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Parasite Mediated Sexual Selet Essay, Research Paper

Why do some birds have long tails, others have bright plumage or patterns and others possess beautifully complicated songs. Many animal behaviourists have agreed that all these characteristics are developed as a result of female sexual selection, but that is where the agreement stops.

The many theories on the evolution of secondary sexual characters, are an evolution in their own right. The purpose of this paper is to analyse the evolution of the research and hypotheses on this subject. How did each come about and lead to another, which was proven and find the answer to the question:

How do females benefit from choosing a male with an enhanced morphological feature?

What is sexual selection?

Sexual selection is a corollary of natural selection and the two processes work in the same manner. Natural and sexual selection often create different selection pressures: natural selection results in traits that enhance reproduction and survival, where as sexual selection results in traits that enhance reproductive success, but at the expense of survival (Ghiselin(1969) of Darwin in Birkenhead & Moller 1991).

Sexual selection occurs when the members of one sex mate disproportionately with members of the opposite sex on the basis of secondary traits, i.e. behaviours or structures other than reproductive organs or gametes. Sexual selection can occur in two ways; as a result of competition between members of the same sex (intrasexual) or by female choice of the fittest male following intra sexual competition or salesmanship (intersexual) (Clayton 1991).

One of the most important factors determining the intensity of sexual selection is the relative amount of parental investment by each sex (Trivers 1972 in Krebs & Davies).

The sex investing least are more involved in intrasexual competition, where as the sex investing most are more discriminating in their choice of partner (Trivers 1972 in Birkenhead & Moller 1991). Trivers (1972) suggested that because sperm were minute relative to the size of the ova, males invest less in gametes than females. Dewsbury (1982)argued that although they are smaller, there are a high number transferred in an ejaculate or spermataphore, the strategy possibly evolved as a result of sperm competition. E.g the more lottery tickets you have the higher the chance of winning (Birkenhead & Moller 1991).

There are three main hypothesis that run through the whole subject of sexual selection, within which are many more hypothesis.

Sexy son hypothesis


One of Darwin’s greatest achievements was the recognition that variation in characters such as antlers or elaborate plumage resulted in variance in reproductive success (Birkenhead & Moller 1991). Darwin suggested that evolution of elaborate plumage is due to female choice (Darwin in Petrie 1991). Darwin (1871) defined ’sexual selection’ as the effects of ‘a struggle between the individuals of one sex, generally the males, for possession of the other sex’(Darwin in Birkenhead & Moller 1991). Although the idea that traits such as antlers are involved in male – male competition was and remains relatively uncontroversial, Darwin’s views on the role of female choice were hotly debated from the start (Howlett 1993). Darwin suggested that many apparently deleterious secondary sexual characters in males such as bright colours, elaborate ornaments and conspicuous displays, evolved as a result of female choice. Darwin never tried to explain the crucial agent of selection, that females have preferences for exaggerated male traits. Rather, he took it for granted that females of many species possess a ’sense of beautiful’, akin to the sense in humans (Enquist & Arak 1993).Although Darwin did not have a satisfactory explanation for the origin and maintenance of the female choosiness assumed to drive the evolution of male traits his theory continued and was revitalised in 1915 by the English geneticist R.A. Fisher(Howlett 1993)


Fisher’s basic idea was that a novel but essentially arbitrary male trait such as a long tail might appear in a population and enjoy, for whatever reason, a slight advantage.

Fisher (1930) (Fisher in Krebs 1981) first suggested that females selected males with an enhanced feature as it indicated something about the male. Maybe they were better at flying and collecting food. It may also indicate something about male quality. Longer tails for example could simply be easier to detect and a point for species recognition. Longer tailed males may pass on an advantage to the females sons, hence the ’sexy son hypothesis’. The off-spring would both possess the genes that grow long tails (males) and those that choose them (females). Eventually the co-evolving genes for enlarged features would become so large that it would endanger the species, so the species would be checked by natural selection. (Howlett 1993). This is what Fisher called the ‘Runaway’ hypothesis (R.A. Fisher in Krebs & Davies 1993)Fisher then simplified his hypothesis to state that elaborate displays may be sexually selected simply because it makes males attractive to females. (R.A. Fisher in Krebs & Davies 1993).

Females prefer to mate with colourful males, plumage brightness correlates with the males capacity for parental care and perhaps their genotypic quality. The most successful males are those with the most extreme plumage, vocalisations and displays. (Kirkpatrick & Ryan 1991) Previous experimental studies on polygynous birds by Andersson (1982) And Von Schantz et al (1989)(in Petrie 1991) suggest that females may prefer males with enhanced morphological features. Lekking species are mainly polygamous, therefore males are in direct competition to obtain mates. Enlarging a critical morphological feature can increase mating success. If all train ornaments are intact it reflects good fighting ability and gene viability. (Atatalo et al 1991)

The train is a classic example of sexual selection, as with many species where the male is brightly coloured, it is employed to attract females (Hill 1991). Marion Petrie et al. (1991) undertook many experiments to prove that male peacocks, Pavo cristatus, with longer tails enjoy more successful copulation’s in a season. They investigated two hypothesis for the evolution of an extravagant morphological feature i.e. the peacocks train: (1) the train could have evolved because of its importance in competition between males, or (2) females could prefer males with more elaborate trains, either because the train indicates something about the male that is important to the female, or because females have developed an arbitrary preference for males possessing the character. Both hypotheses predict that variance in mating success would be accounted for by variance in train morphology (Petrie et al. 1991). The experiment results showed that there is non-random mating in peafowl, that a large part of the variance in mating success can be attributed to variance in train morphology and that females choose to mate with those males that have the most elaborate trains of those sampled (Petrie et al. 1991).

Another classic experimental study is that of Malte Andersson (1982) who showed that females of the long-tailed widow bird Euplectes progne in Kenya prefer males with long tails. The highly polygamous species is an ideal candidate for sexual selection; the male is a sparrow sized bird with a tail up to 50cm long, the female tail only 7cm long, presumably close to the optimum for flight purposes. Andersson studied 36 males which he divided into four groups. In one group he docked the tails to about 14cm, while in another group he attached the severed bits of group 1 tails with superglue. This increased the tail length of group 2 males by an average of 25cm. The remaining two groups were controls: one lot were left untouched and the others had their tails cut and glued without altering length. By counting the number of nests in each territory, Andersson showed that before his experimental manipulations there was no difference in mating success of the different groups, while afterwards the long tailed males did significantly better than the controls or the shorter tailed birds (Krebs & Davies 1993).

‘Good genes’ hypothesis

The handicap hypothesis suggested by Amotz Zahavi (1975) pointed out that a male with a long train has a handicap in day to day survival, therefore the ability to escape predators and survive, would indicate that the male was genetically viable. A female who chose that male would be passing on the genes to her off-spring who would, in turn have a long train and better chance of survival and reproduction. Males with shorter tails or lost tail feathers are more susceptible to predation and would pass weaker genes to the next generation. (McFarland 1993) It is therefore an indicator of ‘good genes’.(Zahavi in Birkenhead & Moller 1991) In the good genes models it is proposed that females obtain favourable genes or gene complexes by mating with particular males. This occurs because: (1) the secondary sexual character reveals either the males phenotypic quality (Andersson 1982, 1986 in Birkenhead & Moller 1991), (2) the traits females choose is a handicap to the male i.e. the trait reveals the males superior quality and his ability to survive despite having to bear the burden of the trait (Zahavi 1975, 1977 in Birkenhead & Moller 1991), (3) it reveals the male’s ability to resist parasites and disease (Hamilton & Zuk in Birkenhead & Moller 1991).

The difference between Fisher and Zahavi’s hypothesis is that the ‘good genes’ are genes for the utilitarian aspects of survival and reproduction, rather than genes purely for attracting females as Fisher thought Krebs & Davies 1993). Although Zahavi’s theory is widely accepted, following theoreticians pointed out that the male does not carry the handicap all year round but only during the mating season when in good condition (Krebs & Davies 1993). There are different variants of the flexible handicap idea, the ‘condition dependant’ or ‘revealing’ handicap is a degree of expression of the male sexual display to tell the female of his genetic quality (Krebs & Davies 1993). *

Parasite Mediated Sexual Selection

In 1982 Hamilton and Zuk proposed a provocative solution for the unexplained fact that the males of many species exhibit ’showy’ traits such as brightly coloured plumage or vigorous courtship displays. They suggested that showy traits are fully expressed only by males who are resistant to parasites and that females examine such traits in order to choose resistant males as mates (Clayton 1991). A male who is of ‘good genes’ would be resistant to parasites and therefore always have a perfect display (Hamilton & Zuk in Krebs & Davies 1993). The Hamilton Zuk hypothesis is a good genes model of sexual selection, which assumes that choice of resistant males benefits females indirectly through the inheritance of resistance by offspring. Parasite based mate choice could also lead to more direct fitness benefits (Clayton 1991). Females might choose unparasitised males to protect themselves and/or offspring from parasite transmission(transmission avoidance model), or to provide resources such as parental care (resource provisioning model). These models differ from the good genes model in that they require no assumptions about the genetics of host parasite interactions. Like the good genes model, they predict that female choice will occur on the basis of parasite-indicative traits. All three models of parasite mediated sexual selection (PMSS) predict that showy males benefit through enhanced mating success, but the models differ with respect to the postulated benefits acquired by choosy females (Clayton1991).

Fluctuating Asymmetry

Fluctuating asymmetry (FA) refers to the random, stress induced deviations from perfect symetry that develop during the growth of bilaterally symmetrical traits. Individual differences in the level of FA may influence mate choice: in a number of species, females prefer to mate with males that have more symmetrical sexual ornaments. As the degree of FA has been shown to reflect the ability of individuals to cope with a wide variety of environmental stresses, it has been suggested that mating preferences for symmetry evolve for adaptive reasons, because the degre of FA provides honest information about male quality (Johnstone 1994).

Moller suggested that females use asymmetry in secondary sexual characters to determine male quality. Fluctuating asymmetry reflects the susceptibility of an individual to environmental stress and genetic stress, with increased asymmetry in stressed individuals (Jennions 1993). Moller and other researchers claim that symmetry is a badge of biological fitness. Only those with the best genes and food supplies are likely to shrug off the parasites and other environmental hazards that can disturb healthy, symmetrical development (Concar 1995). Moller concluded “Symmetry good , asymmetry bad” (Moller in Concar 1995).

Moller has undertaken many experiments manipulating the tails of barn swallows (Hirundo rustica) to prove his argument. Male swallow tails were individually manipulated to effect the size and symmetry to determine whether length or asymmetry were used as cues in mate choice. The results showed that males with elongated and symmetric tails mated sooner and enjoyed larger annual success than those with shorter or asymmetric tails. This also suggests that females prefer males with such ornaments as indicators of male quality (Moller 1992).

These results were argued by Andrew Blamford and Adrian Thomas of Cambridge University. Their alternative explanations for the evolution and maintenance of tail symmetry are those of aerodynamics. The Cambridge zoologists set out to discover if long tails really are a burden which can be shouldered only by the fittest males. Using computer models, they calculated the lift and drag that birds tails of different sizes and shapes produced (Mason 1993). They investigated three main shapes found in bird tails, (1) tails which are triangular in shape during flight and forked at rest (swallow) (2) tails that taper down from the widest point (pheasant) (3) tails that have long streamer like extension (pintailed duck) (Mason 1993). The results showed that streamers and tapered tails caused a lot of drag. They do not make the tail wider or larger in surface area providing extra lift, they serve only to make flying more difficult (Blamford & Thomas 1993). Such tails could be explained by the good genes idea because a male would have to be strong and healthy to carry this type of tail and survive (Mason 1993). Long tails with a shallow fork are the most aerodynamic, the longer they are the more surface area they have causing more lift than drag (Blamford & Thomas 1993). Any asymmetry would affect the stability of flight (Blamford & Thomas 1993).

So tail length may reliably reflect male quality as predicted by ‘good gene’ models. The female preference for symmetric tails is more convincing evidence for good-gene processes, but it could also be due to sensory bias in animals toward symmetric shapes (Jennions 1993).

Magnus Enquist and Anthony Arak, of the University of Stockholm, looked into the mechanisms concerned with signal recognition. Using computer neural networks to model the evolution of female preferences for long tailed conspecifics. By training female birds to recognise controlled patterns that were representations of males and then showing them similar patterns. The females recognised patters that were similar to the controls but also opted for those that were exaggerated in size (Enquist & Arak 1993). Their results strongly suggest that all recognition systems contain ‘hidden’ female preferences (Jennions 1993).

‘Darwin’s idea that a ’sense of the beautiful’ is an inherent, aesthetic property of animal nervous systems may not be far from the truth. In Darwin’s own words “When we behold a male bird elaborately displaying his graceful plumes or splendid colours, it is impossible to doubt that the female admires the beauty of her male partner”‘ (Enquist & Arak of Darwin 1993).


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Atatalo et al. (1991) General lekking systems. Nature 352, 155 – 156.

Alatalo, R.V. & Osmo, R (1995) Sexy son hypothesis – controversial once more. TREE vol 10 p52.

Birkhead, T.C & Moller, A.P (1992) Sperm competition in birds, evolutionary causes and consequences. Academic Press.

Blamford, A & Thomas, A. (1993)Swallowing ornamental symmetry. Nature 361 628

Brookes M. & Pomiankowski A. (1994) Symmetry is in the eye of the beholder. TREE vol 9 no 6 p201.

Clayton D.H. (1991) The influence of parasites on host sexual selection. PARASITOLOGY TODAY vol 7 no1 p329.

Concar, D. (1995) Bees get a buzz out of symmetry. New Scientist 18

Concar D. (1995) Sex and the symmetrical body. New Scientist. April 22.

Enquist M. & Arak A. (1993) Selection of exaggerated male traits by female aesthetic senses. Nature 361 446-448

Guilford, T.(1995) Animal signals: all honesty and light? TREE vol 10 p100.

Hedenstrom. A. (1995) Swallows unhandicapped by long tails? TREE vol10 p140

Hill, G.E (1991) Plumage colouration is a sexually selected indicator of male quality.

Nature 350 337- 339.

Howlett R. (1993) Beauty on the brain. Nature. 361 398-399

Keller L. & Krieger M.J.B. (1996) Mating success of male birds. NATURE vol 380.

Kirkpatrick, M & Ryan M.J (1991) The evolution of mating preferences and the paradox of the lek. Nature 350 33-38.

Jennions. M.D. (1993)Female choice in Birds and the cost of big tails. TREE vol8 p230.

Johnstone. R. A. (1994) Female preference for symmetrical males as a by-product of selection for mate recognition. NATURE vol 372 p172

Krebs, J.R & Davies, N.B (1981 ) An Introduction to Behavioural Ecology.

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Petrie, M, Halliday, T & Sanders, C (1991) Peahens prefer peacocks with elaborate trains. Animal Behaviour 41, 323 – 331.

Petrie, M, Halliday, T & Sanders, C (1992) Peacocks with low mating success are more likely to suffer predation. Animal Behaviour 44, 585 – 586.

Petrie, M, Halliday, T & Sanders, C (1994) Improved growth and survival of off-spring of peacocks with more elaborate trains. Animal Behaviour 371, 598 – 599.

Ridley M. (1992) Swallows and Scorpion Flies Find Symmetry is Beautiful. Science 257 327-328

Swaddle. P. & Cuthill. I. C. (1994) Preference for symmetrical males by female zebra finches. NATURE vol 367 p165

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