The Comb (October 2012)

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We look forward to bringing you further exciting updates over the next month! View the latest posts on the BMC Series blog homepage. Latest posts by Emily Crow see all Call for papers: Paving the desire paths of health information needs: The phenotype-linked fertility hypothesis suggests that the honesty of these signals may be maintained through a link with fertility. Indeed, in some recent studies, bright coloration has been linked with better fertility and also indicates semen characteristics that influence fertility, such as sperm performance.

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However, most of these studies were performed on species with ornamental feather coloration, in which the ornament is produced months prior to reproduction. We tested the phenotype-linked fertility hypothesis in the domestic rooster, a species where comb characteristics fluctuate on a short-term basis according to environmental and physiological conditions and thus may serve as immediate signals of quality to hens.

We examined the relationships of comb size and color with measures of sperm quality, including concentration, mobility, and viability. We found that comb size was inversely correlated, whereas comb color was positively correlated with sperm viability. Thus, males with the smallest reddest combs had the highest percentage of viable sperm.

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Our finding that comb color signals better quality sperm supports the phenotype-linked fertility hypothesis. However, the negative relationship between comb size and sperm viability indicates that females who choose larger ornaments also choose males with lower fertility, a counterintuitive behavior in terms of the phenotype-linked fertility hypothesis.

We examined the relationships of comb size and color with measures of sperm quality, including concentration, mobility, and viability. We found that comb size was inversely correlated, whereas comb color was positively correlated with sperm viability. Thus, males with the smallest reddest combs had the highest percentage of viable sperm.

Our finding that comb color signals better quality sperm supports the phenotype-linked fertility hypothesis. However, the negative relationship between comb size and sperm viability indicates that females who choose larger ornaments also choose males with lower fertility, a counterintuitive behavior in terms of the phenotype-linked fertility hypothesis.


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To understand these signals in an adaptive context, it is important to determine how each individual comb characteristic is interpreted and utilized by females during mate choice. Many male birds exhibit dramatic and conspicuous sexual signals that likely provide information to females about male quality because these traits are condition dependent. Decades of work have been devoted to uncovering the environmental, behavioral, and physiological factors responsible for honest signal expression in birds.

In his phenotype-linked fertility hypothesis, Sheldon suggested that the honesty of avian sexual signals may be maintained by a link between a male's sexual display and his functional fertility. The abundance of infertile males within wild avian populations has been questioned due to the likelihood of strong selection against infertility.

However, evidence suggests that there is, in fact, variation in the ability of male birds to produce fertile offspring Wetton and Parkin ; Sheldon and Burke ; McGary et al.


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Recent studies have provided additional support for Sheldon's phenotype-linked fertility hypothesis. Male mallards with brighter bill coloration during pairing produced sperm with accelerated swimming speeds during mating several months later Peters et al. Likewise, a study on domestic chickens showed a strain-specific relationship between comb size and fertilizing ability of sperm McGary et al.

Domestic chickens are particularly appropriate for studies testing the phenotype-linked fertility hypothesis because, while they are maintained in captivity, they retain many of the behavioral characteristics and social structure of their Asian ancestors, red jungle fowl Gallus gallus. Both sexes move in small flocks consisting of one dominant and several subordinate roosters and several hens Zuk, Johnson, et al. Hens may mate with several roosters and choose males based on a composite assessment of multiple secondary sexual characteristics, including bright red combs and wattles Zuk, Johnson, et al.

Comb size and color appear to be most important in mate choice decisions by hens Zuk, Johnson, et al. In addition, their large body size and their domesticity make repeated sperm collection relatively easy.


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A large body of work has been devoted to developing assays to assess several aspects of sperm quality, including sperm concentration Froman and Feltmann , mobility Froman and McLean , viability Blom , and ability to penetrate the perivitelline layer of the ovum Bramwell et al. Previous studies show positive relationships between comb size and characteristics of sperm quality, however, these results are strain specific Gebriel et al.

In 3 studies, no relationship was found between comb size and measures of sperm quality, such as sperm concentration or mobility Pizzari et al.

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Thus, the nature of the relationship between comb size and sperm quality remains unclear in domestic chickens. In addition, to date, no study has examined the relationship between comb color and characteristics of sperm quality in any species or strain. We tested whether comb size and comb color, the 2 primary criteria used by domestic hens in mate choice, were significant predictors of sperm quality, using measures of sperm concentration, mobility, and viability.

We predicted that both comb size and color would serve as honest signals of sperm quality. Twenty Cobb broiler breeder roosters were reared under standard industry conditions in floor pens until maturity. When the roosters were 25 weeks of age, we transferred them into cages where they were housed under a light cycle of Measures of comb size, comb color, and sperm quality were taken once weekly for 2 consecutive weeks starting when roosters were 36 weeks old.

Both semen concentration and mobility were measured using an assay outlined in Froman and Feltmann A clean semen sample, free of contaminants, was collected from each rooster via abdominal massage and was held in a prewarmed test tube.

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The unknown concentration was then calculated by comparing with a standard curve. Subsequently, mobility was determined using a modified version of the Accudenz assay designed by Froman and Mclean Although this technique was initially validated using Accudenz, we utilized Nycodenz, which is equivalent in composition Sbracia et al. This procedure was repeated for each of the 20 roosters. Neat semen was diluted 1: Magenta-stained sperm cells were indicative of dead, or nonviable, sperm that possess permeable cell membranes and permit eosin to enter and stain the sperm.

A percentage of live: Comb sizes were measured in 2 ways. Using a set distance of 3 m and standardized lighting, we then took a digital picture of the comb against a cm 2 grid containing 1 cm 2.

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We used the same pictures and software to perform color measurements on the combs by selecting 3 single-pixel points across the length of the comb at each end and in the middle and recording red, green, and blue intensity values for each pixel. Because color metrics are all interrelated, it is possible to use red, green, and blue intensity values to calculate the hue, saturation, and brightness.

We used the online RGB to HSB calculator Gardner to obtain values of hue, saturation, and brightness for each color pixel selected. This calculator works using the following simple equations: Brightness is the maximum intensity value divided by , saturation is the difference between the maximum and minimum intensity values divided by the maximum intensity value, and hue is the difference between the 2 lowest intensity values divided by the maximum intensity value and expressed in degrees 0— Hue, saturation, and brightness were averaged for all 3 pixel points measured for each comb.

Mobility measures therefore had to be corrected for the influences of sperm concentration before analyses, and we did this using the residuals of the regression described above. Because sperm viability is calculated as a percentage, sperm viability data were arcsine transformed. Sperm concentration and the residual sperm mobility values were normally distributed and required no further transformation. To reduce the number of comparisons conducted, both the comb color measurements and the comb size measurements were condensed using separate principal components PCs analyses, resulting in a single PC each for comb color and comb size.

Hue and saturation loaded positively, whereas brightness loaded negatively for measurements taken on both occasions For weeks 1 and 2, respectively: A single component for each week was labeled PC1 size and was used as our assessment of comb size. The relationship between comb size PC1 size and color PC1 color was examined using a simple regression during both weeks of data collection. For data collected during week 1, the relationships of PC1 color and PC1 size with sperm concentration, viability, and mobility were analyzed using simple regressions.