Polyandry is shaped by male mating behavior in house mice living in semi-natural conditions.

Polyandry is widespread across animals, yet its underlying causes and consequences remain unclear in natural populations. Recent work suggests that spatial and temporal variation in the environment shape the causes and consequences of polyandry. Here, we extend this research by testing whether one consequence of polyandry, multiple paternity, is influenced by females' social environment, specifically the prevalence of male alternative reproductive tactics (ARTs), territorials and roamers. We studied semi-natural populations of house mice (Mus musculus domesticus) under different food quality treatments. Doing so allowed us to explore whether the relationship between male ARTs and multiple paternity depends on environmental conditions, which are expected to mediate the relative costs and benefits of reproductive investment. Using microsatellite genotyping, we assigned paternity to 329 litters produced by 257 females across 5 generations. We then classified the sires of these litters as territorials or roamers based on their reproductive behavior around conception and quantified each sire's reproductive contribution to each litter. Multiple paternity was strongly associated with the presence of roamers, which were twice as likely to occur in multiple-sired litters compared with territorials. Territorials still sired most offspring, an effect consistent with roamers engaging in sneaky/opportunistic matings. However, we found no clear evidence that environmental conditions shift how ARTs contribute to multiple paternity. Together, our results demonstrate a tight link between female mating outcomes and male ARTs across environments, implying that the drivers of polyandry depend on the social environment. By explicitly linking ARTs to multiple paternity, our study provides a mechanistic understanding on how social dynamics shape reproductive outcomes in mating systems where both sexes mate multiply.