Comb size and color relate to sperm quality: a test of the phenotype-linked fertility hypothesis

In many avian species, males display dramatic and conspicuous ornaments that serve as honest signals of quality to females. 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. 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. 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. 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.