation was being actively chosen for by sexual selection (Wiens 2001). This implies that either other forces are able to overcome sexual selection or sexual selection may become weaker over time. The reason for loss of male traits is not known for sure, but there are many factors that contribute such as predation risk, loss of female preference, and genetic drift (Wiens 2001). There is, however, not enough data to prove that loss of female preference leads to loss of the trait in males (Wiens 2001).

It is easy to see that sexual selection is complicated. The evolution of female choice in peafowl has a number of different hypotheses (Loyau, Jalme, and Sorci 2005). In peacocks, it has recently become highly debated how peahens exhibit choice and what they are choosing for. Some believe that peahens prefer peacocks with larger, more elaborate trains that have more eyespots. However, many others have conducted research to disprove this and propose that females do not actually have a preference for elaborate trains and may not assess mates based on trains at all. These hypotheses are not mutually exclusive, and therefore multiple hypothesis could be correct in determining the mechanism, as multiple forces are at play. Understanding the mechanisms of sexual selection and female choice will help us better our understanding of the evolution of the peacock’s train through further research.

11/13/14 Edit to wiki page:

https://en.wikipedia.org/wiki/Peafowl

Sexual selection is the ability of male and female organisms to exert selective forces on each other with regard to mating activity. The strongest driver of sexual selection is gamete size. In general, eggs are bigger than sperm and females produce fewer gametes than males. This leads to eggs being a bigger investment, and therefore to females being choosey about the traits that will be passed on to her offspring by males. The peahen’s reproductive success and the likelihood of survival of her chicks is partly dependent on the genotype of the mate. Females generally have more to lose when mating with an inferior male due to her gametes being more costly than the male’s.

How did female choice evolve? There are multiple hypotheses that explain the evolution of female choice. These hypotheses include direct benefits to the female. In some instances, females are provided with benefits such as protection, shelter, or nuptial gifts that sway the female’s choice of mate. Another possible hypothesis is that females choose mates with good genes. Males with more exaggerated secondary sexual characteristics, such as bigger, brighter peacock trains, tend to be more fit and therefore have better genes in the peahen’s eyes. These better genes will directly benefit her offspring as well as her fitness and reproductive success. Runaway selection also seeks to clarify the evolution of the peacock’s train. In the case of runaway sexual selection, linked genes in males and females code for sexually dimorphic traits in males and preference for that trait in females. The linkage disequilibrium of the genes causes a positive feedback loop that exaggerates the trait in males and the preference in females. Another hypothesis for the evolution of female choice is sensory bias, in which females have a preference for a trait in a non-mating context that becomes transferred to mating. It is important to remember that there may be multiple causations for the evolution of female choice.

9/29/14

https://en.wikipedia.org/wiki/Peafowl

3 improvements: 1.'This theory may be contrasted with Fisher's theory that male sexual traits, such as the peacock's train, are the result of selection for attractive traits because these traits are considered attractive.' This statement can be expanded on as follows: Fisher's theory was based on runaway selection, which is when females choose mates based on less useful or detrimental traits that they find attractive. This may clarify some information for the readers.

2.The discussion of the "eyes" of the plumage can be expanded on. The "eyes" are called ocelli and vary in size and number for each peacock. This will help clarify the use of the term ocelli later in the article.

3.'Takahashi determined that the peacock's train was not the universal target of female mate choice, showed little variance across male populations, and, based on physiological data collected from this group of peafowl, do not correlate to male physical conditions.' This statement can be expanded on as follows for clarity: Takahashi concluded that calling, specifically male acoustic signaling, may affect the mating success of peacocks. This will help clarify the results of Takahashi's study.

Edit to page: Merle Jacobs proposes a food-courtship theory, which states that females are attracted to males due to features that resemble territorial foods. He claims peahens are attracted to peacocks due to the resemblance of their eye spots to blue berries. [12] citation 12 = Jacobs, M. March 10, 1999. A New Look at Darwinian Sexual Selection. Natural Science. [9/14/2014]; http://naturalscience.com/ns/articles/01-11/ns_mej.html.

Amanda Mills 9/14/14 Paul Larson Tues 1:50

Topic: The evolution of the peacock’s train through sexual selection. This topic has recently been debated as to whether it arose from sexual selection and whether bigger trains exhibit higher fitness.

5 Primary Sources:

1. Jacobs, M. March 10, 1999. A New Look at Darwinian Sexual Selection. Natural Science. [9/14/2014]; http://naturalscience.com/ns/articles/01-11/ns_mej.html

This article discusses male adornment and the possible reasons why they are advantageous or selected for. The author argues that what seemed to be choice of an adorned male by a female was actually a mutual attraction of both sexes to a particular type of reproductive site. She claims that mate selection is related directly to adaptive niche specialization. A food-courtship theory that she devised is discussed in relation to peafowls.

2. Loyau, A. Perie, M. Saint Jalme, M. Sorci, G. September 7, 2008. Do peahens not prefer peacocks with more elaborate trains. ScienceDirect. [9/14/2014]; 2008, 76, e5-e9. doi: 10.1016/j.anbehav.2008.07.021. http://www.adeline-loyau.net/publications/Loyau_etal_AnimBehav2008.pdf.

This article discusses the peacock’s train and whether or not the peafowl prefers more elaborate trains. It discusses how the peacock’s train is a complex structure that cannot be summarized with only three morphological traits, as it often is when experiments are done on the topic. It discusses possible reasons as to why the peafowl isn’t attracted to bigger, more complex trains.

3. Manning, JT. December 19, 2002. Age-advertisement and the evolution of the peacock’s train. Jornal of Evolutionary Biology. [9/14/14]; 2(5), 379-384. http://onlinelibrary.wiley.com/doi/10.1046/j.1420-9101.1989.2050379.x/pdf.

This article discusses the costly structures of male birds and how they evolved. The author discusses four possibilities which invoke female preference for male ornaments. It then discusses each possible way more in depth and gives data that supports or denies. The author also shares his research on the subject and discusses the results, stating that peahens tend to choose peacocks with more ocelli because this is correlated with age (the more ocelli, the older). And this finding suggests that peahens prefer older peacocks due to the fact that they have survived longer and have a higher fitness.

4. Takahashi, M. Arita, H. Hiraiwa-Hasegawa, M. Hasegawa, T. February 20, 2008. Peahens do not prefer peacocks with more elaborate trains. ScienceDirect. [9/14/14]; 2008, 75, 1209-1219. doi: 10.1016/j.anbehav.2007.10.004. http://z4.ifrm.com/12418/66/0/p1028200/Peacockmain.pdf.

This article discusses a study done by the authors in which they investigated the role of the male train in mate choice of Indian peafowl. They used male and female centered observations in a feral population of Indian peafowl in Japan for 7 years. The study found no evidence that peahens prefer peacocks with more elaborate trains. The article then goes on to discuss the results and possible reasons why. They conclude that peahens are more likely to exercise choice based on cues rather than the peacock’s train.

5. Wiens, J. September 9, 2001. Widespread loss of sexually selected traits: how the peacock lost its spots. Trends in Ecology and Evolution. [9/14/14]; 16(9), 517-523. http://courses.washington.edu/ccab/Wiens%20on%20sexual%20selection%20-%20TREE01.pdf.

This article discusses the evolution of elaborate male traits and female preferences for these traits. It uses studies to show that losses of elaborate male traits are widespread and can be more common than gains. It also claims that female preferences for male traits can be reduced, lost, or reversed. It then discusses the implications for competing models of sexual selection due to these losses of traits and preferences.