We investigated benefits of polyandry and phenotypic correlates of male mating success in black ratsnakes. f h

,We used 10 microsatellite loci to identify the sires of 375 hatchlings from 34 clutches with known mothers. Multiple paternity occurred in 88 % of clutches, despite the fact that ratsnakes mate when dispersed. The incidence of multiple paternity did not vary with female size or with clutch size. Hatching success tended to be higher in multiply sired clutches, but offspring from these clutches were not longer or heavier. We identified 34 males as the sires of 144 hatchlings. Most instances where sires could not be identified resulted from exclusion of all 426 potential sires that we screened. Unidentified sires were probably males that hibernated outside our study area and thus were not included in the intensive sampling at hibernacula in the study area. Among males that sired offspring, body size but not tail length contributed to success, consistent with male-biased sexual size dimorphism and male–male combat shown by this species. Large males were more successful primarily by siring more offspring per clutch rather than by siring offspring in more clutches. This may be a result of greater success in sperm competition. Neither body condition nor genetic similarity to the female was related to male mating success. Our results, in conjunction with published data on movement patterns, suggest that snakes of both genders can benefit from actively seeking multiple matings. 2004 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved