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Directional postcopulatory sexual selection revealed by artificial insemination


Postcopulatory sexual selection comprises both sperm competition, where the sperm from different males compete for fertilization1, and cryptic female choice, where females bias sperm use in favour of particular males2. Despite intense current interest in both processes as potential agents of directional sexual selection3, few studies have attributed the success of attractive males to events that occur exclusively after insemination. This is because the interactions between pre- and post-insemination episodes of sexual selection can be important sources of variation in paternity4. The use of artificial insemination overcomes this difficulty because it controls for variation in male fertilization success attributable to the female's perception of male quality, as well as effects due to mating order and the relative contribution of sperm from competing males5. Here, we adopt this technique and show that in guppies, when equal numbers of sperm from two males compete for fertilization, relatively colourful individuals achieve greater parentage than their less ornamented counterparts. This finding indicates that precopulatory female mating preferences can be reinforced exclusively through postcopulatory processes occurring at a physiological level. Our analysis also revealed that relatively small individuals were advantaged in sperm competition, suggesting a possible trade-off between sperm competitive ability and body growth.

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Figure 1: Expected versus observed paternity distribution.
Figure 2: Effect of phenotypic traits on the proportion of offspring sired.


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We thank T. Birkhead, J. Kelley, B. Kempenaers, F. Neat, T. Pizzari, L. Simmons and A. Skinner for comments on an earlier draft of the manuscript, C. Romualdi for statistical advice and A. Ludlow for assistance with the preliminary artificial insemination trials. This research was supported by a Marie Curie Independent Research Fellowship from the EU and was carried out in conformity with the relevant Italian laws governing the care of animals in research.

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Correspondence to Jonathan P. Evans.

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Evans, J., Zane, L., Francescato, S. et al. Directional postcopulatory sexual selection revealed by artificial insemination. Nature 421, 360–363 (2003).

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