Sperm competition mechanisms in birds: models and data

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Abstract
This study presents three models to explain the mechanism of last male sperm precedence in birds. Because passive loss of sperm from the female reproductive tract occurs, all models incorporate this process. The three models are passive sperm loss alone, stratification with passive sperm loss, and displacement with passive sperm loss. With two inseminations containing the same number of sperm, the models make the following predictions. For passive sperm loss alone, (1) differential paternity is positively and linearly related to the time interval between inseminations, (2) with a slope that is equal to rate of loss of sperm from the female reproductive tract, (3) with an intercept that is the same as the differential fertilizing capacity between the semen of the two inseminations, and (4) the ratio of offspring from two inseminations remains constant over time. For stratification, (1) the relationship between differential paternity and the interval between inseminations is nonlinear and exhibits a “broken stick” pattern, with a substantial first-insemination precedence for short intervals, and (2) the proportion of offspring fathered by the first insemination increases over time. For displacement, the relationship between differential paternity and the interval between inseminations is nonlinear and also exhibits a “broken stick” pattern, but in contrast to the stratification model, sperm from the last insemination have precedence. Data from three experimental studies of the domestic fowl and one for the turkey provide the opportunity to test these models, albeit to different extents. The data from all studies are consistent with the passive sperm-loss model, except that one aspect of one data set provided ambiguous support for stratification. None of the data provided any support for the displacement model.

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