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To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.anbehav.2006.09.016 Byline: A. Pilastro, M. Mandelli, C. Gasparini, M. Dadda, A. Bisazza Abstract: In polyandrous species, females may influence paternity by biasing sperm usage in favour of particular males. In the guppy, Poecilia reticulata, the number of sperm inseminated in a copulation depends on the female's perception of male attractiveness. We videorecorded copulations in the laboratory to test the hypothesis that there is a positive correlation between the number of sperm inseminated and the duration of the copulation. Duration was positively correlated with the number of sperm retrieved from the female's gonoduct. Once copulation duration was statistically controlled for, more sperm were retrieved from the gonoduct in courtship copulations than in coercive copulations, and in postpartum females than in virgin females. Copulation duration was positively correlated with the degree of carotenoid coloration of the male, suggesting that copulation duration might be the proximate mechanism of cryptic female choice in this species. The intensity of 'jerking', a postcopulatory behaviour of male guppies, was positively correlated with the number of sperm inseminated, suggesting a possible signalling function of this display. Author Affiliation: (a) Department of Biology, University of Padova, Italy (a ) Department of General Psychology, University of Padova, Italy Article History: Received 16 March 2006; Revised 28 April 2006; Accepted 4 September 2006 Article Note: (miscellaneous) MS. number: 8883