Previous studies have attempted to assess the reproductive benefits of polyandry in mammals but have failed to account for variation in mating frequency across experimental treatments. Recently, it has been shown that multiply sired litters are common in natural populations of house mice, suggesting that female mice are actively polyandrous and sperm competition occurs in this species. Here we show that female house mice mated to 3 males (polyandry) had greater postbirth pup survival than females mated 3 times to the same male (monandry). Remote behavioral observations revealed that copulation frequency did not differ across our experimental treatments. We discuss this result with reference to genetic benefit hypotheses for the evolution of polyandry. Furthermore, by genotyping parents and offspring in the polyandrous treatment, we assessed which male reproductive traits contributed to paternity. We found that the first principal component describing variation in 10 male reproductive variables explained a significant proportion of the variation in paternity when males were second to mate. This component was weighted predominantly by measures of sperm length, suggesting that males with short sperm may be more successful when engaging in a disfavored role of sperm competition.