Vincent C. Slater

April 20, 2003

Avian Sexual Selection

Introduction

"...[I]f a man can in a short time give elegant carriage and beauty to his bantams, according to his standard of beauty, I can see no good reason to doubt that female birds, by selecting, during thousands of generations, the most melodious or beautiful males, according to their standard of beauty, might produce a marked effect." Charles Darwin, Origin of Species (1859). Itπs thought to be wrong to give human characteristics to things that are not in fact human, but in this case Darwin was correct in his statement. Almost 150 years ago, Darwin recognized the fact that animals selected their mates and explained the process to be the "struggle between individuals of one sex, usually males, for the possession of the other sex," (Darwin 1859). Since its inception, during Darwinπs time, research into sexual selection has become one of the most intensely studied fields of evolution over the past century (Moller 1994). Not only birds and other animals but also humans participate in the same evolutionary processes of sexual selection.

Characteristics of bird species have evolved over the centuries resulting from sexual selection. Such changes include the development of immense fan-like tails of the

male Peacock, bright colored plumage, puffy throat sacs, large spurs, to even the beautiful melodic songs of other birds. Darwin defined sexual selection in his 1871 writing The Descent of Man, and Selection in Relation to Sex describing two methods of sexual selection. Male vs. Male contests, involving a competition between two males, from which the loser will lose its opportunity to mate with a certain female, is the first type of sexual selection. This is most commonly seen in male deer whose antler size is one of the determining factors in these battles, but male to male contests are also seen in birds. For example some species of galliform males have large spurs on their legs or wings which are often used during these fights (Moller 1994). The second form of sexual selection described by Darwin is known as Mate Choice (Darwin 1871). Aesthetically pleasing or beautiful traits of birds like brilliantly colored feathers, large tail feathers and even the melodic songs heard are all traits female mates may choose for mating purposes. Although this process seems some what strange (mainly because it personifies birds), it has been found that females tend to make selections for some of these traits to varying degrees due to their close association with fitness provided the costs of such displays do not outweigh the benefits of this selection. There is at times an over lap of these two methods of selection, for example, the spurs of male-ring necked pheasants are used for male to male contests but females also tend to select males with larger spurs (von Schantz et al. 1989).

Natural selection occurs when some individuals out reproduce others, as a result the genetic material of select individuals is passed down to the next generation of individuals and so on. Sexual selection is a means by which this process occurs. Those individuals are selected via the processes from above resulting in what Mills and Beatty (1979) defined as fitness; the ability of an organism to ≥survive and reproduce≤ the most viable offspring that will propagate the species. Darwin (1859) referred to evolution as the survival of the fittest which was added to by Mills and Beatty.

Body

Bright plumage in birds has become the standard example of sexual selection, in terms of birds. The bright vibrant colors are not only attractive to avid bird watchers but to other individuals in their species. Darwin found there to be a positive correlation between plumage brightness in males and the degree of polygyny observed due to the effects of sexual selection (1871). Those males with the brighter plumage will experience a higher rate of mating success in male to male contests and in mate choice over those individuals who are phenotypically less vibrant. These males are often times chosen because they are less profitable to predators (Baker and Parker 1979) and less susceptible to parasites and pathogens (Hamilton and Zuk 1982). Hoelzer (1989) suggested that these males are better parents but in some polygynous species such as the red-winged blackbird. The preferred male provides very little if any parental care while males who engage in fewer copulation events seem to be much better at parenting. The reason for such a selection is described by Weatherhead and Robertson (1979) as the åsexy son hypothesisπ. This hypothesis suggested that females selected the preferred males because the next generation of would possess the desirable traits; making them either able to out compete more dominant birds or simply being more attractive to females for reproduction.

Most species of birds however donπt follow the polygynous model, rather they form monogamous relationships that last typically for the breeding season. In a group of individuals where there is an unbiased sex ratio exists, in theory each member of the community should be able to form a monogamous relationship with another individual. Males in such societies would not be expected to develop the overly exaggerated phenotypic characteristics displayed by polygynous males. When the motivation for variation is removed, we would expect not to see such characteristics. Instead we find that to be untrue. Male mallards that form monogamous bonds prior to winter have an incredibly shinny green head and grey body as compared to the dull female mallards. Regardless of a polygynous or monogamous lifestyle the display of bright colors, large fan tails, etc., always carries a cost. What would be more obvious to a predator then a bright colored bird perched upon a tree branch? These traits that expose even the strongest of male birds is viewed as a handicap. Due to the obvious increased predation this typically results in a decrease in the expression of this

exaggerated trait (Getty 1998). In a contradicting study, Anderson (1982) found that by altering the tail lengths of long-tailed widowbirds, females tended to choose males with the altered tails over short and control tailed birds. One suggested reason for this discrepancy is the fact that noticeably, the large tailed males are much more prominent due to this large tail (Merry 1999). From this hypothesis, males of the highest fitness display or even advertise their exaggerated features as a display of this fitness. The åunprofitable prey hypothesisπ suggested by Baker and Parker (1979) suggests that predators have taken notice to the increased fitness of bright colored birds and tend to avoid preying on them for the more drab colored birds. To prove that predators avoided attacking such brightly colored birds, Gˆtmark (1996) placed fake specimens of pied flycatchers on mounts in a field. He placed the bright males only several meters from the drab females. The result from the experiment was the females were attacked by sparrowhawks at a greater rate than the males due to their bright feathers suggesting greater fitness.

Thompson et al (1997) found there to be a correlation in house finches and the amount of parasite load. They did not try to indicate that parasites were the ≥cause or consequence of poor host physiological condition≤(Thompson et al 1997) but rather that parasites negatively effect the biological and biochemical mechanisms involving metabolism, etc. Their study looked at post-molting plumage color, feather length and parasite infection rates to determine how sexual selection was affected. Thompson et al (1997) determined from their experiment that long, red flight feathers and plumage were indicators of high fitness, low parasite infection while males with a high parasite infection rate were drab-yellowish in color with shorter flight feathers. The reason for smaller feathers is considered a direct result of the parasitic infection leading to less energy being expended to grow the feathers and being redirected to fight the infection (Thompson et al 1997).

Songbirds are perhaps some of the most enchanting birds there are. There melodic songs captivate bird watchers around the world because of their beautiful songs from today and yesterday during Darwinπs time. Itπs no wonder that due to this singing trait female birds tend to choose males with the most vibrant, melodic songs. Similar to the tails of peacocks, the complexity of songs is used in some species as a gauge of fitness. Moller (1994) suggested that due to the resistance of fit birds against disease and parasitic infection, those which are healthier are able to devote more time and metabolic energy towards time spent on learning/imitating new songs. ≥Experiments in which male condition was manipulated, through food supplements, have shown that singing rate may honestly reflect a male's current physiological condition≥(Nowicki 1998).

Mutation is one of the several driving factors influencing evolution but typically we think of it as a random event resulting in changes to the genetic material of an organism occurring naturally. In April, 1986 the meltdown of a nuclear reactor in reactor no. 4 of the Chernobyl nuclear power plant caused radiation to spread across Europe. The effects of the radiation caused mutations in the genetic material of many animals, plants, etc across the region resulting in changes in certain allele frequencies effecting phenotype. A study conducted by Moller and Mousseau (2001) found that the mutation rate for albinism in the barn swallows had increased from the normal rate of ~1% up to 13-15% in those affected by the Chernobyl disaster. ≥Because germline mutations arise every generation, we should expect a steady decrease in mean phenotypic values with time and an increase in partial albinism in barn swallows from the Chernobyl region, because an increasing fraction of the population would be comprised of mutants, unless small, albinistic mutant phenotypes were selected against≤ (Moller and Mousseau 2001). From this hypothesis it was determined using collected data from barn swallows that selection against the smaller sized, albino swallows was occurring resulting in a lower probability of survival compared to non-albino birds (Moller and Mousseau 2001).

Conclusions

Darwin (1871) hypothesized that sexual selection was one of the many forces driving evolution and natural selection. Whether it be from changes in plumage color to genetic mutations resulting from human interactions (Chernobyl for example), selection for the fittest individuals with the most desirable phenotypes is something that has remained constant throughout the study of sexual selection. Although not always the most logical way of for protection, the use of bright colors, etc. seems to not only signal predators of their higher fitness levels but also the female birds; whom the bright advertisements are being directed towards of the same thing. Darwin said it best with the quote used at the beginning of this paper: "...[I]f a man can in a short time give elegant carriage and beauty to his bantams, according to his standard of beauty, I can see no good reason to doubt that female birds, by selecting, during thousands of generations, the most melodious or beautiful males, according to their standard of beauty, might produce a marked effect." Charles Darwin, Origin of Species (1859). Birds do the same as man, select for the traits they like, we do it for their beauty, but birds do it for fitness, beauty is a secondary relationship indicating the fitness they search for.

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