Sperm mobility determines the outcome of sperm competition in the domesticfowl

The aim of this study was to establish whether the mobility of sperm of the domestic fowl, as measured by an in vitro assay, predicted the outcome of sperm competition. Thirteen pairs of New Hampshire roosters, comprising one male categorized as having high-mobility sperm and the other as having ave rage-mobility sperm, were used. Each male provided 25 x 10(6) sperm, which were mixed and artificially inseminated into between four and seven New Ham pshire hens, each of which produced 2-11 offspring. The experiment was cond ucted twice, such that the same pair of males inseminated the same females. Paternity was assigned by using microsatellite markers. There was a clear effect of sperm-mobility phenotype on the outcome of sperm competition: in all 13 pairs the high-mobility male fathered the majority of offspring (73. 3% overall; p < 0.0001). The proportion of offspring fathered by the high-m obility male within pairs varied significantly between male pairs (p < 0.00 05). This effect was associated with the difference in sperm-mobility score s between males within pairs: there was a significant positive relationship between the proportion of offspring fathered by the high-mobility male and the ratio of mobility scores between males (p < 0.05). In addition, compar ed with their success predicted from the non-competitive situation, in the competitive situation high-mobility males were disproportionately successfu l in fertilizing eggs compared with average-mobility males. This may occur because female sperm storage is limited in some way and a greater proportio n of high-mobility sperm gain access to the female's sperm storage tubules. There was no evidence that female effects accounted for any of the variati on in paternity.