Passive sperm loss and patterns of sperm precedence in Muscovy Ducks (Cairina moschata)

Abstract

We investigated the importance of passive sperm loss in the outcome of sperm competition in captive, wild-type Muscovy Ducks (Cairina moschata). In nature, Muscovy Ducks can be expected to experience more intense sperm competition than most other waterfowl because of their non-monogamous mating system. We estimated the instantaneous rate of sperm loss from the reproductive tracts of female Muscovy Ducks as 0.0235 ± 0.0018 h-1 [SE], a fairly typical rate in comparison to the few other species of birds in which passive sperm loss has been measured. We also measured sperm precedence in trials in which a captive female was allowed to mate with two males in succession, with either a 24-h or a 72-h lag between matings. Paternity was determined with microsatellite markers. The mean proportion of a female's eggs fertilized by the second male (P2) was 0.72 ± 0.14 in trials with the 24-h lag and 0.42 ± 0.13 in trials with the 72-h lag. The last-male precedence observed in the 24-h trials can be explained by a quantitative model in which passive sperm loss alone determines average success, but this model is not consistent with the outcome of the 72-h trials. Other factors, including perhaps postcopulatory female choice, must be acting in addition to passive sperm loss in the trials with the longer lag. © 2010 by The American Ornithologists' Union. All rights reserved.